Snow and Weather Glossary

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[edit] A

Above mean sea level (AMSL) refers to the elevation (on the ground) or altitude (in the air) of any object, relative to the average sea level datum. AMSL is used extensively in radio (both in broadcasting and other telecommunications uses) by engineers to determine the coverage area a station will be able to reach. It is also used in aviation, where all heights are recorded and reported with respect to AMSL (though also see flight level), and in the atmospheric sciences.


Accretion is an atmospheric science term for when an ice crystal or snowflake hits a supercooled liquid droplet, which then freeze together. This increases the size of the water particle. A common example of this that is visible to people is graupel.


Air Mass An extensive body of air throughout which the horizontal temperature and moisture characteristics are similar.


AIR This is considered the mixture of gases that make up the earth's atmosphere. The principal gases that compose dry air are Nitrogen (N2) at 78.09%, Oxygen (O2) at 20.946%, Argon (A) at 0.93%, and Carbon Dioxide (CO2) at 0.033%. One of the most important constituents of air and most important gases in meteorology is water vapor (H2O).


Albedo The albedo of an object is the extent to which it diffusely reflects light from the Sun. It is therefore a more specific form of the term reflectivity. Albedo is defined as the ratio of diffusely reflected to incident electromagnetic radiation. It is a unitless measure indicative of a surface's or body's diffuse reflectivity. The word is derived from Latin albedo "whiteness", in turn from albus "white". The range of possible values is from 0 (dark) to 1 (bright).


Albedo feedback A climate feedback involving changes in the Earth’s albedo. It usually refers to changes in the cryosphere' which has an albedo much larger (~0.8) than the average planetary albedo (~0.3). In a warming climate, it is anticipated that the cryosphere would shrink, the Earth’s overall albedo would decrease and more solar energy would be absorbed to warm the Earth still further.


Alcohol is any organic compound in which a hydroxyl functional group (-OH) is bound to a carbon atom, usually connected to other carbon or hydrogen atoms. An important class are the simple acyclic alcohols, the general formula for which is CnH2n+1OH. Of those, ethanol (C2H5OH) is the type of alcohol found in alcoholic beverages, and in common speech the word alcohol refers specifically to ethanol.


Alpine climate is the average weather (climate) for a region above the tree line. This climate is also referred to as mountain climate or highland climate. The climate becomes colder at high elevations—this characteristic is described by the lapse rate of air: air tends to get colder as it rises, since it expands. The dry adiabatic lapse rate is 10 °C per km of elevation or altitude. Therefore, moving up 100 meters on a mountain is roughly equivalent to moving 80 kilometers (45 miles or 0.75° of latitude) towards the pole. This relationship is only approximate, however, since local factors such as proximity to oceans can drastically modify the climate. The main form of precipitation is often snow, often accompanied by stronger winds. Compared to a polar climate, there is more sunlight during the winter.


Alpine permafrost (most permafrost is located in high latitudes (i.e. land in close proximity to the North and South poles)) may exist at high altitudes in much lower latitudes.


Altimeter An instrument used to determine the altitude of an object with respect to a fixed level. The type normally used by meteorologists measures the altitude with respect to sea level pressure.


Altitude In meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level.


Ambient temperature simply means "the temperature of the surroundings".


Anemometer The general name for instruments designed to measure either total wind speed or the speed of one or more linear components of the wind vector. These instruments may be classified according to the transducer employed; those commonly used in meteorology include the cup, propeller, Pitot-tube, hot-wire or hot-film, and sonic anemometers.


Aspect generally refers to the direction to which a mountain slope faces. The aspect of a slope can make very significant influences on its local climate (microclimate).


Atmosphere A gaseous envelope gravitationally bound to a celestial body (e.g., a planet, its satellite, or a star). Different atmospheres have very different properties. For instance, the atmosphere of Venus is very thick and cloudy, and is responsible for producing the very high surface temperatures on that planet by virtue of its greenhouse effect. On the other hand, the Martian atmosphere is very sparse. Earth's atmosphere is intermediate between these two extremes. It is distinguished from all other known atmospheres by its very active hydrologic cycle. One need merely examine pictures of Earth from space to appreciate the intricate cloud structures. Water in Earth's atmosphere plays a very important energetic role. Because of its chemical composition, most incoming sunlight passes through Earth's atmosphere and is absorbed at the ground. This heat is transported to the atmosphere through sensible heat and moisture fluxes. Upon condensation, this heat is then released into the atmosphere. The thermodynamics of water vapor is the crucial factor to the existence of severe storms in Earth's atmosphere. Since more solar radiation is absorbed in the Tropics than at high latitudes, the atmosphere (and the ocean) transports heat poleward. These motions are heavily altered by the effects of planetary rotation to determine the atmospheric general circulation. Fluid dynamical instabilities play a large role in this circulation and are crucial in determining the fluctuations in this circulation that we call “Weather.”The atmosphere may be conceptually divided into several layers, according to its thermal and ionization structure. The region where the temperature decreases because of the upward heat flux is called the troposphere. Above it, there is a layer in which temperature increases upward because of ozone absorption of solar radiation, the stratosphere. Above this, the temperature decreases in the mesosphere, and above this, in the thermosphere, the extremely energetic radiation causes temperature to increase with height out to the outer reaches of Earth's atmosphere, the exosphere. Within the mesosphere and thermosphere, solar radiation is sufficiently energetic to ionize gases.


Atmospheric convection is the result of a parcel-environment instability, or temperature difference, layer in the atmosphere. It is often responsible for adverse weather throughout the world.


Atmospheric pressure (Also called barometric pressure.) The pressure exerted by the atmosphere as a consequence of gravitational attraction exerted upon the “column” of air lying directly above the point in question. As with any gas, the pressure exerted by the atmosphere is ultimately explainable in terms of bombardment by gas molecules; it is independent of the orientation of the surface on which it acts. Atmospheric pressure is one of the basic meteorological elements. It is measured by many varieties of barometer and is expressed in several unit systems. The most common unit used is the millibar (1 millibar equals 1000 dynes cm−2). Unique to the science of meteorology is the use of inches (or millimeters) of mercury, that is, the height of a column of mercury that exactly balances the weight of the column of atmosphere the base of which coincides with that of the mercury column. Also employed are units of weight per area and units of force per area.


Azimuth is the angle from a reference vector in a reference plane to a second vector in the same plane, pointing toward, (but not necessarily meeting), something of interest. For example, with the sea as your reference plane, the azimuth of the Sun might be the angle between due North and the point on the horizon the Sun is currently over. An imaginary line drawn along the surface of the sea might point in the direction of the Sun, but would obviously never meet it. Azimuth is usually measured in degrees (°). The concept is used in many practical applications including navigation, astronomy, mapping, mining and artillery. The word azimuth is derived from the Arabic word السمت as-simt, which means direction, referring to the ways or directions a person faces.

[edit] B

Barometer An instrument for measuring atmospheric pressure. Two types of barometers are commonly used in meteorology: the mercury barometer and the aneroid barometer.

Barometer Pressure The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg).

Blizzard is a severe storm condition characterized by low temperatures, strong winds, and heavy snow. The difference between blizzard and a snowstorm is the strength of the wind.

Boot Packing The mechanical reworking of snow to harden it and to prevent depth hoar formation, usually by a large number of people walking up and down the slopes. This affects deeper layers than ski-cutting. This compresses any depth hoar which has formed and helps prevent its formation by decreasing pore space. Boot packing increases snow density and strength. This method is generally limited to small areas due to the manpower required.

