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Orographic Rain

A Weather Forecasting Lesson Plan

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Orographic Rain
Contributed by Paul Warburton

A range of hills or mountains can encourage the processes which lead to precipitation. The diagram below shows how an upland area can force warm, moist air to rise. As this air rises it cools until it reaches a point where the water vapor it contains condenses and forms clouds. This can eventually lead to rainfall or other precipitation (like snow) occurring.

This process of relief rainfall happens in many parts of the world. The photograph shows part of Tenerife in the Canary Islands. Prevailing winds come from the west over the Atlantic and rise over the western side of the island. There is often cloud on this side of Tenerife and it is the wetter side of the island.

The eastern side of Tenerife is drier and is in an area called the rain shadow. Rainfall is released over the uplands in the center of the island. As the air descends over the eastern side not only is it drier but it also warms. Warm air holds more moisture than cold air so little condensation if any will occur.

Terms

  • Condensation – when water vapor cools and turns from a gas to a liquid.
  • Precipitation – a collective term for the different ways moisture falls from the atmosphere – this includes snow, rain, hail and sleet.
  • Prevailing winds – the direction from which winds most frequently come.

Activities

  1. Complete two rainfall graphs using the data in this table. The bars must be shaded in light blue.
  2. Explain the differences between the two graphs on the handout referring to the map at the bottom.
  3. This lesson is also available in PDF format here.

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