Unit 8 - Geography

Upper atmosphere

Atmospheric level above the surface friction layer.

Windward

Exposed, upwind side of the topographic barrier that faces the winds that flow across it.

Barometer

Instrument that measures atmospheric pressure; invented by Torricelli in 1643.

Pressure-gradient force

The difference in surface pressure divided by the distance between two locations is called the __________. ___________ gives rise to a force that causes air to move (as wind) from the place of higher pressure to that of lower pressure.

Mountain breeze

A breeze that flows downward; results from a greater radiative heat loss from the mountain slopes at night and the resulting increase in density.

Surface layer

Also known as the friction layer, the lower troposphere where frictional forces are important, generally the lowest 500 to 2000 m (1640 to 6562 ft) of the atmosphere. Above this layer frictional effects on wind flow are negligible.

Anticyclone

Atmospheric high-pressure cell. Low-level anticyclones are characterized by sinking motion and spiraling airflow out of the central area of high pressure. In the Northern Hemisphere, winds flow clockwise around an anticyclone; in the Southern Hemisphere, winds flow counterclockwise around an anticyclone.

Cyclone

Atmospheric low-pressure cell involving the convergence of air at low levels, which flows into and spirally rises at the center. The isobars around the cyclone are generally circular in shape, with their values decreasing toward the center. In the Northern Hemisphere, winds flow counterclockwise around a cyclone; in the Southern Hemisphere, winds flow clockwise around a cyclone.

Cold-air drainage

Category of local-scale wind systems governed by the downward oozing of heavy, dense, cold air along slopes under the influence of gravity; produces catabolic winds (such as southeastern France's mistral) that are fed by massive pools of icy air that accumulate over such major upland regions as the Alps and the Rocky Mountains.

Frictional force

Drag that slows the movement of air molecules in contact with, or close to, the Earth's surface; varies with the "roughness" of the surface. There is less friction with movement across a smooth water surface than across the ragged skyline of a city center.

Coriolis force

Force that, owing to the rotation of the Earth, tends to deflect all objects moving over the surface of the Earth away from their original paths. In the absence of any other forces, the deflection is to the right tin the Northern Hemisphere and to the left in the Southern Hemisphere; the higher the latitude, the stronger the deflection.

Santa Ana wind

Hot, dry, foehnh-type wind that occasionally affects Southern California. Its unpleasantness is heightened by the downward funneling of this air flow from the high inland desert through narrow passes in the mountains that line the Pacific Coast.

Isobar

Line connecting all points having identical atmospheric pressure.

Surface wind

Low-level winds within the friction layer. Such winds blow across the isobars instead of parallel to them, thus producing a flow of surface air out of high-pressure areas and into low-pressure areas at an oblique angle to the isobars. Surface wind speed is determined by the pressure gradient and the roughness of the surface.

Wind

Movement of air relative to the Earth's surface. Winds are always named according to the direction from which they blow.

Chinook wind

Name given to the foehn winds that affect the leeward areas of mountain zones in the western plateaus of North America.

Valley breeze

Occurs as mountain terrain heats up rapidly in the sun, causing lower pressure to develop over the mountain ridges, spawning a breeze that flows upslope during the day.

Land breeze

Offshore air flow affecting a coastal zone, resulting from a nighttime pressure gradient that steers local winds from the cooler (higher-pressure) land surface to the warmer (lower-pressure) sea surface.

Sea breeze

On shore airflow affecting a coastal zone, resulting from a daytime pressure gradient that steers local winds from the cooler (higher-pressure) sea surface onto the warmer (lower-pressure) land surface.

Leeward

Protected side of a topographic barrier with respect to the winds that flow across it; often refers to the area downwind from the barrier as well, which is said to be in the "shadow" of that highland zone.

Gravitational force

The primary force exerting an influence on air molecules. The atmosphere is "held" against Earth by this force, and our planet would have no atmosphere at all were it not for gravity.

Pressure (atmospheric)

Weight of a column of air per unit area at a given location, determined by the acceleration of gravity and the mass of atmosphere at that location. Standard sea-level air pressure produces a reading of 760 mm (29.92 in) on the mercury barometer. In terms of pressure, it is also given as 1,013.25 millibars (mb), or 14.7 pounds/square inch.

Geostrophic wind

Wind that results when the Coriolis and pressure-gradient forces balance each other out. Geostrophic wind has a constant speed and direction and blows parallel to straight isobars.

Katabatic wind

Winds that result from cold-air drainage; especially prominent under clear conditions where the edges of highlands plunge sharply toward lower-lying terrain.