Snow Glossary

ablation

the process of being removed. Snow ablation usually refers to removal by melting

accretion

growth of precipitation particles by collision of ice crystals with supercooled liquid droplets which freeze on impact

blizzard

winds of at least 35 miles per hour along with considerable falling and/or blowing snow reducing visibility to less than one-quarter mile for a period of at least three hours. (extremely cold temperatures often are associated with dangerous blizzard conditions, but are not a formal part of the modern definition.)

climatology

the science and study of climate; the aggregate of all weather conditions at a point over a long period of time

condensation

the process in which water vapor becomes liquid

dendrite

hexagonal ice crystals with complex and often fernlike branches.

depth hoar

large (one to several millimeters in diameter), cohesionless, coarse, faceted snow crystals which result from the presence of strong temperature gradients within the snowpack

evaporation (water)

the physical process in which liquid water changes into a gas

firn

rounded, well-bonded snow that is older than one year

graupel

snowflakes that become rounded pellets due to riming. Typical sizes are two to five millimeters in diameter (0.1 to 0.2 inches). Graupel is sometimes mistaken for hail.

metamorphism

changes in the structure and texture of snow grains which results from variations in temperature, migration of liquid water and water vapor, and pressure within the snow cover

precipitation

the accumulated depth of rain or drizzle and also the melted water content of snow or other forms of frozen precipitation, including hail

polycrystal

a snowflake composed of many inddidual ice crystals

rime

a deposit of ice formed when supercooled water droplets freeze on contact with an object

saturation vapor pressure (water)

the maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water. This is the maximum water vapor the air can hold for any given combination of temperature and pressure

snowboard

a solid, flat, white material, such as painted plywood, approximately two feet on each side, that is laid on the ground or on the surface of the snow by weather observers to obtain more accurate measurements of snowfall and water content

snow core

a sample of snow, either just the freshly fallen snow or the combined old and new snow on the ground, obtained by pushing a cylinder down through the snow layer and extracting it

snowbursts

very intense showers of snow, often of short duration, that greatly restrict visibility and produce periods of rapid snow accumulation

snow density

the mass of snow per unit volume which is equal to the water content of snow ddided by its depth

snowfall

the depth of new snow that has accumulated since the previous day or since the previous observation

snow depth

the combined total depth of both old and new snow on the ground

snowflake

a cluster of ice crystals that falls from a cloud

snow flurries

snow that falls for short durations and which often changes in intensity. Flurries usually produce little accumulation

snow load

the downward force on an object or structure caused by the weight of accumulated snow

snow water equivalent

the water content obtained from melting

snowpack

the total snow and ice on the ground, including both new snow and the previous snow and ice which has not melted

snow squall

a brief, but intense fall of snow that greatly reduces visibility and which is often accompanied by strong winds

sublimation

the process in which ice changes directly to water vapor without melting, but also in meteorology the opposite process in which water vapor is transformed to ice (also called deposition)

supercooled

the condition when a liquid remains in the liquid state even through its temperature is below its freezing point

supersaturation

the condition which occurs in the atmosphere when the relative humidity is greater than 100 percent

surface hoar

the deposition (sublimation) of ice crystals on a surface which occurs when the temperature of the surface is colder than the air above and colder than the frost point of that air

vapor pressure

the pressure exerted by water vapor molecules in a given volume of air