Page 273 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

AQuantitativesolution Adiabatic Warming and the Rain Shadow
When Europeans first came to the “New World,” they encoun- tered a place of abrupt climate zonation. Nowhere else on the globe are great mountain chains such as the Andes, Cascades, Sierra Nevada, Coast Mountains, and Rocky Mountains found in the zone of the prevailing westerly winds, adjacent to low interior plains or basins to the east. Intense rain-shadow deserts in middle latitudes—for example, the Palliser Triangle in southern Alberta— are unique to the Americas. For many immigrants seeking the paradise of west coasts by the overland route, these rain-shadow regions were deadly. The cause of these rain shadows lies in the adiabatic process, working first on air rising up a slope and then on air sinking down a slope. It is nature’s process of wringing out the available moisture by forced ascent of air over a mountain barrier.
Imagine a mountain barrier, 4000 m in elevation, rising from sea level and located in the prevailing westerly winds that blow onshore from the ocean. The initial air temperature is 28°C as the air mass strikes the mountain along the windward side (westward-facing slope). The air mass then rises to the top of the mountain, where it crosses the crest and descends the leeward side (eastward-facing slope) of the mountain barrier. What happens to the air temperature as it rises up the slope? How much change occurs? What lapse rate applies? If the initial air had a dew-point temperature of 12°C, using the dew-point lapse rate, at what altitude will condensation begin?
Refer to Figure 8.6a. The air temperature decreases as it rises up the windward slope. If the air is unsaturated, there is no con- densation and the temperature decreases by 10 C° per 1000 m, so at 1000 m the temperature is 18°C, at 2000 m the temperature is 8°C, and so forth.
However, we are given the initial dew-point temperature at sea level (12°C) and the dew-point lapse rate (2 C° ∙ 1000 m−1). So at sea level, the dew-point temperature is 16 C° lower than the air temperature (indicating that the air is unsaturated). At 1000 m, the dew-point temperature is 10°C (8 C° lower than air tempera- ture), and at 2000 m, 8°C. Thus, the temperature of the air and the dew-point temperature have converged to the same value
at 2000 m. Cloud formation begins at this altitude, the lifting condensation level—an altitude at which air temperature is at the dew point—and there is a strong possibility that precipitation will occur, taking moisture out of the air.
Continued lifting will no longer be at the dry adiabatic rate. Instead, because condensation releases latent heat that warms the atmosphere, the lower, moist adiabatic rate must be used. From 2000 m upward, the air will cool at an average rate of 6 C° per 1000 m. So at 3000 m the air will be at 2°C and at 4000 m, −4°C.
Now, imagine the leeward-slope descent. Starting with an air temperature of −4°C at an elevation of 4000 m, what will happen to the air on the leeward side of the mountain?
Cold, dry air at the mountaintop will subside and warm adiabatically—its temperature will increase at the dry adiabatic rate it descends. At 3000 m, the air temperature will be 6°C; at 2000 m, 16°C; at 1000 m, 26°C; and at 500 m, 31°C. At 500 m on the leeward side of the mountain barrier, the air is warmer and, because no moisture has been added during its descent, the drier air produces a rain shadow.
concepts review
Key leArning
Weather is the short-term condition of the atmosphere; meteorology is the scientific study of the atmosphere. The spatial implications of atmospheric phenomena and their relationship to human activities strongly link meteorol- ogy to physical geography.
weather (p. 208) meteorology (p. 208)
■ Describe air masses that affect North America, and relate their qualities to source regions.
An air mass is a regional volume of air that is homog- enous in humidity, stability, and cloud coverage and that may extend through the lower half of the troposphere. Air masses are categorized by their moisture content—m for maritime (wetter) and c for continental (drier)—and their temperature, a function of latitude—designated A (arctic), P (polar), T (tropical), E (equatorial), and AA (Antarctic). Air masses take on the characteristics of their source region and carry those characteristics to new re- gions as they migrate. As air masses move, their charac- teristics change to reflect their underlying regions.
air mass (p. 208)
1. How does a source region influence the type of air mass that forms over it? Give specific examples of each basic classification.
2. Of all the air mass types, which are of greatest sig- nificance to Canada and the United States? What happens to them as they migrate to locations differ- ent from their source regions? Give an example of air mass modification.
■ Identify and describe four types of atmospheric lifting mechanisms, and give an example of each.
Air masses can rise through convergent lifting (airflows conflict, forcing some of the air to lift); convectional lift- ing (air passing over warm surfaces gains buoyancy); orographic lifting (air passes over a topographic bar- rier); and frontal lifting. Orographic lifting creates wet- ter windward slopes and drier leeward slopes situated in the rain shadow of the mountain. In North America, chi- nook winds (called föhn or foehn winds in Europe) are the warm, downslope airflows characteristic of the leeward side of mountains. Conflicting air masses may produce a cold front (and sometimes a zone of strong wind and rain) or a warm front. A zone right along or slightly ahead of the front, called a squall line, is characterised by turbulent and wildly changing wind patterns and intense precipitation.
convergent lifting (p. 210) convectional lifting (p. 210)
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