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I know that forests can change the local temperature, but what about rainfall?. Do forests affect rainfall in their region? I have read that if a forest is very big, it can cause minor changes to local rainfall and temperature. How does that work?

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For rain to fall, water vapour must exist in sufficient quantities in the atmosphere. The main source of atmospheric water vapour is evaporation from oceans. However, evaporation from large bodies of water such as lakes and dams also contributes. Another significant contributor, on a local level, is transpiration from forests.

the Amazon alone creates 50-80 percent of its own rainfall through transpiration

Removing forests has

the biggest impact on cross-continental transport of water vapour

Removal of trees from an area of forest immediately stops transpiration in the that area. This causes less rain to fall in that area. It changes the way light is reflected from the Earth's surface, which changes the heating characteristics of the ground, which changes air movements. It will also have an influence on ground water levels and depending on local geology can bring salt to the surface causing salt scars and eventually desertification.

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  • $\begingroup$ Trees can also emit particulates or VOCs around which water vapor coalesces into rainfall. $\endgroup$ – cphlewis May 6 '15 at 4:24
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    $\begingroup$ So the historical saying that "Forest bring rain" is not totally false? $\endgroup$ – Sab May 6 '15 at 9:16
  • $\begingroup$ It is not just forests. Large fields of corn cause a lot of evaporation (transpiration) - I have heard it said that it is no coincidence that "tornado alley" and the corn belt have such overlap... $\endgroup$ – Floris May 6 '15 at 12:08
  • $\begingroup$ Also now there is research showing that pollen can influence cloud formation. onlinelibrary.wiley.com/doi/10.1002/2015GL064060/abstract $\endgroup$ – farrenthorpe May 6 '15 at 14:48

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