[edit] C

Carbon Dioxide — Colorless gas, formula CO2, the fourth most abundant gas in dry air. The end product of the combustion or oxidation of organic material, including fossil fuels, CO2 is a very strong greenhouse gas and has very important radiative effects.


Civil Twilight The time between the moment of Sunset, when the sun's apparent upper edge is just at the horizon, until the center of the sun is 6° directly below the horizon.


Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and numerous other meteorological elements in a given region over longer periods of time. Climate can be contrasted to weather, which is the present condition of these same elements over periods up to two weeks.


Clouds 1. A visible aggregate of minute water droplets and/or ice particles in the atmosphere above the earth's surface. Cloud differs from fog only in that the latter is, by definition, close (a few meters) to the earth's surface. Clouds form in the free atmosphere as a result of condensation of water vapor in rising currents of air, or by the evaporation of the lowest stratum of fog. For condensation to occur at a low degree of supersaturation, there must be an abundance of cloud condensation nuclei for water clouds, or ice nuclei for ice-crystal clouds, at temperatures substantially above −40°C. The size of cloud drops varies from one cloud type to another, and within any given cloud there always exists a finite range of sizes. Generally, cloud drops (droplets) range from 1–100 μm in diameter, and hence are very much smaller than raindrops. See cloud classification. 2. Any collection of particulate matter in the atmosphere dense enough to be perceptible to the eye, as a dust cloud or smoke cloud.


Cloud albedo is a measure of the Albedo of a Cloud - higher values mean that the cloud reflects more solar radiation, or more radiation is Transmittance|transmitted. Cloud albedo varies from less than 10% to more than 90% and depends on drop sizes, liquid water or ice content, thickness of the cloud, and the sun's zenith angle. The smaller the drops and the greater the liquid water content, the greater the cloud albedo, if all other factors are the same. Low, thick clouds (such as stratocumulus) primarily reflect incoming solar radiation, whereas high, thin clouds (such as Cirrus) tend to transmit it to the surface but then trap outgoing infrared radiation, contributing to the greenhouse effect.


Cold refers to the condition or perception of having low temperature; it is the absence of heat or warmth. People intuitively associate various notions and images with cold, such as ice and the color blue. The coldest theoretically possible temperature is absolute zero, which is 0 K on the Kelvin scale, a thermodynamic temperature scale, and −273.15 °C on the Celsius scale. Absolute zero is also 0 °R on the Rankine scale, another thermodynamic temperature scale, and −459.67 degrees on the Fahrenheit scale.


Cold front is where a cold air mass is replacing warm air mass at some location. Because the frontal surface (boundary between the cold and warm air) is steep, air is pushed up rapidly creating cumulus-type clouds. Precipitation along a cold front is usually intense but short in duration. (See also: Warm front.)


Cold wave is a weather phenomenon that is distinguished by a cooling of the air. Specifically, as used by the U.S. National Weather Service, a cold wave is a rapid fall in temperature within a 24 hour period requiring substantially increased protection to agriculture, industry, commerce, and social activities. The precise criterion for a cold wave is determined by the rate at which the temperature falls, and the minimum to which it falls. This minimum temperature is dependent on the geographical region and time of year.


Corduroy The resulting pattern on the snow after grooming is known as corduroy in ski and snowboard slang, and is widely regarded as a good - albeit unexciting - surface on which to ski or ride.


Condensation In general, the physical process by which a vapor becomes a liquid or solid; the opposite of evaporation, although on the molecular scale, both processes are always occurring. In meteorological usage, this term is applied only to the transformation from vapor to liquid; any process in which a solid forms directly from its vapor is termed deposition, and the reverse process sublimation. In meteorology, condensation is considered almost exclusively with reference to water vapor that changes to dew, fog, or cloud. Condensation in the atmosphere occurs by either of two processes: cooling of air to its dewpoint, or addition of enough water vapor to bring the mixture to the point of saturation (that is, the relative humidity is raised to 100 percent).


Conduction Transport of energy (charge) solely as a consequence of random motions of individual molecules (ions, electrons) not moving together in coherent groups. Conduction of energy is a consequence of temperature gradients; conduction of charge (electrical conduction) is a consequence of electric potential gradients. Conduction is distinguished from convection in which energy (or charge) is transported by molecules (ions, electrons) moving together in coherent groups.


Constructive metamorphosis is caused by the upward movement of water vapor within the snow pack. Warmer temperatures are found closer to the ground because it receives heat from the core of the earth. Snow has a low thermal conductivity so this heat is retained creating a temperature gradient between the air underneath the snow pack and the air above it. Warmer air holds more water vapor. Through the process of sublimation the newly formed water vapor travels vertically by way of diffusion from a higher concentration (next to the ground) to a lower concentration (near the snow pack surface) by traveling through the air spaces between ice crystals. When the water vapor reaches the top of the snow pack it is subjected to much colder air causing it to condense and refreeze, forming ice crystals at the top of the snow pack that can be seen as the layer of crust on top of the snow.


Cooling The process by which temperature decreases due to an excess of emitted radiation over absorbed radiation.


Corn Snow (colloquial) Quasi-Corn, Spring Snow which is coarse, granular wet snow. During the spring and summer diurnal cycle of melting and refreezing the corn snow skiing is at its best in mid to late morning, after a layer has begun to melt but before it is too wet and sloppy. The term quasi-corn to refer to snow which is very much like corn snow but not truly from extensive melt-freeze metamorphism.


Convection is the movement of molecules within fluids (i.e. liquids, gases and rheids). It cannot take place in solids, since neither bulk current flows or significant diffusion can take place in solids. Convection is one of the major modes of heat transfer and mass transfer. Convective heat and mass transfer take place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid. In the context of heat and mass transfer, the term "convection" is used to refer to the sum of advective and diffusive transfer.


Cryosphere One of the earth's spheres of irregular form existing in the zone of interaction of the atmosphere, hydrosphere and lithosphere, distinguished by negative or zero temperature and the presence of water in the solid or super-cooled state; the term refers collectively to the portions of the earth where water is in solid form, including snow cover, floating ice, glaciers, ice caps, ice sheets, seasonally frozen ground and perennially frozen ground (permafrost).


Crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.


Crystal Faceting; when water freezes into ice, the water molecules stack together to form a regular crystalline lattice, and the ice lattice has six-fold symmetry. It is this hexagonal crystal symmetry that ultimately determines the symmetry of snow crystals.


Crown snow-load is snow and hard rime accumulating on tree crowns and structures in a cold climate. Hard rime is formed when droplets of fog or low level cloud (Stratus) freezes to the windward (wind-facing) side of tree branches, buildings, or any other solid objects, usually with high wind velocities and air temperatures between -2 °C (28 °F) and -8 °C (18 °F). Especially when a warm front brings wet snow, the surface of the tree is colder than the snowflakes, and the latter attach to the tree crown.

[edit] D

Dawn The first appearance of light in the eastern sky before Sunrise. It marks the beginning of morning Twilight. The visual display is created by the scattering of light reaching the upper atmosphere prior to the sun's rise to the observer's horizon.


Daylight or the light of day is the combination of all direct and indirect sunlight outdoors during the daytime (and perhaps twilight). This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected from the Earth and terrestrial objects. Sunlight scattered or reflected from objects in outer space (that is, beyond the Earth's atmosphere) is generally not considered daylight.


