Working with Snow

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Air Mass




Creating a Snow Patch


Barometer Pressure


Civil Twilight


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.

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.

HUMIDITY 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.

Ice Crystal

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.


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.

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%.



Snow Flakes


FAQ Snow

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.

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