Page 186 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

150 part I The energy–atmosphere System
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North Pole
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Gander
During the airplane flight, Earth rotated eastward, placing Gander farther east than it was at takeoff – this change in position produces the apparent deflection in flight path. The navigator must correct for this Coriolis deflection.
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Flight path deflected by Coriolis force
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Flight path on a nonrotating Earth
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Earth. Note the deflection to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Several factors contribute to the Coriolis force on Earth. First, the strength of this deflection varies with the speed of Earth’s rotation, which varies with latitude (please review Table 2.1). Remember that rotational speed is 0 km·h−1 at the poles, where Earth’s surface is closest to its axis, and 1675 km·h−1 at the equator, where Earth’s sur­ face is farthest from its axis. Thus, deflection is zero along the equator, increases to half the maximum deflection at 30° N and 30° S latitude, and reaches maximum deflec­ tion for objects moving away from the poles. Second, the deflection occurs regardless of the direction in which the object is moving, as illustrated in Figure 6.7, and does not change the speed of the moving object. Third, the deflec­ tion increases as the speed of the moving object increases; thus, the faster the wind speed, the greater its apparent de­ flection. Although the Coriolis force affects all moving ob­ jects on Earth to some degree, its effects are negligible for small­scale motions that cover insignificant distance and time, such as a Frisbee or an arrow.
How does the Coriolis force affect wind? As air rises from the surface through the lowest levels of the atmo­ sphere, it leaves the drag of surface friction behind and increases speed (the friction force is discussed just ahead). This increases the Coriolis force, spiraling the winds to the right in the Northern Hemisphere or to the left in the Southern Hemisphere, generally producing upper­air west­ erly winds from the subtropics to the poles. In the upper troposphere, the Coriolis force just balances the pressure gradient force. Consequently, the winds between higher­ pressure and lower­pressure areas in the upper troposphere flow parallel to the isobars, along lines of equal pressure.
Friction Force
In the boundary layer, friction force drags on the wind as it moves across Earth’s surfaces, but decreases with height above the surface. Without friction, surface winds
Maximum at poles
60°N
Earth's rotation
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Northern Hemisphere
Equator
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Deflection to right
No deflection
Deflection to left
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Animation
▲Figure 6.7 The Coriolis force—an apparent deflection.
Coriolis Force
S
plane in flight (Earth rotation speed + plane speed) be­ comes so great that it cannot be balanced by the gravi­ tational force pulling toward Earth’s axis. Therefore, the plane experiences an overall movement away from Earth’s axis, observed as a right­hand deflection toward the equator. Unless the pilot corrects for this deflective force, the flight will end up somewhere further south (Figure 6.7b). In contrast, flying westward on a return flight opposite Earth’s rotation direction decreases the centrifugal force (Earth rotation speed – plane speed) so that it is less than the gravitational force. In this case, the plane experiences an overall movement toward Earth’s axis, observed in the Northern Hemisphere as a right­ hand deflection toward the pole.
Distribution and Significance Figure 6.7c summarizes the distribution of the effects of the Coriolis force on
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