Deconstructive metamorphosis begins as the snow makes its way to the ground often melting, refreezing, and settling. Water molecules become reordered causing the snowflakes to become more spherical in appearance. These melting snowflakes fuse with others around them becoming larger until all are uniform in size. While the snow is on the ground the melting and joining of snow flakes reduces the height of snow pack by shrinking air spaces and causing the density and mechanical strength of the snow pack to increase. Freshly fallen snow with a density of 0.1/cm3 has very good insulating properties; however as time goes on, due to destructive metamorphism the insulating property of the snow pack decreases because the air spaces between snowflakes disappear. Snow that has been residing on the ground for a long period of time has an average density of 0.40g/cm3 and conducts heat well; however, once a base of 50 cm of snow with a density around 0.3g/cm3 has accumulated, temperatures under the snow remain relatively constant because the greater depth of snow compensates for its density. Destructive metamorphosis is a function of time, location, and weather. It occurs at a faster rate with higher temperatures, in the presence of water, under larger temperature gradients (e.g., warm days followed by cold nights), at lower elevations and on slopes that receive large amounts of solar radiation. As time goes on snow settles compacting air spaces, a process expedited by the packing force of the wind.


Deep Hoar Snow Large-grained, faceted, cup-shaped crystals near the ground. Depth hoar forms because of large temperature gradients within the snowpack.


Density refers to Mass per volume, usually specified in kilogram per cubic meter (kg/m^3). The density of water is 1000 kg/m^3 and snow density is usually measured as a ratio to this. So snow which is 100 kg/m^3 is specified as 100/1000, or 10 percent (of the density of water). New snowfall is typically between 7% and 12% but can be lower or higher than this sometimes. Wind exposure often increases the density to 20% to 30%. Higher density (heavier) snow typically results from warmer temperatures and/or winds while lower density (lighter snow) usually results from colder air with less wind. The density will increase over time due to snow settlement. Old snow may reach 40% to 50% density and firn can reach 60%. The highest density of ice known is fully mature glacier ice.

Dew Condensation in the form of small water drops that forms on grass and other small objects near the ground when the temperature has fallen to the dew point, generally during the nighttime hours.


Dew Point The temperature to which air must be cooled at a constant pressure to become saturated.


Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material. In a phase with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing or a state of equilibrium. Basically, it is the movement of molecules from an area of high concentration to a lower area.


Doppler Radar Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar. Using the Doppler effect, it measures the velocity of particles. Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away.


Downslope Effect The warming of an air flow as it descends a hill or mountain slope.


Drifting Snow Snow particles blown from the ground by the wind to a height of less than six feet or 2 meters.


Drifts Normally used when referring to snow or sand particles are deposited behind obstacles or irregularities of the surface or driven into piles by the wind.


Drizzle is light precipitation consisting of liquid water drops smaller than those of rain, and generally smaller than 0.5 mm (0.02 in.) in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates due to drizzle are on the order of a millimeter per day or less at the ground. Owing to the small size of drizzle drops, under many circumstances drizzle largely evaporates before reaching the surface, and so may be undetected by observers on the ground.


Dry Bulb Thermometer A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperature. One of the two thermometers that make up a psychrometer.


Dry ice, sometimes referred to as "Cardice" or as "card ice" is the solid form of carbon dioxide. It is commonly used as a versatile cooling agent because in normal conditions it is about 78.5K colder than frozen water, and a half denser than frozen water. Thus, dry ice carries almost four times the cooling effect as an ice block of the same size. More, whereas ice melts into an oft inconvenient puddle of water, dry ice dissipates into "thin air" of harmless carbon dioxide. Dry ice costs about a dollar a pound (US) versus ten cents a pound for water ice in blocks. So, the convenience of dry ice costs over twice as much as water ice for the same amount of cooling.


Dusk The period of waning light from the time of sunset to dark.

[edit] E

Earth Systems "Earth system science (ESS)," the key term is "system." A system is a collection of interdependent parts enclosed within a defined boundary. Within the boundary of the earth is a collection of four interdependent parts called "spheres." Earth's spheres include:the lithosphere, the hydrosphere, the biosphere, and the atmosphere.


Elevation The measure of height with respect to a point on the earth's surface above mean sea level.


Equator is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole that divides the Earth into a Northern Hemisphere and a Southern Hemisphere. The equators of other planets and astronomical bodies are defined analogously.


Eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition. This composition is known as the eutectic composition and the temperature is known as the eutectic temperature. The intersection of the eutectic temperature and the eutectic composition gives the eutectic point.


Evaporation The physical process by which a liquid, such as water is transformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.

[edit] F

Faceted snow Angular snow with poor bonding created from large temperature gradients within the snowpack.

Felt air temperature (or apparent air temperature) is a quantitative measure in degrees Celsius that indicates the amount of heat the human body loses outdoors in a given time and place. It is an indication of the air temperature perceived by the body, which may differ from the actual temperature due to other climactic conditions affecting the exchange of heat between the body and the air.


Firn — Old snow that has become granular and compacted (dense) as the result of various surface metamorphoses, mainly melting and refreezing but also including sublimation. The resulting particles are generally spherical and rather uniform.


Firnification (Snow to ice conversion) As snow is progressively buried by further snowfall it becomes compacted and eventually metamorphoses into ice. Key change = air passages are sealed off at a density of ~ 830 kg m-3. At depth depending on water, temperature, pressure.


Forecast A statement of expected future occurrences. Weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist.


Fluid is defined as a substance that continually deforms (flows) under an applied shear stress. All gases are fluids, but not all liquids are fluids. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids.


Freezing Point / Freeze The process of changing a liquid to a solid. The temperature at which a liquid solidifies under any given set of conditions. Pure water under atmospheric pressure freezes at 0°C or 32°F.


Freezing-point depression describes the phenomenon in which the freezing point of a liquid (a solvent) is depressed when another compound is added, meaning that a solution has a lower freezing point than a pure solvent. This happens whenever a solute is added to a pure solvent, such as water. The phenomenon may be observed in sea water, which due to its salt content remains liquid at temperatures below 0 °C, the freezing point of pure water.


Freezing Rain Is Rain that falls as liquid and freezes upon impact to form a coating of glaze on the colder ground or other exposed surfaces.


Fresh water or (the predominant nontechnical spelling) fresh water is naturally occurring water on the Earth's surface in bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Freshwater is characterized by having low concentrations of dissolved salts and other total dissolved solids. The term specifically excludes seawater and brackish water.


Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapor available. Frost is also usually translucent in appearance. There are many types of frost, such as radiation and window frost.


Frozen Ground Soil or Rock in which part or all of the pore water has turned into ice.


Frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the soil and adjacent materials, and on nearby heat sources. For example, snow cover and asphalt insulate the ground and homes can heat the ground. Alternatively, in Arctic and Antarctic locations the freezing depth is so deep that it becomes year-round permafrost, and the term "thaw depth" is used instead.


