Page 210 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

174 part I The Energy–Atmosphere System
WINDSOR
KAPUSKASING
WEST
EAST
WEST
EAST
WIND SPEED (m/s)
> = 8.5 7.0 – 8.5 5.5 – 7.0 4.0 – 5.5 2.5 – 4.0 1.0 – 2.5
Calms: 10.01%
NORTH
2%
SOUTH
5%
NORTH
2%
5%
4% 3%
1%
4% 3%
1%
WIND SPEED (m/s)
> = 8.5 7.0 – 8.5 5.5 – 7.0 4.0 – 5.5 2.5 – 4.0 1.0 – 2.5
Calms: 4.98%
SOUTH
▲Figure AQS 6.1 Annual wind roses for Windsor and Kapuskasing, Ontario, 1976–2005. A wind rose is a visual summary of the distribu- tion of wind directions over a given time period. In this case, the compass is divided into 36 categories, each with a range of 10°; and the dis- tribution of wind speeds within each category is shown with coloured segments on the petals. For Windsor, the mean annual wind speed was 4.4 m · s−1 and the wind resultant was 256°. The corresponding values for Kapuskasing were 3.5 m · s−1 and 286°. Resultant directions are shown with a red line. [Wind rose plotting software program: WRPLOT View (v. 6.5.2) from Lakes Environmental, www.weblakes.com/products/wrplot/index.html.]
concepts review
KEY LEARNINg
■ Define the concept of air pressure, and describe instruments used to measure air pressure.
The weight of the atmosphere in terms of force per unit area is air pressure, created by motion, size, and num- ber of molecules. A mercury barometer measures air pressure at the surface (mercury in a tube—closed at one end and open at the other, with the open end placed in a vessel of mercury—that changes level in response to pressure changes), as does an aneroid barometer (a closed cell, partially evacuated of air, that detects changes in pressure).
air pressure (p. 145) mercury barometer (p. 146) aneroid barometer (p. 146)
1. How does air exert pressure? Describe the basic instrument used to measure air pressure. Compare the operation of two different types of instruments discussed.
2. What is the relationship between air pressure and density, and between air pressure and temperature?
3. What is normal sea-level pressure in millimetres? Millibars? Kilopascals?
■ Define wind, and explain how wind is measured, how wind direction is determined, and how winds are named.
Wind is the horizontal movement of air across Earth’s surface; turbulence adds updrafts and downdrafts, and
thus a vertical component, to the definition. Wind speed is measured with an anemometer (a device with cups that are pushed by the wind) and its direction with a wind vane (a flat blade or surface that is directed by the wind).
wind (p. 146) anemometer (p. 146) wind vane (p. 147)
4. What is a possible explanation for the beautiful sunrises and sunsets during the summer of 1992 in North America? Relate your answer to global circulation.
5. Explain this statement: “The atmosphere social-
izes humanity, making the world a spatially linked society.” Illustrate your answer with some examples.
6. Define wind. How is it measured? How is its direction determined?
■ Explain the four driving forces within the atmosphere—gravity, pressure gradient force, Coriolis force, and friction force—and locate the primary
high- and low-pressure areas and principal winds.
The pressure that Earth’s gravitational force exerts on the atmosphere is virtually uniform worldwide. The pressure gradient force drives winds, as air moves from areas of high pressure to areas of low pressure. Maps portray air pressure patterns using the isobar—an isoline that con- nects points of equal pressure. The Coriolis force causes an apparent deflection in the path of winds or ocean cur- rents, owing to the rotation of Earth. This force deflects objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The friction force drags winds along Earth’s varied surfaces in opposition to