Theory question 305
Weather(a) Explain four reasons for which temperature decreases with height within the troposphere. (b) With the aid of a diagram, describe the mode of formation of orographic rainfall.
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(a) The atmosphere is heated mainly from the ground by terrestrial radiation, so air far from the ground is cooler; air at height is thinner (lower density) and holds less heat; there is less water vapour and dust at height to absorb heat; rising air expands under lower pressure and cools; so temperature falls about 6.5°C per 1,000 m.
(b) Diagram: moist wind from the sea, a mountain, clouds and rain on the windward side, dry descending air on the leeward (rain shadow) side. Moist air is forced to rise over a mountain barrier; it cools as it rises; it reaches dew point and condenses into clouds; rain falls on the windward slope; the air descends the leeward side, warms and is dry, producing a rain shadow.
Label windward and leeward sides.