Fog Water droplets suspended in the atmosphere in the vicinity the earth's surface that affect visibility. According to international definition, fog reduces visibility below 1 km (0.62 miles). Fog differs from cloud only in that the base of fog is at the earth's surface while clouds are above the surface. When composed of ice crystals, it is termed ice fog. Visibility reduction in fog depends on concentration of cloud condensation nuclei and the resulting distribution of droplet sizes. Patchy fog may also occur, particularly where air of different temperature and moisture content is interacting, which sometimes make these definitions difficult to apply in practice. Fogs of all types originate when the temperature and dewpoint of the air become identical (or nearly so). This may occur through cooling of the air to a little beyond its dewpoint (producing advection fog, radiation fog or upslope fog), or by adding moisture and thereby elevating the dewpoint (producing steam fog or frontal fog). Fog seldom forms when the dewpoint spread is greater than 4°F. In aviation weather observations fog is encoded F, and ground fog GF. Fog is easily distinguished from haze by its higher relative humidity (near 100%, having physiologically appreciable dampness) and gray color. Haze does not contain activated droplets larger than the critical size according to Köhler theory. Mist may be considered an intermediate between fog and haze; its particles are smaller (a few μm maximum) in size, it has lower relative humidity than fog, and does not obstruct visibility to the same extent. There is no distinct line, however, between any of these categories.


Fusion The change of state from a solid to a liquid at the same temperature. The heat of fusion is the number of gram calories of heat necessary to change one gram of a substance from the solid to the liquid state. It is the opposite of freezing.

[edit] G

Gas is a state of matter, consisting of a collection of particles (molecules, atoms, ions, electrons, etc.) without a definite shape or volume that are in more or less random motion.


Geomorphology (from Greek: γῆ, ge, "earth"; μορφή, morfé, "form"; and λόγος, logos, "study") is the scientific study of landforms and the processes that shape them. Geomorphologists seek to understand why landscapes look the way they do: to understand landform history and dynamics, and predict future changes through a combination of field observation, physical experiment, and numerical modeling. Geomorphology is practiced within geography, geology, geodesy, engineering geology, archaeology, and geotechnical engineering. Early studies in geomorphology are the foundation for pedology, one of two main branches of soil science.


Glacier is a perennial mass of ice which moves over land. A glacier forms in locations where the mass accumulation of snow and ice exceeds ablation over many years. The word glacier comes from French via the Vulgar Latin glacia, and ultimately from Latin "glacies" meaning ice.


Graupel A form of frozen precipitation consisting of snowflakes or ice crystals and supercooled water droplets frozen together.


Gust A sudden significant increase in or rapid fluctuations of wind speed. Peak wind must reach at least 16 knots (18 miles per hour) and the variation between peaks and lulls is at least 10 knots (11.5 miles per hour). The duration is usually less twenty seconds.

[edit] H

Hail Precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls or irregular pieces of ice, which comes in different shapes and sizes. Hail is considered to have a diameter of 5 millimeter or more; smaller bits of ice are classified as ice pellets, snow pellets, or graupel.


Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky. The WMO manual of codes includes a classification of horizontal obscuration into categories of fog, ice fog, steam fog, mist, haze, smoke, volcanic ash, dust, sand and snow. Sources for haze particles include farming (ploughing in dry weather), traffic, industry, and wildfires.


Heat is the process of energy transfer from one body or system to another due to thermal contact, which in turn is defined as an energy transfer to a body in any other way than due to work performed on the body.


Heat transfer is the transition of thermal energy from a hotter object to a cooler object ("object" in this sense designating a complex collection of particles which is capable of storing energy in many different ways). When an object or fluid is at a different temperature than its surroundings or another object, transfer of thermal energy, also known as heat transfer, or heat exchange, occurs in such a way that the body and the surroundings reach thermal equilibrium.


Height is the measurement of vertical distance, but has two meanings in common use.


High-altitude is sometimes defined to begin at 2,400 metres (8,000 ft) above sea level. Regions on the Earth's surface (or in its atmosphere) that are high above mean sea level are referred to as high altitude.


Horizon is the apparent line that separates earth from sky. It is the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting intersection of earth and sky is called the visible horizon. When looking at a sea from a shore, the part of the sea closest to the horizon is called the offing.


Humidity is the amount of water vapor in the air. Relative humidity is defined as the ratio of the partial pressure of water vapor in a parcel of air to the saturated vapor pressure of water vapor at a prescribed temperature. Humidity may also be expressed as specific humidity. Relative humidity is an important metric used in forecasting weather. Humidity indicates the likelihood of precipitation, dew, or fog. High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by reducing the evaporation of perspiration from the skin. This effect is calculated in a heat index table.

[edit] I

Ice The solid form of water. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail for example.


Iceberg is a large piece of ice from freshwater that has broken off from a snow-formed glacier or ice shelf and is floating in open water. It may subsequently become frozen into pack ice. Alternatively, it may come to rest on the seabed in shallower water, causing ice scour (also known as ice gouging) or becoming an ice island.


Ice Crystals Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as halos, coronas, and sun pillars. May be called "diamond dust." It is reported as "IC" in an observation and on the METAR.


Ice fog is a type of fog consisting of fine ice crystals suspended in the air. It can happen only in cold areas of the world since water can remain liquid down to -40 °C (-40 °F). It should be distingued from diamond dust, a precipitation of sparse ice crystal falling from clear sky.


Ice nucleus is a particle which acts as the nucleus for the formation of an ice crystal in the atmosphere. The presence of ice nuclei increase the temperature that ice will form in the atmosphere from around −42°C to about −10°C. There are many processes that can take place in the atmosphere to form ice particles, the simplest is by water vapor depositing directly onto the solid particle. The presence of an ice nucleus can also cause a previously supercooled water droplet to freeze through contact, immersion or dissolution within the water that would otherwise have stayed in the liquid phase at a given temperature.


Ice Pellets Precipitation in the form of transparent or translucent pellets of ice, which are round or irregular in shape. They have a diameter of 0.2 inches (5 mm) or less. They are classified into two types: hard grains of ice consisting of frozen rain drops or largely melted and refrozen snowflakes; pellets of snow encased in a thin layer of ice which have formed from the freezing of droplets intercepted by pellets or water resulting from the partial melting of pellets.


Ice sheet is a mass of glacier ice that covers surrounding terrain and is greater than 50,000 km² (20,000 mile²), thus also known as continental glacier.


Ice Storm A severe weather condition characterized by falling freezing precipitation. Such a storm forms a glaze on objects, creating hazardous travel conditions and utility problems.


Insolation is a measure of solar radiation energy received on a given surface area in a given time. The name comes from a portmanteau of the words incident solar radiation. It is commonly expressed as average irradiance in watts per square meter (W/m2) or kilowatt-hours per square meter per day (kW·h/(m2·day)) (or hours/day). In the case of photovoltaics it is commonly measured as kWh/(kWp·y) (kilowatt hours per year per kilowatt peak rating).


Isotherm, represents the altitude in which the temperature is at 0°C (the freezing point of water) in a free atmosphere (i.e. allowing reflection of the sun by snow, etc.). Any given measure is valid for only a short period of time, often less than a day. Above the freezing level, the temperature of the air is below freezing. Below it, the temperature is above freezing. The profile of this frontier, and its variations, are studied in meteorology, and are used for a variety of forecasts and predictions. Whilst not given on general weather forecasts, it is used on bulletins giving forecasts for mountainous areas.


Inversion A departure from the usual increase or decrease of an atmospheric property with altitude. It usually refers to an increase in temperature with increasing altitude, which is a departure from the usual decrease of temperature with height.


Ice Crystal Any one of a number of macroscopic, crystalline forms in which ice appears, including hexagonal columns, hexagonal platelets, dendritic crystals, ice needles, and combinations of these forms.

