The following statements summarize and describe many of the key terms and concepts presented
in the chapter.
- Air has weight: At sea level it exerts a pressure of 1 kilogram per
square centimeter (14.7 pounds per square inch). Air
pressure is the force exerted by the weight of air above. With increasing altitude
there is less air above to exert a force, and thus air pressure decreases with altitude,
rapidly at first, then much more slowly. The unit used by meteorologists to measure
atmospheric pressure is the millibar. Standard sea-level
pressure is expressed as 1013.2 millibars. Isobars are lines on a
weather map that connect places of equal air pressure.
- A mercury barometer measures air pressure using a column of mercury in
a glass tube that is sealed at one end and inverted in a
dish of mercury. As air pressure increases, the mercury in the tube rises; conversely, when
air pressure decreases, so does the height of the column of mercury. A mercury barometer
measures atmospheric pressure in inches of mercury, the height of the column
of mercury in the barometer. Standard atmospheric pressure at sea level equals 29.92 inches
of mercury. Aneroid (without liquid) barometers consist of
partially evacuated metal chambers that compress as air pressure increases and expand as
- Wind is the horizontal flow of air from areas of higher pressure toward areas of
lower pressure. Winds are controlled by the following
combination of forces: (1) the pressure-gradient force (amount of pressure
change over a given distance); (2) Coriolis effect (deflective effect of
Earth's rotation to the right in the Northern Hemisphere and to the left in the Southern
Hemisphere); and (3) friction with Earth's surface (slows the movement of air
and alters wind direction).
- Upper-air winds, called geostrophic winds, blow parallel to the isobars
and reflect a balance between the pressure-gradient force
and the Coriolis effect. Upper-air winds are faster than surface winds because friction is
greatly reduced aloft. Friction slows surface winds, which in turn reduces the Coriolis
effect. The result is air movement at an angle across the isobars toward the area of lower
- The two types of pressure centers are (1) cyclones, or
lows (centers of low pressure), and (2) anticyclones, or
highs (high-pressure centers). In the Northern Hemisphere, winds around a low
(cyclone) are counterclockwise and inward. Around a high (anticyclone), winds are clockwise
and outward. In the Southern Hemisphere, the Coriolis effect causes winds to move clockwise
around a low and counterclockwise around a high. Because air rises and cools adiabatically
in low-pressure centers, cloudy conditions and precipitation are often associated with their
passage. In high-pressure centers, descending air is compressed and warmed; therefore, cloud
formation and precipitation are unlikely, and "fair" weather is usually expected.
- Earth's global pressure zones include the equatorial low,
sub-tropical high, subpolar low, and polar high. The
global surface winds associated with these pressure zones are the trade winds,
westerlies, and polar easterlies.
- Particularly in the Northern Hemisphere, large seasonal temperature differences over
continents disrupt the idealized, or zonal, global patterns of pressure and
wind. In winter, large, cold landmasses develop a seasonal high-pressure system from which
surface airflow is directed off the land. In summer, landmasses are heated and low pressure
develops over them, which permits air to flow onto the land. The seasonal changes in wind
direction are known as monsoons.
- In the middle latitudes, between 30 and 60 degrees latitude, the general west-to-east
flow of the westerlies is interrupted by migrating cyclones and anticyclones.
The paths taken by these pressure systems are closely related to upper-level airflow and the
polar jet stream. The average position of the polar jet stream, and hence the
paths followed by cyclones, migrates equatorward with the approach of winter and poleward as
- Local winds are small-scale winds produced by a locally generated
pressure gradient. Local winds include sea and land breezes
(formed along a
coast because of daily pressure differences caused by the differential heating of land and
water); valley and mountain breezes (daily wind similar to sea
and land breezes except in a mountainous area where the air along slopes heats differently
from the air at the same elevation over the valley floor); and chinook and
Santa Ana winds (warm, dry winds created when air descends the leeward side of
a mountain and warms by compression).
- The two basic wind measurements are direction and speed.
Winds are always labeled by the direction from which they blow. Wind direction
measured with a wind vane, and wind speed is measured using a cup
- El Niño is the name given to the periodic warming of the ocean
that occurs in the central and eastern Pacific. It is associated with periods when a
weakened pressure gradient causes the trade winds to diminish. A major El Niño event
triggers extreme weather in many parts of the world. When surface temperatures in the
eastern Pacific are colder than average, a La Niña event is triggered.
A typical La Niña winter blows colder-than-normal air over the Pacific Northwest and
the northern Great Plains while warming much of the rest of the United States.
- The global distribution of precipitation is strongly influenced by the global pattern
of air pressure and wind, latitude, and distribution of land and water.