Page 184 - Geosystems An Introduction to Physical Geography 4th Canadian Edition
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148 part I The energy–atmosphere System
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forces operate on moving air and ocean currents at Earth’s surface and influence global windcirculation patterns.
Pressure Gradient Force
The pressure gradient force drives air from areas of higher barometric pressure (moredense air) to areas of lower barometric pressure (lessdense air), thereby caus ing winds. A gradient is the rate of change in some prop erty over distance. Without a pressure gradient force, there would be no wind.
High and lowpressure areas exist in the atmosphere principally because Earth’s surface is unequally heated. For example, cold, dry, dense air at the poles exerts greater pressure than warm, humid, lessdense air along the equa tor. On a regional scale, high and lowpressure areas are associated with specific masses of air that have varying characteristics. When these air masses are near each other, a pressure gradient develops that leads to horizontal air movement.
In addition, vertical air movement can create pres sure gradients. This happens when air descends from the upper atmosphere and diverges at the surface or when air converges at the surface and ascends into the upper atmosphere. Strongly subsiding and diverging air is as sociated with high pressure, and strongly converging and rising air is associated with low pressure. These horizon tal and vertical pressure differences establish a pressure gradient force that is a causal factor for winds.
An isobar is an isoline (a line along which there is a constant value) plotted on a weather map to connect points of equal pressure. The pattern of isobars provides a portrait of the pressure gradient between an area of higher pressure and one of lower pressure. The spacing between isobars indicates the intensity of the pressure difference, or pressure gradient.
Just as closer contour lines on a topographic map indicate a steeper slope on land and closer isotherms on a temperature map indicate more extreme temperature gradients, so closer isobars denote steepness in the pres sure gradient. In Figure 6.6a, note the spacing of the iso bars. A steep gradient causes faster air movement from a highpressure area to a lowpressure area. Isobars spaced wider apart from one another mark a more gradual pres sure gradient, one that creates a slower airflow. Along a horizontal surface, a pressure gradient force that is act ing alone (uncombined with other forces) produces move ment at right angles to the isobars, so wind blows across the isobars from high to low pressure. Note the location of steep (“strong winds”) and gradual (“light winds”) pressure gradients and their relationship to wind inten sity on the weather map in Figure 6.6b.
Coriolis Force
The Coriolis force is a deflective force that makes wind travelling in a straight path appear to be deflected in relation to Earth’s rotating surface. This force is an ef fect of Earth’s rotation. On a nonrotating Earth, surface
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▲Figure 6.5 Sixteen wind directions identified on a wind compass. Winds are named for the direction from which they originate. For example, a wind from the west is a westerly wind.
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CRITICALthinking 6.1 Measure the Wind
estimate wind speed and direction as you walk across campus on a day with wind. you might access the Url given on page 147 or go to MasteringGeography to view the Beaufort scale to assist you. record your estimates at least twice during the day—more often if you note chang- ing wind patterns. For wind direction, moisten your finger, hold it in the air, and sense evaporative cooling on one side of your finger to indicate from which direction the wind is blowing. Check the internet to find a weather station on your campus or at a nearby location, and compare your measurements to actual data. What changes in wind speed and direction do you notice over several days? How did these changes relate to the weather you experienced? •
Driving Forces within the Atmosphere
Four forces determine both speed and direction of winds. The first of these is Earth’s gravitational force, which ex erts a virtually uniform pressure on the atmosphere over all of Earth. Gravity compresses the atmosphere, with the density decreasing as altitude increases. The gravitational force counteracts the outward centrifugal force acting on Earth’s spinning surface and atmosphere. (Centrifugal force is the apparent force drawing a rotating body away from the centre of rotation; it is equal and opposite to the centripetal, or “centreseeking” force.) Without gravity, there would be no atmospheric pressure—or atmosphere, for that matter.
The other forces affecting winds are the pressure gra dient force, Coriolis force, and friction force. All of these
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