The crystal lattice of ice is hexagonal in its symmetry under most atmospheric conditions. Varying conditions of temperature and vapor pressure can lead to growth of crystalline forms in which the simple hexagonal pattern is present in widely different habits (a thin hexagonal plate or a long thin hexagonal column). In many ice crystals, trigonal symmetry can be observed, suggesting an influence of a cubic symmetry. The principal axis (c axis) of a single crystal of ice is perpendicular to the axis of hexagonal symmetry. Planes perpendicular to this axis are called basal planes (a axes related to the prism facets) and present a hexagonal cross section.

[edit] J

Jet Stream An area of strong winds that are concentrated in a relatively narrow band in the upper troposphere of the middle latitudes and subtropical regions of the Northern and Southern Hemispheres. Flowing in a semi-continuous band around the globe from west to east, it is caused by the changes in air temperature where the cold polar air moving towards the equator meets the warmer equatorial air moving polarward. It is marked by a concentration of isotherms and strong vertical shear.


Joule–Thomson effect In thermodynamics, the Joule–Thomson effect describes the Temperature change of a gas or liquid when it is forced through a valve or porous plug while kept insulated so that no heat is exchanged with the environment.

[edit] K

Kelvin (symbol: K) is a unit increment of temperature and is one of the seven SI base units. The Kelvin scale is a thermodynamic (absolute) temperature scale referenced to absolute zero, the theoretical absence of all thermal energy. So by definition, the temperature of a substance at absolute zero is zero kelvin (0 K).

[edit] L

Landscape comprises the visible features of an area of land, including the physical elements of landforms, water bodies such as rivers, lakes and the sea, living elements of land cover including indigenous vegetation, human elements including land uses, buildings and structures, and transitory elements such as lighting and weather conditions.


Latent heat refers to the amount of energy released or absorbed by a chemical substance during a change of state that occurs without changing its temperature, meaning a phase transition such as the melting of ice or the boiling of water. The term was introduced around 1750 by Joseph Black as derived from the Latin latere, to lie hidden.


Latitude The location north or south in reference to the equator, which is designated at zero (0) degrees. Parallel lines that circle the globe both north and south of the equator. The poles are at 90° North and South latitude.


Liquid Is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material.


Longitude The location east or west in reference to the Prime Meridian, which is designated as zero (0) degrees longitude. The distance between lines of longitude are greater at the equator and smaller at the higher latitudes, intersecting at the earth's North and South Poles. Time zones are correlated to longitude.


Lux A lux meter for measuring illuminances in work places. The lux (symbol: lx) is the SI unit of illuminance and luminous emittance. It is used in photometry as a measure of the apparent intensity of light hitting or passing through a surface. It is analogous to the radiometric unit watts per square metre, but with the power at each wavelength weighted according to the luminosity function, a standardized model of human brightness perception.

[edit] M

Magnesium chloride Approximately 51% of world output is now used by cold countries to de-ice roads in winter Calcium chloride is preferred over sodium chloride, since CaCl releases energy upon forming a solution with water, heating any ice or snow it is in contact with. It also lowers the freezing point, depending on the concentration. NaCl does not release heat upon solution; however, it does lower the freezing point. It is also more readily available and does not have any special handling or storage requirements, unlike calcium chloride. The salinity (S) of water is measured as grams salt per kilogram (1000g) water, and the freezing temperatures are as follows.

S(g/kg) 0 10 20 24.7 30 35
T(freezing) (C) 0 -0.5 -1.08 -1.33 -1.63 -1.91


Mass (from Ancient Greek: μᾶζα) commonly refers to any of three properties of matter, which have been shown experimentally to be equivalent: inertial mass, active gravitational mass, and passive gravitational mass. In everyday usage, "mass" is often used interchangeably with weight.


Melt Freeze Metamorphism Metamorphism which results from one or (more commonly) more cycles of melting followed by refreezing. One sequence of melting conditions followed by freezing conditions is called a melt-feeze cycle. This is usually diurnal with melt occuring starting in mid-morning or early aftrenoon and freezing starting when the sun goes down. A period of melt-freeze metamorphism leads to corn snow.


Melting Level The altitude at which ice crystals and snow flakes melt as they descend through the atmosphere. Related term: bright band


Melting Point The temperature at which a solid substance undergoes fusion, changing from a solid to a liquid state. Contrast with freezing point.


Metamorphism The process where deposited snow changes its form. Changes in the grain structure are caused by varying pressure and temperature conditions.


Microclimate is a local atmospheric zone where the climate differs from the surrounding area. The term may refer to areas as small as a few square feet (for example a garden bed) or as large as many square miles (for example a valley).


Mist A collection of microscopic water droplets suspended in the atmosphere. It does not reduce visibility as much as fog and is often confused with drizzle.


Mixed Precipitation Any of the following combinations of freezing and frozen precipitation: snow and sleet, snow and freezing rain, or sleet alone. Rain may also be present.


Moisture Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.


Mountain is a large landform that stretches above the surrounding land in a limited area usually in the form of a peak. A mountain is generally steeper than a hill. The adjective montane is used to describe mountainous areas and things associated with them. The study of mountains is called Orography.


Mountain Breeze A katabatic wind, it is formed at night by the radiational cooling along mountainsides. As the slopes become colder than the surrounding atmosphere, the lower levels of air cool and drain to the lowest point of the terrain. It may reach several hundred feet in depth, and extreme cases, attain speeds of 50 knots or greater. It blows in the opposite direction of a valley breeze.


Mountain Wave A wave in the atmosphere caused by a barrier, such as a mountain. Sometimes it is marked by lenticular clouds to the lee side of mountain barriers. May be called a standing wave or a lee wave.

[edit] N

Nadir (from Arabic نظير nadhir, "opposite") is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface there. Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous terms. Specifically, in astronomy, geophysics and related sciences (e.g., meteorology), the nadir at a given point is the local vertical direction pointing in the direction of the force of gravity at that location. The direction opposite of the nadir is the zenith.


New Snow 1. Recently fallen snow in which the original form of the snow crystals is recognizable. 2. The amount of snow fallen within the previous 24 hours.


Newton The unit of force giving a mass of about one kilogram (2.205 pounds) an acceleration of about one meter (1 yard) per second per second.


Night The period of the day between dusk and dawn.


Nitrogen (N2) A colorless, tasteless, odorless gas that is the most abundant constituent of dry air. It comprises 78.09%.


Northern Hemisphere is the half of a planet that is north of equator—the word hemisphere literally means 'half sphere'. It is also that half of the celestial sphere north of the celestial equator. Earth's northern hemisphere contains most of its land area and most of its human population (about 90%).


Nucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form via nucleation in liquids are gaseous bubbles, crystals, or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation.

[edit] O

Oceanic climate, also called marine west coast climate, maritime climate, Cascadian climate and British climate for Köppen climate classification Cfb and subtropical highland for Köppen Cfb or Cwb, is a type of climate typically found along the west coasts at the middle latitudes of some of the world's continents. This climate has cool summers and warm winters, with a narrow annual temperature range. It typically lacks a dry season, as precipitation is more evenly dispersed through the year. It is the predominant climate type across much of Europe, coastal northwestern North America, portions of southern South America and Africa, southeast Australia, New Zealand, as well as isolated locations elsewhere.


