Page 211 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 6 Atmospheric and Oceanic Circulations 175
the pressure gradient. The pressure gradient and Coriolis force in combination (absent the friction force) produce geostrophic winds, which move parallel to isobars, char- acteristic of winds above the surface frictional layer.
In a high-pressure system, or anticyclone, winds de- scend and diverge, spiraling outward in a clockwise di- rection in the Northern Hemisphere. In a low-pressure system, or cyclone, winds converge and ascend, spiraling upward in a counterclockwise direction in the Northern Hemisphere. (The rotational directions are reversed for each in the Southern Hemisphere.)
The pattern of high and low pressures on Earth in generalized belts in each hemisphere produces the dis- tribution of specific wind systems. These primary pres- sure regions are the equatorial low, the weak polar highs (at both the North and the South Poles), the subtropical highs, and the subpolar lows.
All along the equator, winds converge into the equa- torial low, creating the intertropical convergence zone (ITCZ). Air rises in this zone and descends in the sub- tropics in each hemisphere. The winds returning to the ITCZ from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere produce the trade winds.
The subtropical high-pressure cells on Earth are gen- erally between 20° and 35° in each hemisphere. In the Northern Hemisphere, they include the Bermuda High, Azores High, and Pacific High. Winds flowing out of the subtropics to higher latitudes produce the westerlies in each hemisphere.
In January, two low-pressure cells known as the Aleutian Low and Icelandic Low dominate the North Pacific and Atlantic, respectively. The region of contrast between cold polar air and the warmer air toward the equator is the polar front. The weak and variable polar easterlies diverge from the high-pressure cells at each pole, the stronger of which is the Antarctic High.
pressure gradient force (p. 148) isobar (p. 148)
Coriolis force (p. 148)
friction force (p. 150) geostrophic wind (p. 151) anticyclone (p. 151)
cyclone (p. 151)
equatorial low (p. 153)
polar high (p. 153)
subtropical high (p. 153)
subpolar low (p. 153)
intertropical convergence zone (ITCZ) (p. 153) trade winds (p. 153)
Bermuda High (p. 155)
Azores High (p. 155)
Pacific High (p. 155)
westerlies (p. 155)
Aleutian Low (p. 158)
Icelandic Low (p. 158)
polar front (p. 158)
polar easterlies (p. 158)
Antarctic High (p. 158)
7. What does an isobaric map of surface air pressure portray? Contrast pressures over North America for January and July.
8. Describe the effect of the Coriolis force. Explain how it appears to deflect atmospheric and oceanic circulations.
9. What are geostrophic winds, and where are they encountered in the atmosphere?
10. Describe the horizontal and vertical air motions in a high-pressure anticyclone and in a low-pressure cyclone.
11. Construct a simple diagram of Earth’s general circu- lation; begin by labeling the four principal pressure belts or zones, and then add arrows between these pressure systems to denote the three principal wind systems.
12. How is the intertropical convergence zone (ITCZ) related to the equatorial low? How does the ITCZ appear on the satellite images of accumulated pre- cipitation for January and July in GIA 6.2?
13. Characterise the belt of subtropical high pressure on Earth: Name several specific cells. Describe the generation of westerlies and trade winds and their effects on sailing conditions.
14. What is the relationship among the Aleutian Low, the Icelandic Low, and migratory low-pressure cyclonic storms in North America? In Europe?
■ Describe upper-air circulation, and define the jet streams.
A constant isobaric surface—a surface that varies in alti- tude from place to place according to where a given air pressure, such as 500 mb, occurs—is useful for visualizing geostrophic wind patterns in the middle and upper tropo- sphere. The variations in altitude of this surface show the ridges and troughs around high- and low-pressure systems. Areas of converging upper-air winds sustain surface highs, and areas of diverging upper-air winds sustain surface lows.
Vast wave motions in the upper-air westerlies are known as Rossby waves. Prominent streams of high- speed westerly winds in the upper-level troposphere are the jet streams. Depending on their latitudinal position in either hemisphere, they are termed the polar jet stream or the subtropical jet stream.
constant isobaric surface (p. 158) Rossby waves (p. 158)
jet stream (p. 158)
15. What is the relation between wind speed and the spacing of isobars?
16. How is the constant isobaric surface, especially the ridges and troughs, related to surface-pressure sys- tems? To divergence aloft and surface lows? To con- vergence aloft and surface highs?
17. Relatethejet-streamphenomenontogeneralupper-air circulation. How is the presence of this circulation related to airline schedules for the trip from New York to San Francisco and for the return trip to New York?
■ Explain the regional monsoons and several types of local winds.
Intense, seasonally shifting wind systems occur in the tropics over Southeast Asia, Indonesia, India, northern Australia, equatorial Africa, and southern Arizona. These winds are associated with an annual cycle of returning precipitation with the summer Sun and named using the Arabic word for season, mausim, or monsoon. The