Relief rainfall, also known as orographic rainfall, occurs when moist air is forced to rise over a mountain range or elevated terrain. The process can be described in several key steps:
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Moist Air Movement: Warm, moist air is typically carried by prevailing winds towards a mountain range. This air can originate from oceans or other bodies of water, where it picks up moisture.
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Ascent Over Mountains: When the moist air reaches the mountains, it is forced to ascend due to the barrier created by the terrain. As the air rises, it moves into regions of lower atmospheric pressure.
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Cooling and Condensation: As the air rises, it cools adiabatically (without heat exchange with the environment). The decrease in temperature causes the water vapor in the air to condense into tiny water droplets, forming clouds. This process occurs because cooler air cannot hold as much moisture as warmer air.
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Precipitation: When enough water vapor condenses, the droplets coalesce and grow larger. Eventually, these droplets fall as precipitation (rain, snow, etc.) on the windward side of the mountain.
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Leeward Effects: After the air has released moisture on the windward side, it descends on the leeward side (the side sheltered from the wind). As the air descends, it warms up again and can lead to dry conditions, often resulting in phenomena like rain shadows, where the leeward side receives significantly less precipitation compared to the windward side.
In summary, relief rainfall results from the interaction of moist air with topographical features, leading to cooling and condensation as the air is lifted, followed by precipitation.