Orography (from the Greek όρος, hill, γραφία, to write) is the study of the formation and relief of mountains and can more broadly include hills, and any part of a region's elevated terrain. Orography (also known as oreography, orology or oreology) falls within the broader discipline of geomorphology.


Oxygen (O2) A colorless, tasteless, odorless gas that is the second most abundant constituent of dry air, comprising 20.946%.


Ozone (O3) A nearly colorless gas and a form of oxygen (O2). It is composed of an oxygen molecule made up of three oxygen atoms instead of two.


oxygen-16 The major stable isotope of oxygen, having eight protons and eight neutrons; it amounts to over 99% of the element in nature.


Oxygen isotope ratio cycle are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substances, such as polar ice or calcite in ocean core samples. The ratio is linked to water temperature of ancient oceans, which in turn reflects ancient climates. Cycles in the ratio mirror climate changes in geologic history.


Ozone Layer An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface.

[edit] P

Permafrost or permafrost soil is soil at or below the freezing point of water (0 °C or 32 °F) for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of the ground material.


Precipitation Any of all of the forms of water particles, whether liquid or solid, that fall from the atmosphere and reach the ground. The forms of precipitation are: rain, drizzle, snow, snow grains, snow pellets, diamond dust, hail, and ice pellets.


Pressure The force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface.


Pressure Change The net difference between the barometric pressure at the beginning and ending of a specified interval of time, usually the three hour period preceding an observation.


Prevailing Wind A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.

[edit] R

Rain Precipitation in the form of liquid water drops that have diameters greater than 0.5 mm, or, if widely scattered, the drops may be smaller. The only other form of liquid precipitation, drizzle, is to be distinguished from rain in that drizzle drops are generally less than 0.5 mm in diameter, are very much more numerous, and reduce visibility much more than does light rain. For observing purposes, the intensity of rainfall at any given time and place may be classified as

  1. “light,” the rate of fall varying between a trace and 0.25 cm (0.10 in.) per hour, the maximum rate of fall being no more than 0.025 cm (0.01 in.) in six minutes;
  2. “moderate,” from 0.26 to 0.76 cm (0.11 to 0.30 in.) per hour, the maximum rate of fall being no more than 0.076 cm (0.03 in.) in six minutes;
  3. “heavy,” over 0.76 cm (0.30 in.) per hour or more than 0.076 cm (0.03 in.) in six minutes.


Rain crust A type of snow crust formed by refreezing after surface snow crystals have been melted and wetted by liquid precipitation.


Radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body. Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature.


Radiational Cooling The cooling of the earth's surface and the adjacent air. Although it occurs primarily at night, it happens when the earth's surface suffers a net loss of heat due to outgoing radiation.


Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves. The law of reflection says that for specular reflection the angle at which the wave is incident on the surface equals the angle at which it is reflected. Mirrors exhibit specular reflection.


Rime The rapid freezing of supercooled water droplets as they touch an exposed object, forming a white opaque granular deposit of ice. It is one of the results of an ice storm, and when formed on aircraft it is called rime icing.

[edit] S

Sastrugi are sharp irregular grooves or ridges formed on a snow surface by wind erosion and deposition, and found in polar and temperate snow regions. They differ from sand dunes in that the ridges are parallel to the prevailing winds. These words are plural: the singular is sastruga or zastruga. The word was taken from Russian.


Sea ice is largely formed from ocean water that freezes. Because the oceans consist of saltwater, this occurs at about -1.8 °C (28.8 °F). Sea ice may be contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into the ocean. Icebergs are compacted snow and hence fresh water.


Seawater or Saltwater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%. This means that every kilogram, or every litre, of seawater has approximately 35 grams (1.2 oz) of dissolved salts (mostly, but not entirely, the ions of sodium chloride: Na+, Cl−). The average density of seawater at the ocean surface is 1.025 g/ml; seawater is denser than freshwater (which reaches a maximum density of 1.000 g/ml at a temperature of 4 °C (39 °F)) because of the salts’ added mass. The freezing point of sea water decreases with increasing salinity and is about −2 °C (28.4 °F) at 35 g/L.[1].


Seasonally frozen ground, ground that freezes and thaws annually.


Sky is the part of the atmosphere or of outer space visible from the surface of any astronomical object. It is difficult to define precisely for several reasons. During daylight, the sky of Earth has the appearance of a deep blue surface because of the air's scattering of sunlight. The sky is sometimes defined as the denser gaseous zone of a planet's atmosphere. At night the sky has the appearance of a black surface or region scattered with stars.


Sleet may refer to: Rain and snow mixed, particularly in countries where British English is spoken or Ice pellets, mainly within the United States.


Slip stations A defined location where the slip crew is stationed. The slip crew is directed by the section chief. It must also be clearly visible to incoming slippers. Instructions regarding where to slip in the area below must be passed across quickly and clearly. Section leaders will coordinate slippers and any special slipping needs during training and the competition.


Slush Snow or ice on the ground that has been reduced to a softy watery mixture by rain and/or warm temperatures.


Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. The process of this precipitation is called snowfall. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure.


Snow Accumulation (also called snow depth.) A measurement of the depth of snow on the ground made either since the snow began falling or since a previous observation. The total accumulation is equivalent to the total snow depth during a storm, or after any single snowstorm or series of storms. Snow accumulation can vary due to settling and melting and will therefore vary depending on how often it is measured. For example, if new snow is measured every hour during a relatively long duration storm, it is likely that the summed accumulations may exceed a total snow accumulation measured only once at the end of the storm.


Snow Advisory A statement or advisory issued when snow is expected to create hazardous travel conditions. It warns of less severe weather conditions than a winter storm.


Snowball is a spherical object made from snow, usually created by scooping snow with the hands, and compacting it into a roughly fist-sized ball. The snowball is often used to engage in games, such as snowball fights.


Snowball maker A device used to make snowballs. Reduces issues with cold wet hands, soggy gloves or poorly packed snowballs.


Snow Banner A plume of snow blown off a mountain crest, resembling smoke blowing from a volcano.


Snow Blindness Temporary blindness or impaired vision that results from bright sunlight reflected off the snow surface. The medical term is niphablepsia.


Snowburn A burn of the skin, like a sunburn, but caused by the sun's rays reflected off the snow surface.


Snow Cover The areal extent of ground covered by the snow. It is usually expressed as a percent of the total area of a given region.


Snow Cornice is a Snow drift feature that forms along a break in slope, typically along ridgelines in exposed mountain areas. These can range from less than one meter to tens of meters in width and depth and from a few meters to more than a kilometer in length. Cornices can present serious hazards to mountain travelers including climbers, skiers, hikers, and snowmobilers.


Snow Creep A continuous, extremely slow, downhill movement of a layer of snow.


Snow Crust The crisp, almost icy, surface on fallen snow, usually formed by the slight melting and refreezing of the surface snow.


Snow Crystals Any of several types of ice crystal found in snow. A snow crystal is a single crystal, in contrast to a snowflake, which is usually an aggregate of many single snow crystals.


Snow Depth The actual depth of snow on the ground at any instant during a storm, or after any single snowstorm or series of storms.


Snow Devil A small, rotating wind that picks up loose snow instead of dirt (like a dust devil) or water (like a waterspout). Formed mechanically by the convergence of local air currents. May be called a snowspout.


Snow Eater Any warm downslope wind, or foehn, that blows over snowy terrain and melts the snow.


Snowfall The rate at which snow falls, usually expressed in inches or centimeters of snow depth over a six hour period.


