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The change from the liquid state of water to the water vapor requires a large latent heat to loosen the hydrogen bonds. The process of transpiration in the plant involves exactly this change of state. Here, the water evaporates from the lower leaf surface and the leaf gives up some of its heat. In this way, the leaf is prevented from overheating that would result from exposure to sunlight. Sweating in animals involves this same latent heat aspect of water.

If trees use transpiration to decrease their temperature, shouldn't the air around them be warmer? The leaves are cooled as water evaporates, and the air should be warmer.

I always experience the opposite as I approach a forest. I'm not sure if the effect could be attributed to the reduced irradiation due to foliage.

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    $\begingroup$ I am not a botanist but I don't think trees use transpiration primarily to decrease their temperature. In general they don't regulate their temperature in an active way (they may use antifreeze compounds though), in that sense they are poikilotherms. $\endgroup$ – Buck Thorn Oct 22 at 9:15
  • $\begingroup$ Related: chemistry.stackexchange.com/questions/112043/… $\endgroup$ – Karsten Theis Oct 22 at 15:31
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You seem to misunderstand the thermodynamics of evaporation: "Evaporation is an endothermic process, in that heat is absorbed during evaporation." It appears you are confusing evaporative cooling with a heat pump. In a heat pump, the compression half of the cycle changes the vapor back to liquid, which does make things warmer.

Consider boiling water at sea level: Water liquid at 100°C becomes water vapor at 100°C, requiring ~540 cal/g energy to change state from liquid to vapor at the same temperature. At lower temperatures, as water evaporates, thermal energy is withdrawn from the liquid, cooling it below ambient temperature. "Swamp coolers" work on that principle.

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The air at ground level in a forest will be shady - therefore considerably cooler than in full sun. That is probably the major cause/effect of the noticeable temperature change on entering a forest.

The leaves will be warmed by sunlight, cooled by evaporation. At the first bit of evaporation, the air could be warmed a little as the leaves are cooled a little. But it's not at all clear that evaporation of water could become the driving process and cool the leaves and warm the surrounding air to a noticeable degree.

You could wet a dishtowel and swing it around to see how much it warms the air. The towel will become noticeably cooler as water evaporates, but the air is so much larger a reservoir of energy that a change in its temperature will be dispersed over a large volume and will be insensible. Perhaps the effect could be measured in a confined volume with no stirring and very sensitive thermometers.

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