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Acid Rain or Snow:  Rain or Snow that has a pH value of 5.6 or lower.

Active front:  A front, or portion thereof, which produces appreciable cloudiness and precipitation.

Actual elevation:  The vertical distance above mean sea level of the ground at a meteorological station.

Advection fog:  A type of fog caused by the flow of moist air over a cold surface, and the consequent cooling of that air to below its dew point.

Afer:  A southwest wind in Italy.

Afterglow:  A broad high arch of radiance seen occasionally in the western sky above the highest clouds in deepening twilight.

Air:  The mixture of gases comprising the earth’s atmosphere.

Air mass:  A large body of air where the horizontal distribution of temperature, pressure, and moisture is nearly uniform.

Air mass shower:  A shower that is produced by local convection within an unstable air mass. Such showers are not associated with a front or instability line.

Air mass source region:  An extensive area of the Earth’s surface over which bodies of air frequently remain for a sufficient time to acquire characteristic temperature and moisture properties imparted by that surface.

Air parcel:  An imaginary body of air to which may be assigned any or all of the basic dynamic and thermodynamic properties of atmospheric air.

Aircraft ceiling:  The ceiling classification applied when the reported ceiling value has been determined by a pilot while in flight within one and one-half miles of any runway of the airport.

Aircraft weather reconnaissance:  The making of detailed weather observations or investigations from aircraft in flight.

AIREP:  Abbreviation for aircraft report.

Airport elevation:  The officially designated elevation of an airport above mean sea level. It is the highest point on any of the runways of the airport.

Alaska current:  an ocean current, the northward flowing division of the Aleutian current. It has the character of a warm current in spite of the fact that it carries sub-arctic water.

Alberta Clipper:  A small, fast moving storm which forms on a quasi-stationary Pacific front, often in the lee of the Rocky Mountains in Alberta, Canada. The low normally travels quickly southeast across the northern or central United States and is followed by arctic air. It is now recognized that lows moving inland from the Pacific are the actual parent systems.

Aleutian current:  an eastward flowing ocean current which lies north of the North Pacific current. As the current approaches the coast of North America it divides to form the northward-flowing Alaska current, and the southward flowing California current.

Altimeter:  An instrument which determines the altitude of an object with respect to a fixed level. Two types are pressure altimeters and radio altimeters.

Altocumulus:  A principal cloud type, white or gray in color, comprised of either water droplets or ice crystals. Altocumulus occur as a layer or patch with waved or rolled aspects, found anywhere from the 6000 to 20,000 foot level.

Altostratus:  A principal cloud type, gray in color. Altostratus appear in a sheet or layer with uniform appearance, found anywhere from the 6500 to 20,000 foot level.

Anabatic wind:  An upslope wind. The opposite of katabatic wind.

Anafront:  A weather front at which the warm air ascends the frontal surface up to high altitudes.

Anemograph:  A device which measures and records the strength of the wind.

Anemometer:  An instrument used to measure the strength of the wind.

Annual flood:  The highest flow at any point on a stream during any particular calendar year or water year. The annual flood need not exceed the established flood stage.

Antarctic air:  A type of air whose characteristics are developed in an Antarctic region. Antarctic air appears to be colder at the surface in all seasons, and at all levels in fall and winter, than arctic air.

Anticyclogenesis:  Any strengthening or development of anticyclonic circulation in the atmosphere; the opposite of anticyclolysis.

Anticyclolysis:  Any weakening of anticyclonic circulation in the atmosphere; the opposite of anticyclogenesis.

Anticyclone:  A weather system with a clockwise circulation in the northern hemisphere and counterclockwise circulation in the southern hemisphere. Anticyclones are normally associated with high atmospheric pressure and thus are called High Pressure Systems.

Anticyclonic:  Having a sense of rotation about the vertical opposite to that of the Earth’s rotation, that is clockwise in the northern hemisphere, counter-clockwise in the southern hemisphere.

Anvil cloud:  The cirrus outflow cloud found capping a thunderstorm.

Arctic Air:  A type of air mass that develops during the winter over surfaces of ice and snow. The air mass is deep but surface temperatures are usually higher than those of a polar air mass.

Arctic front:  The semi-permanent, semi-continuous front between the deep, cold arctic air and the shallower, basically less cold polar air of northern latitudes.

Arid climate:  Generally, any extremely dry climate.

Astronomical twilight:  The interval of incomplete darkness between sunrise (or sunset) and the moment when the true position of the sun is 18 degrees below the horizon, at which there is no discernible horizon glow over the sun’s azimuth.

Atmosphere:  The envelope of gases surrounding the earth and bound to it more or less permanently by the virtue of the earth’s gravitational attraction.

Atmospheric pressure:  The force exerted by the atmosphere as a consequence of gravitational attraction exerted upon the column of air lying directly above the point in question. Higher atmospheric pressure means more air is within the column of air and the opposite remains true for lower atmospheric pressure. Expressed in a variety of units, the most common being the millibar, but also measured in inches of mercury.

Aurora:  The sporadic radiant emission from the upper atmosphere over middle and high latitudes. Auroras are related to magnetic storms and the influx of charged particles from the sun.

Aurora Australis:  The aurora of southern latitudes.

Aurora Borealis:  The aurora of northern latitudes.

Auroral zone:  A circle around either geomagnetic pole within which there is a maximum of auroral activity. This zone lies about ten to fifteen degrees of geomagnetic latitude from the geomagnetic pole. The auroral zone broadens and extends equatorward during intense auroral displays.

Autumn:  The season of the year which is the transition period from summer to winter, occurring as the sun approaches the winter solstice. In popular and in meteorological usage autumn includes the months of September, October and November in the Northern Hemisphere and March, April and May in the Southern Hemisphere. Astronomically, the period extending from the autumnal equinox to the winter solstice.

Autumnal equinox:  The point at which the sun passes directly over the equator on its journey south. The time of this occurrence is approximately September 22.

Avalanche:  A mass of snow moving rapidly down a steep mountain slope.

Avalanche wind:  The rush of air produced in front of an avalanche of dry snow or in front of a landslide. The most destructive form, the avalanche blast, occurs when an avalanche is stopped abruptly, as in the case of an almost vertical fall into a valley floor. such blasts may have very erratic behavior, leveling one house without damaging its neighbor.

Azores High:  The semi-permanent subtropical high over the North Atlantic Ocean, so named especially when it is located over the eastern part of the ocean. The same high, when displaced to the western part of the Atlantic, or when it develops a separate high there, is known as the Bermuda High.