Science / Weather Glossary

Absolute Humidity: A type of humidity that considers the mass of water vapor present per unit volume of space. Also considered as the density of the water vapor. It is usually expressed in grams per cubic meter.

Absolute Instability: When the lapse rate of a column of air is greater than the dry adiabatic lapse rate. The term absolute is used because this applies whether or not the air is dry or saturated.

Absolute Temperature Scale: A temperature scale with a freezing point of +273°K (Kelvin) and a boiling point of +373°K.

Absolute Zero: Considered to be the point at which theoretically no molecular activity exists or the temperature at which the volume of a perfect gas vanishes. The value is 0° Kelvin, -273.15° Celsius and . . . View Full Definition

Absorption: The process in which incident radiant energy is retained by a substance. The absorbed radiation is then transformed into molecular energy.

Abyssal Plain: The flat, gently sloping or nearly level region of the sea floor.

Adiabatic Process: A thermodynamic change of state in a system in which there is no transfer of heat or mass across the boundaries of the system. In this process, compression will result in warming and expansi . . . View Full Definition

Advection: The horizontal transfer of any property in the atmosphere by the movement of air (wind). Examples include heat and moisture advection.

Advection Fog: Fog that develops when warm moist air moves over a colder surface, cooling that air to below its dew point.

Advisory: Statements that are issued by the National Weather Service for probable weather situations of inconvenience that do not carry the danger of warning criteria, but, if not observed, could lead . . . View Full Definition

Afos: Acronym for Automation of Field Operations and Services. It is the computer system that links National Weather Service offices together for weather data transmission.

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%, . . . View Full Definition

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

Air Mass Thunderstorm: A thunderstorm that is produced by convection within an unstable air mass through an instability mechanism. Such thunderstorms normally occur within a tropical or warm, moist air mass during . . . View Full Definition

Air Pollution: The soiling of the atmosphere by contaminants to the point that may cause injury to health, property, plant, or animal life, or prevent the use and enjoyment of the outdoors.

Air Quality Standards: The maximum level which will be permitted for a given pollutant. Primary standards are to be sufficiently stringent to protect the public health. Secondary standards must protect the public . . . View Full Definition

Alaskan Winds: The downslope air flow that blows through the Alaskan valleys. It is usually given local names, such as Knik, Matanuska, Pruga, Stikine, Taku, Take, Turnagain, or Williwaw.

Albedo: The ratio of the amount of radiation reflected from an object's surface compared to the amount that strikes it. This varies according to the texture, color, and expanse of the object's surfa . . . View Full Definition

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