(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 o...

(a) Reasons temperature decreases with height in the troposphere

1. The atmosphere is heated mainly from below. The Earth's surface absorbs incoming solar radiation and reradiates heat as terrestrial radiation. Air nearest the ground is therefore warmed most directly, while air higher up receives less of this heat.
2. Terrestrial radiation decreases with altitude. As distance from the heated land or sea surface increases, the amount of long-wave radiation available to warm the air becomes less; consequently, upper air is cooler.
3. Expansion of rising air causes adiabatic cooling. Atmospheric pressure decreases with height. When air rises into lower pressure, it expands, using its internal heat energy in the process, and its temperature falls.
4. There is less water vapour, dust and other heat-retaining materials aloft. Most water vapour, dust and carbon dioxide are concentrated in the lower troposphere. These absorb and retain outgoing terrestrial radiation, so the lower layers are warmer than the drier, cleaner air at greater heights.

(b) Formation of orographic rainfall

Orographic, or relief, rainfall occurs when a mass of warm, moist air is forced to rise over a hill or mountain barrier.

1. Moist onshore wind reaches the windward slope of a mountain.
2. The mountain forces the air to rise. As it rises, pressure falls and the air expands and cools adiabatically.
3. On reaching its dew point, the water vapour condenses around condensation nuclei to form clouds.
4. Continued uplift produces thick cloud and rain falls on the windward slope.
5. After crossing the summit, the air has lost much of its moisture. It descends the leeward slope, becomes compressed and warms adiabatically.
6. The descending air has a lower relative humidity and produces little or no rain. The dry leeward area is called a rain-shadow region.