Air Pressure and Wind opens with a description of the units and instruments used for measuring atmospheric pressure. A definition of wind is followed by an analysis of the factors that affect it—pressure gradient force, Coriolis effect, and friction. A discussion of cyclones and anticyclones includes associated movements of air and weather patterns, also presented are the general, global patterns of pressure and wind. A more detailed discussion of atmospheric circulation in the mid-latitudes is followed by descriptions of several local winds. Wind measurement and instruments are also briefly mentioned. The chapter closes with an examination of the relations between global precipitation and Earth’s pressure and wind belts.
Learning Objectives
After reading, studying, and discussing this chapter, you should be able to;
•Describe air pressure, how it is measured, and how it changes with altitude.
•Explain how the pressure gradient force, Coriolis effect, and friction influence wind.
•Describe the movements of air associated with the two types of pressure centers.
•Describe the idealized global patterns of pressure and wind.
•Discuss the general atmospheric circulation in the mid-latitudes.
•List the names and causes of the major local winds.
•Describe the global distribution of precipitation.
Chapter Summary
•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 1015.2 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 pressure decreases.
•Wind is the horizontal flow of air from areas of higher pressure to 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 force 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 (the most prominent feature being the jet streams) generally flow parallel to the isobars and are called geostrophic winds.
•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) they are clockwise and outward. In the Southern Hemisphere, the Coriolis effect causes winds to be clockwise around a low and counterclockwise around a high. Since air rises and cools adiabatically in a low pressure system, cloudy conditions and precipitation are often associated with their passage. In a high pressure system, descending air is compressed and warmed, therefore, cloud formation and precipitation are unlikely in an anticyclone, and “fair” weather is usually expected.
•Earth’s global pressure zones include the equatorial low, subtropical 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 air flow is directed off the land. In summer, landmasses are heated and a low—pressure system develops over them, which permits air to flow onto the land. These seasonal changes in wind direction are known monsoons.
•In the middle latitudes, between 50 and 60 degrees latitude, the general west-to-east flow of the westerlies is interrupted by the migration of cyclones and anticyclones. The paths taken by these
cyclonic and anticyclonic systems is closely correlated to upper-level air flow and the polar jet stream. The average position of the polar jet stream, and hence the paths of cyclonic systems, migrates equator ward with the approach of winter and poleward as summer nears.
•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 over 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 than the air at the same elevation over the valley floor), 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 speed is measured using a cup anemometer. The instrument most commonly used to measure wind direction is the wind vane. When the wind consistently blows more often from one direction than from any other, it is termed a prevailing wind.
•In general, regions influenced by high pressure, with its associated subsidence and divergent winds, experience relatively dry conditions. Conversely, regions under the influence of low pressure and its converging winds and ascending air receive ample precipitation. In addition to latitudinal variations in precipitation related to the global pattern of pressure and temperatures, the distribution of land and water also influences Earths precipitation pattern.