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In some places in the world it can be observed that spots which used to be forest (or with the potential to become a forest) were changed to fields of solar panels.

I've been close to solar panels on a sunny day on top of buildings and felt the warmth. So I am wondering how much the near-ground average temperature rises when changing from a forested area.

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The answer is almost certainly 'depends'. The dependencies are a) around the forest - what kind of tree, what density of plants, what season? Then around the solar panels - how densely packed, how efficient etc? But we can make some estimates using albedo as a measure of the ratio of light reflected vs converted to heat.

If a solar panel is 15% efficient that would be broadly equivalent to an albedo of 0.15 . Wikipedia suggests that deciduous forest has an albedo of 0.15 to 0.18, with conifer forest varying from 0.09 to 0.18. This implies that replacing forest with solar panels shouldn't have a great effect on albedo.

Albedo won't be the only effect on temperature as a forest will cool through evapotranspiration, which may reduce temperatures by more than 1 or 2 $^\circ$C. (Though a solar farm may well have grass under the panels which will also cool through evapotranspiration.)

There will also be a perception effect - when you stand beside a solar panel the heat is released close to you, whereas if you were in a forest the bulk of radiated heat is going to be released in the canopy.

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