# Working with Snow

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 Revision as of 09:24, 13 April 2009 (edit)Joe (Talk | contribs)← Previous diff Revision as of 09:25, 13 April 2009 (edit) (undo)Joe (Talk | contribs) Next diff → Line 1: Line 1: - == '''[[Snow and Weather Glossary]]'''== + == '''Snow and Weather Glossary'''== '''[[Air Mass]]''' '''[[Air Mass]]''' Line 17: Line 17: '''[[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). '''[[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). - '''[[Clouds]]''' + '''[[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. - *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. + 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. - + - 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. + '''[[Civil Twilight]]''' '''[[Civil Twilight]]'''

## Snow and Weather Glossary

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).

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.

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

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

Barometer PressureThe 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).

Clouds1. 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.

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.

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.

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.

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.

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

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

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.

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.

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

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.

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.

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.

Humidty The amount of water vapor in the air. It is often confused with relative humidity or dew point. Related terms: absolute humidity, relative humidity, and specific humidity

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.

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 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 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.

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.

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.

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 90Â° North and South latitude.

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.

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.

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 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.

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%.

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.

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.

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.

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.

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.

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

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

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 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 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. Related term: Chinook and Dave's Dictionary

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

SNOWFLAKES An ice crystal or an aggregate of ice crystals which fall from clouds.

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 Frozen precipitation in the form of very small, white, opaque grains of ice. The solid equivalent of drizzle. It is reported as "SG" in an observation and on the METAR.

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.

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

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. It is reported as "GS" in an observation and on the METAR.

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

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.

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.

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.

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.

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

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,

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.

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 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.

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, or nautical miles per hour. The unit most often used in the United States is 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 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.

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.

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

## FAQ Snow

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