climatology and meteorology

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ablation

(1) combined processes (such as sublimation, fusion or melting, evaporation) which remove snow or ice from the surface of a glacier or from a snow-field; also used to express the quantity lost by these processes (2) reduction of the water equivalent of a snow cover by melting, evaporation, wind and avalanches.

absolute humidity

(1) the ratio of the mass of water vapor to the volume occupied by a mixture of water vapor and dry air (2) mass of water contained in a unit volume of moist air.

accretion

growth of a cloud or precipitation particle by the collision and union of a frozen particle (ice crystal or snowflake) with a super-cooled liquid droplet which freezes on impact.

accumulation

all processes by which snow or ice are added to a glacier, this is typically the accumulation of snow, which is slowly transformed into ice; other accumulation processes can include avalanches, wind-deposited snow, and the freezing of rain within the snow pack.

accuracy

the extent to which the readings of an instrument approach the true value of the calculated or measured quantities, supposing that all possible corrections are applied.

acid precipitation

rain or snow containing acidic substances, resulting from the atmospheric pollution mainly with sulfur and nitrogen; acid precipitation has a lower pH than unpolluted rain.

advection

the horizontal transfer of air mass properties by the velocity field of the atmosphere.

advection fog

fog which forms in the lower part of a warm moist air mass moving over a colder surface (land or water).

aerosol

non-gaseous substances, divided into solid particles or liquid droplets, held in suspension in the atmosphere.

air freezing index

the cumulative number of degree-days below 0° C for the air temperature during a given time period.

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