Page 159 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 5 global Temperatures 123
Salvador (Bahia), Brazil 13° S lat., 9 m elev.
New Orleans, Louisiana 30° N lat., 3 m elev.
Edinburgh, Scotland 56° N lat., 134 m elev.
Montréal, Québec 45.5° N lat., 57 m elev.
Barrow, Alaska 71° N lat., 9 m elev.
35°
30°
25°
20°
15° Figure 4.17 – net radiation
Located on maps in:
10°
5°
Figure 1.15 – latitudinal zones Figure 4.20 – sensible heat
snowfall exceeds the amount of snow lost through summer melting and evaporation.
0° The snow line’s location is a function both
–5°
–10°
of latitude and of elevation, and to a lesser extent, is related to local microclimatic con- ditions. Even at low latitudes, permanent ice fields and glaciers exist on mountain sum-
–20° mits, such as in the Andes and East Africa. In equatorial mountains, the snow line oc- –25° curs at approximately 5000 m. With increas- ing latitude, snow lines gradually lower in elevation from 2700 m in the midlatitudes to
–35° lower than 900 m in southern Greenland. The effects of latitude and elevation –40° combine to create temperature character- istics at many locations. In Bolivia, two cities, both at about 16° S latitude, located less than 800 km apart, have quite different climates (Figure 5.6). Concepción is a low-elevation city at 490 m with a warm, moist climate that is typical for many low-latitude locations with consistent Sun angle and daylength throughout the year. This city has an annual
average temperature of 24°C.
La Paz, at an elevation of 4103 m above sea level,
is situated on a high plateau with a cool, dry climate. La Paz has moderate annual temperatures, averaging about 11°C. Despite its high elevation, people living around La Paz are able to grow wheat, barley, and potatoes—crops characteristic of the midlatitudes—in the fertile highland soils. For comparison, the summit of Mount Robson, the highest peak in the Canadian Rockies, is 3954 m—similar in elevation to La Paz, but at a much higher latitude, resulting in a much harsher climate. La Paz has a mild and hospitable climate re- sulting from its low-latitude location.
Cloud Cover
At any given moment, approximately 50% of Earth is covered by clouds. Examine the extent of cloud cover over Earth on the two images inside the front cover. In Chapter 4, we learned that clouds affect the Earth–atmosphere energy balance by reflecting and absorbing radiation, and that their effects vary with cloud type, height, and density.
The presence of cloud cover at night has a moderat- ing effect on temperature; you may have experienced the colder temperatures outside on a clear night, especially
–30°
JFMAMJJASON D Months
▲Figure 5.5 Latitudinal effects on temperatures. a comparison of five cities from near the equator to north of the arctic Circle demonstrates changing seasonality and increasing differences between average minimum and maximum temperatures with increasing latitude.
with increasing altitude. In fact, the density of the atmo- sphere at an elevation of 5500 m is about half that at sea level. As the atmosphere thins, it contains less sensible heat. Thus, worldwide, mountainous areas experience lower temperatures than do regions nearer sea level, even at similar latitudes.
Two terms, altitude and elevation, are commonly used to refer to heights on or above Earth’s surface. Al- titude refers to airborne objects or heights above Earth’s surface. Elevation usually refers to the height of a point on Earth’s surface above some plane of reference, such as elevation above sea level. Therefore, the height of a flying jet is expressed as altitude, whereas the height of a moun- tain ski resort is expressed as elevation.
In the thinner atmosphere at high elevations in mountainous regions or on high plateaus, surfaces gain and lose energy rapidly to the atmosphere. The result is that average air temperatures are lower, nighttime cool- ing is greater, and the temperature range between day and night is greater than at low elevations. The tempera- ture difference between areas of sunlight and shadow is greater at higher elevation than at sea level, and temper- atures drop rapidly after sunset. You may have felt this difference if you live in or have visited the mountains.
The snow line seen on mountain slopes is the lower limit of permanent snow and indicates where winter
Temperature (°C)