July is the ideal month to assess our need for trees. We may not even want to step out into the garden because of blistering heat, which could easily be mitigated by planting trees that cool the air, however.
It’s not only that trees provide shade but they literally cool the air through “transpiration cooling.” Transpiration has been referred to as botanical sweating. Just as the function of sweat is to cool us down on a hot day, transpiration cools down leaves and the air around them. Transpiration occurs when water passes from soil to roots all the way up to leaves. The final step in water’s transpiring ascent is when it exits leaves but hovers over them in gaseous vapor form. As heat passes through the vapor, there is a cooling effect. We experience this when we get out of a pool on a hot day and feel chilly because as the water on our skin turns to vapor, the hot air passing through the vapor cools the air.
This also explains why plants can wilt on a cool day if a strong wind is blowing. If the thin, cooling layer of water vapor on leaf surfaces is dispersed, the air surrounding leaves dries out. As fast as water is taken up through transpiration, it is immediately lost to the ambient dry air on a windy day. Eventually the leaves lose turgidity and wilt since the rate of transpiration or water ascent in the plant cannot keep up with the rate of foliar water loss.
Of course, the function of transpiration is not merely to keep leaves cool, but also to bring the hydrogen in water molecules up to leaves where photosynthesis, the process by which plants make their own food, takes place. Photosynthesis uses solar energy to