Snow Flakes Colloquially an ice crystal, or more commonly an aggregation of many crystals that falls from a cloud. Simple snowflakes (single crystals) exhibit beautiful variety of form, but the symmetrical shapes reproduced so often in photomicrographs are not found frequently in snowfalls. Broken single crystals, fragments, or clusters of such elements are much more typical of actual snow. Snowflakes made up of clusters of crystals (many thousand or more) or crystal fragments may grow as large as three to four inches in diameter, often building themselves into hollow cones falling point downward. In extremely still air, flakes with diameters as large as 10 inches have been reported.


Snow Flurry / Flurries Light showers of snow, generally very brief without any measurable accumulation.


Snow Garland Snow appearing as a beautiful long thick rope draped on trees, fences and other objects. Formed by the surface tension of thin films of water bonding individual snow crystals.


Snow Grains Snow grains are minute, white and opaque grains of ice. When they hit hard ground, they do not bounce or shatter. They usually fall in very small quantities, and never in the form of a shower. Frozen precipitation in the form of very small, white, opaque grains of ice. The solid equivalent of drizzle.


Snow Level The elevation in mountainous terrain where the precipitation changes from rain to snow, depending on the temperature structure of the associated air mass.


Snow Line The lowest elevation area of a perennial snow field on high terrain, such as a mountain range.


Snowmaggedon When a huge winter blizzard surpasses a previous large winter storm (see snowpocalypse) and dumps copious amounts of snow in a place unprepared for such large snowfall amounts thus grinding entire transportation, retail, school and government systems to nearly a complete halt for several days. Results in extreme cabin fever leading to massive snowball fights and excessive photography.


Snow Pellets Frozen precipitation in the form of white, round or conical opaque grains of ice. Their diameter ranges from 0.08 to 0.2 inch (2 to 5 mm). They are easily crushed and generally break up after rebounding from a hard surface, unlike hail. Sometimes it is called small or soft hail.


Snowpocalypse snow + apocalypse = snowpocalypse When it is predicted to have large amounts of snowfall in a short period of time.


Snowpack The amount of annual accumulation of snow at higher elevations.


Snow removal is the job of removing snow after a snowfall to make competition track and course easier and safer. This is done by both individuals, course crews, and the ski area, with tools and different types of machines and powered equipment.


Snow Roller The product of moist, cohesive snow that when initiated by wind rolls across the landscape, gathering snow until it can no longer move. It is shaped like a rolled sleeping bag, some reaching four feet across and seven feet in diameter.


Snow Shower Frozen precipitation in the form of snow, characterized by its sudden beginning and ending. It is reported as "SHSN" in an observation and on the METAR.


Snow Sintering Crystals lose their points due to molecular motion, wind, and direct pressure. Physically breaking the snow crystals, for instance stomping on them or disturbing them with a shovel, will produce the same effect. The crystal arms are broken and then rounded grains fuse by freezing into larger crystals in a process called sintering.


Snow Squall A heavy snow shower accompanied by sudden strong winds, or a squall.

Snow temperature gradient (Tgrad) The change in the temperature of the snow with depth expressed by the equation: Tgrad = (Ts - Tb) / Zs where Ts: snow surface temperature (°C), Tb: snow bottom temperature (°C), and Zs: snow depth (m). Tgrad is expressed as °C m-1 (°C/m). The snow temperature gradient increases as air temperatures (snow surface temperatures) decrease.

Snow Water Equivalent (SWE) is a common snowpack measurement. It is the amount of water contained within the snowpack. It can be thought of as the depth of water that would theoretically result if you melted the entire snowpack instaneously. To determine the depth of snow using snow water equivalent and density, use the following formula: [SWE] ÷ [Density] = Snow Depth (Density must be in decimal form. For example: 25% = 0.25)


Sodium chloride, also known as common salt, table salt, or halite, is an ionic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms. As the major ingredient in edible salt, it is commonly used as a condiment and food preservative.


Spring crust is a type of snow crust, formed when loose firn is recemented by a decrease in temperature. It is most common in late winter and spring.


Sublimation The process of a solid (ice) changing directly into a gas (water vapor), or water vapor changing directly into ice, at the same temperature, without ever going through the liquid state (water). The opposite of crystallization.


A Sun chart is a graph of the ecliptic of the Sun through the sky throughout the year at a particular latitude. Most sun charts plot azimuth versus altitude throughout the days of the winter solstice and summer solstice, as well as a number of intervening days. Since the movement of the Sun is symmetrical about the solstice, it is only necessary to plot dates from one half of the year. The graph may show the entire horizon or only the half of the horizon closest to the equator. Sky view obstructions can be superimposed upon a Sun chart to obtain the insolation of a location.


Sun crust is a type of snow crust formed by refreezing after surface snow crystals have been melted by the sun.


Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon. When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during a period, that an area receives direct irradiance from the Sun of at least 120 watts per square meter. Sunlight may be recorded using a sunshine recorder, pyranometer or pyrheliometer. Sunlight takes about 8.3 minutes to reach the Earth. Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux, which includes infrared, visible, and ultraviolet light. Bright sunlight provides illuminance of approximately 100,000 lux or lumens per square meter at the Earth's surface.


Sunrise The daily appearance of the sun on the eastern horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun appears on the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunrise is calculated for mean sea level.


Sunset The daily disappearance of the sun below the western horizon as a result of the earth's rotation. In the United States, it is considered as that instant when the upper edge of the sun just disappears below the sea level horizon. In Great Britain, the center of the sun's disk is used instead. Time of sunset is calculated for mean sea level.


Supercooling Is the process of lowering the temperature of a liquid or a gas below its freezing point, without it becoming a solid. A liquid below its standard freezing point will crystallize in the presence of a seed crystal or nucleus around which a crystal structure can form. However, lacking any such nucleus, the liquid phase can be maintained all the way down to the temperature at which crystal homogeneous nucleation occurs.


Supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. It can also refer to a vapor of a compound that has a higher (partial) pressure than the vapor pressure of that compound.

[edit] T

Table salt is refined salt, 99% sodium chloride. It usually contains substances that make it free-flowing (anti-caking agents) such as sodium silicoaluminate or magnesium carbonate.


Temperature The quantity measured by a thermometer. Bodies in thermal equilibrium with each other have the same temperature. In gaseous fluid dynamics, temperature represents molecular kinetic energy, which is then consistent with the equation of state and with definitions of pressure as the average force of molecular impacts and density as the total mass of molecules in a volume. For an ideal gas, temperature is the ratio of internal energy to the specific heat capacity at constant volume.


Thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures temperature or temperature gradient using a variety of different principles.


Temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature gradient is a dimensional quantity expressed in units of degrees (on a particular temperature scale) per unit length. The SI unit is kelvin per meter (K/m). Temperature gradients in the atmosphere are important in the atmospheric sciences (meteorology, climatology and related fields).


Thaw is the period when the snow and ice melt, at the end of the winter, in cold climates.


Thaw depth or thaw line is the level down to which the permafrost soil will normally thaw each summer in a given area. The layer of soil over the thaw depth is called the active layer, while the soil below is called inactive layer.


Thunder Snow A wintertime thunderstorm from which falls snow instead of rain. Violent updrafts and at or below freezing temperatures throughout the atmosphere, from surface to high aloft, discourage the melting of snow and ice into rain. Intense snowfall rates often occur during these situations.


