Ch. 18 Air Pressure and Wind

air pressure

The force exerted by the weight of a column of air above a given point.

mercury barometer

A mercury-filled glass tube in which the height of the mercury column is a measure of air pressure.

aneroid barometer

An instrument for measuring air pressure that consists of evacuated metal chambers that are very sensitive to variations in air pressure.

barograph

A recording barometer.

wind

Air flowing horizontally with respect to Earth’s surface.

isobars

A line drawn on a map that connects points of equal atmospheric pressure, usually corrected to sea level.

pressure gradient force

The amount of pressure change occurring over a given distance.

coriolis effect

The deflective force of Earth’s rotation on all free-moving objects, including the atmosphere and oceans. Deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

geostrophic winds

A wind, usually above a height of 600 meters (2000 feet), that blows parallel to the isobars.

jet streams

Swift (120–240 kilometer per hour), high-altitude winds.

cyclones

A low-pressure center characterized by a counterclockwise flow of air in the Northern Hemisphere. Also called a low.

highs

A high-pressure center characterized by a clockwise flow of air in the Northern Hemisphere. Also called an anticyclone.

convergence

The condition that exists when the distribution of winds in a given area results in a net horizontal inflow of air into the area. Because convergence at lower levels is associated with an upward movement of air, areas of convergent winds are regions favorable to cloud formation and precipitation.

divergence

The condition that exists when the distribution of winds in a given area results in a net horizontal outflow of air from the region. In divergence at lower levels, the resulting deficit is compensated for by a downward movement of air from aloft; hence, areas of divergent winds are unfavorable to cloud formation and precipitation.

pressure or barometric tendency

The nature of the change in atmospheric pressure over the past several hours. It can be a useful aid in short-range weather prediction.

equatorial low

A belt of low pressure that lies near the equator and between the subtropical highs.

intertropical convergence zone (ITCZ)

The zone of general convergence between the Northern and Southern Hemisphere trade winds.

subtropical high

Not a continuous belt of high pressure but rather several semipermanent, anticyclonic centers characterized by subsidence and divergence located roughly between latitudes 25° and 35°.

trade winds

Two belts of winds that blow almost constantly from easterly directions and are located on the equatorward sides of the subtropical highs.

westerlies

The dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs.

polar easterlies

In the global pattern of prevailing winds, winds that blow from the polar high toward the subpolar low. These winds, however, should not be thought of as persistent winds, such as the trade winds.

subpolar low

Low pressure located at about the latitudes of the Arctic and Antarctic Circles. In the Northern Hemisphere the low takes the form of individual oceanic cells; in the Southern Hemisphere there is a deep and continuous trough of low pressure.

polar front

The stormy frontal zone separating air masses of polar origin from air masses of tropical origin.

polar high

Anticyclones that are assumed to occupy the inner polar regions and are believed to be thermally induced, at least in part.

monsoons

Seasonal reversal of wind direction associated with large continents, especially Asia. In winter, the wind blows from land to sea; in summer, from sea to land.

local winds

A small-scale wind produced by a locally generated pressure gradient. Examples include land and sea breezes and mountain and valley breezes.

sea breeze

A local wind blowing from the sea during the afternoon in coastal areas.

land breeze

A local wind blowing from land toward the water during the night in coastal areas.

valley breeze

The daily upslope winds commonly encountered in a mountain valley.

mountain breeze

The nightly downslope winds commonly encountered in mountain valleys.

chinooks

A wind blowing down the leeward side of a mountain and warming by compression.

santa ana

The local name given a chinook wind in southern California.