I can understand that trees produce oxygen in daylight but why is the area with vegetation much cooler than the environment's temperature?

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    $\begingroup$ Is there evidence that this a general phenomenon? $\endgroup$ – kmm Oct 19 '16 at 0:55
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    $\begingroup$ Near trees and weedy places... not only at evening... throughout day it feels much cooler than bare places, perhaps due to moisture content. Rather at evening to early night, sometimes I feel little-bit warmer beneath trees and weedy places which too is plausibly due to high moisture content resisting evaporation. Perhaps plants act as a buffer against extremities of environment. $\endgroup$ – Always Confused Oct 19 '16 at 6:01

Why it can feel chilly close to trees in the evening:

  1. Water coming out of Stomata vaporises, reducing the surrounding temperature.
  2. There is normally no direct sunlight reaching the ground beneath the tree, hence the moisture content of soil is higher, giving it the "chilly feeling"
  3. If there is other vegetation (e.g. grass) growing beneath the tree, then point the first point is repeated for the other vegetation, consequently the combination of point 1 and 2 decreases the temperature noticeably.

An empirical study that estimates the cooling effect of trees from evapotranspiration can be found in Konarska et al (2016). Transpiration of urban trees and its cooling effect in a high latitude city.. In the study, they specifically find a cooling effect at evenings, but less so during nights and at daytime:

With an estimated night-time latent heat flux of 24 W m(-2), tree transpiration significantly increased the cooling rate around and shortly after sunset, but not later in the night. Despite a strong midday latent heat flux of 206 W m(-2), a cooling effect of tree transpiration was not observed during the day.

Why is it more noticeable during the summer?

If the temperature is warm, the atmosphere has a greater capacity to hold water in its vaporous state than if it was cold.

Relative humidity

The ratio between the actual amount of water vapor present to the capacity that the air has at a particular moment. For any given temperature, the air has a particular capacity for water vapor. If that capacity is exceeded, the excess spills out as liquid water.

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| improve this answer | |
  • $\begingroup$ When you meant "the excess spills out as liquid water", I guess you are meaning there will be a state change when water molecules get cohesive. $\endgroup$ – randominstanceOfLivingThing Oct 18 '16 at 7:48
  • $\begingroup$ I added a reference to your answer that felt suitable. Hope that's ok. $\endgroup$ – fileunderwater Oct 24 '16 at 22:31

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