Tree line or timberline is the edge of the habitat at which trees are capable of growing. Beyond the tree line, they are unable to grow because of inappropriate environmental conditions (usually cold temperatures, insufficient air pressure, or lack of moisture). At the tree line, tree growth is often very stunted, with the last trees forming low, densely matted bushes. If it is caused by wind, it is known as krummholz formation, from the German for 'twisted wood'.


Twilight Often called dusk, it is the evening period of waning light from the time of sunset to dark. The time of increasing light in the morning is called dawn. Twilight ends in the evening or begins in the morning at a specific time and can be categorized into three areas of decreasing light. Civil twilight is the time in the evening when car headlights need to be turned on to be seen by other drivers. Nautical twilight is when the bright stars used by navigators have appeared and the horizon may still be seen. Astronomical twilight is when the sunlight is still shining on the higher levels of the atmosphere, yet it is dark enough for astronomical work to begin. During dawn, the reverse order occurs until full daylight.

[edit] V

Volume is how much three-dimensional space a substance (solid, liquid, gas, or plasma) or shape occupies, often quantified numerically using the SI derived unit, the cubic metre. The volume of a container is generally understood to be the capacity of the container, i. e. the amount of fluid (gas or liquid) that the container could hold, rather than the amount of space the container itself displaces.

Three dimensional mathematical shapes are also assigned volumes. Volumes of some simple shapes, such as regular, straight-edged, and circular shapes can be easily calculated using arithmetic formulas. The volumes of more complicated shapes can be calculated by integral calculus if a formula exists for the shape's boundary. One-dimensional figures (such as lines) and two-dimensional shapes (such as squares) are assigned zero volume in the three-dimensional space.

The volume of a solid (whether regularly or irregularly shaped) can be determined by fluid displacement. Displacement of liquid can also be used to determine the volume of a gas. The combined volume of two substances is usually greater than the volume of one of the substances. However, sometimes one substance dissolves in the other and the combined volume is not additive.


Vorticity is a concept used in fluid dynamics. In the simplest sense, vorticity is the tendency for elements of the fluid to "spin." More formally, vorticity can be related to the amount of "circulation" or "rotation" (or more strictly, the local angular rate of rotation) in a fluid.

[edit] W

Water Refers to the chemical compound, H2O, as well as its liquid form. At atmospheric temperatures and pressures, it can exist in all three phases: solid (ice), liquid (water), and gaseous (water vapor). It is a vital, life-sustaining part of life on earth.


Water molecule is formed from two hydrogen atoms and one oxygen atom. The bonding angle of the two hydrogens is almost 105 degrees rather than 180 degrees which would make the molecule symmetrical. This causes it to be dipolar, giving it a positive and negative side which accounts for its unique properties. This allows the formation of hydrogen bonds between adjacent molecules.


Water Cycle The vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas.


Water Vapor Called aqueous vapor, moisture. Water substance in vapor form; one of the most important of all constituents of the atmosphere.


Weather The state of the atmosphere at a specific time and with respect to its effect on life and human activities. It is the short term variations of the atmosphere, as opposed to the long term, or climatic, changes. It is often referred to in terms of brightness, cloudiness, humidity, precipitation, temperature, visibility, and wind.


Weather radar, or weather surveillance radar (WSR), is a type of radar used to locate precipitation, calculate its motion, estimate its type (rain, snow, hail, etc.), and forecast its future position and intensity.


Wet-bulb temperature is a type of temperature measurement that reflects the physical properties of a system with a mixture of a gas and a vapor, usually air and water vapor. Wet-bulb temperature can have several technical meanings: Thermodynamic wet-bulb temperature: the temperature a volume of air would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it, all latent heat being supplied by the volume of air. The temperature read from a wet bulb thermometer. Adiabatic wet-bulb temperature: the temperature a volume of air would have if cooled adiabatically to saturation and then compressed adiabatically to the original pressure in a moist-adiabatic process.


Whiteout When visibility is near zero due to blizzard conditions or occurs on sunless days when clouds and surface snow seem to blend, erasing the horizon and creating a completely white vista.


Wind Air in motion relative to the surface of the earth. Since vertical components of atmospheric motion are relatively small, especially near the surface of the earth, meteorologists use the term to denote almost exclusively the horizontal component. Vertical winds are usually identified as such. Winds; Air that flows in relation to the earth's surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons, rawin, or aircraft reports.


Wind Chill Index The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature.


Wind crust A type of snow crust formed by the packing action of wind on previously deposited snow. Wind crust may break locally, but, unlike wind slab, does not constitute an avalanche hazard.


Wind Direction The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, WNW, etc.).


Wind Shift The term applied to a change in wind direction of 45 degrees or more, which takes place in less than 15 minutes. It may the result of a frontal passage, from katabatic winds, sea breezes, or thunderstorms, and in some instances, the change may be gradual or abrupt.


Windsock or wind cone is a conical textile tube designed to indicate wind direction and relative wind speed. Windsocks typically are used at airports and at chemical plants where there is risk of gaseous leakage. They are sometimes located alongside highways at windy locations.


Wind Speed The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, meters per sec or nautical miles per hour.


Wind Vane An instrument that indicates the wind direction. The end of the vane which offers the greatest resistance to the motion of the air moves to the downwind position.


Windward The direction from which the wind is blowing. Also the upwind side of an object. The opposite of the downwind or leeward side.


Wind Wave is an ocean or lake wave resulting from the action of wind on the water's surface. After it leaves its fetch area, it is considered a swell.


Winter Astronomically, this is the period between the winter solstice and the vernal equinox. It is characterized as having the coldest temperatures of the year, when the sun is primarily over the opposite hemisphere. Customarily, this refers to the months of December, January, and February in the North Hemisphere, and the months of June, July, and August in the Southern Hemisphere.


Winter Storm Any one of several storm systems that develop during the late fall to early spring and deposit wintry precipitation, such as snow, freezing rain, or ice.

[edit] Y

Year The interval required for the earth to complete one revolution around the sun. A sidereal year, which is the time it take for the earth to make one absolute revolution around the sun, is 365 days, 6 hours, 9 minutes, and 9.5 seconds. The calendar year begins at 12 o'clock midnight local time on the night of December 31st-January 1st. Currently, the Gregorian calendar of 365 days is used, with 366 days every four years, a leap year. The tropical year, also called the mean solar year, is dependent on the seasons. It is the interval between two consecutive returns of the sun to the vernal equinox. In 1900, that took 365 days, 5 hours, 48 minutes, and 46 seconds, and it is decreasing at the rate of 0.53 second per century.


Yellow Snow Snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it.

[edit] U

Universal Time Coordinate One of several names for the twenty-four hour time which is used throughout the scientific and military communities.

[edit] Z

Zenith is the direction pointing directly "above" a particular location; that is, it is one of two vertical directions at the location, orthogonal to a horizontal flat surface there. The concept of "above" is more specifically defined in astronomy, geophysics and related sciences (e.g., meteorology) as the vertical direction opposite to the net gravitational force at a given location. The opposite direction, i.e. the direction of the gravitational force is called the nadir. The term zenith also refers to the highest point reached by a celestial body during its apparent orbit around a given point of observation. This sense of the word is often used to describe the location of the Sun, but it is only technically accurate for one latitude at a time and only possible at the low latitudes.


[edit] Weather Gallery

[edit] Also see


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