HOPKINS GLOSSARY OF WEATHER TERMS A

air pressure (or barometric pressure

• A measure of the weight of an entire air column of unit area cross section above the barometer, as a consequence of gravitational attraction, or equivalently, the force exerted by molecules of air acting on a unit area of any surface. See also station pressure and sea level corrected pressure. The units of pressure are the millibar, Pascal or centimeters of mercury in a mercury barometer.

alcohol thermometer

• A liquid-in-glass thermometer containing an alcohol liquid, such as ethanol. Because alcohol has a low freezing point, alcohol thermometers are as minimum thermometers at stations where temperatures routinely reach -40 degrees C (-40 degrees F).

altimeter

• An instrument that indicates the altitude (1) of an object with respect to a reference level such as sea level; examples include pressure altimeters and radio altimeters .

anemometer

• Any instrument that measures wind speed. Several types of anemometers include the hot wire, rotation and pressure-type anemometers .

aneroid barometer

• An instrument that measures the atmospheric (barometric) pressure. and operates on the principle of having changing air pressure bend a flexible metallic canister, which is partially evacuated. An external or internal spring prevents the capsule from collapsing. Changes in the size of this aneroid capsule are proportional to the difference between external and internal pressures, which in turn, move a pointer across an indicator scale or a pen on a recorder chart.

apogee

• Maximum altitude of a rocket above a reference plane, typically mean sea level.

azimuth or azimuth angle

• An angular direction described as the length of arc (in degrees) measured clockwise along the local horizon, from a reference point (usually true north), to that point on the horizon where the particular object or its projection is located; north is defined as having an azimuth of 0 degrees (or 360degrees), east is 90 degrees, and so forth.