This time of year we're always looking for ways to cool down - air conditioning and fans indoors, a breeze outdoors. We tend to forget about what are perhaps nature's biggest air conditioners of all - trees.

Most obviously, trees cool directly by the shade they provide. In addition, trees also cool the air in a more indirect, but actually more significant, way - through a process called evapotranspiration. Simply put, trees provide a huge moist living surface from which water can evaporate, and evaporation cools the air in the same way that your body cools when you emerge from the pool with drops of water on your skin.

Putting an exact dollar figure on the cooling effect of trees can be tricky, but here's a reasonable estimate: a middle-sized shade tree - say half a foot thick - has the cooling ability of a heavy duty air conditioner that uses a few dollars of electricity per day!