THE shortest book of the Old Testament is the Book of Obadiah, which at mere 21 verses is only a little longer than this Weather Eye. The next shortest, by my reckoning, is Haggai, and the third, at 47 verses, is the Book of Jonah. Its two pages are eventful: Jonah, you may remember, was thrown overboard by his companions during a storm to be swallowed and (later) regurgitated by a giant fish. He went next to the town of Nineveh, where he persuaded the citizens to see the error of their ways. After this last episode he rested, and the Lord kindly provided a tree which "came up over Jonah to be a shadow over his head and to ease his discomfort". Not surprisingly, "Jonah was exceeding glad of it."
Now depending on which version of the Bible you happen to consult, this shady shrub may have been a vine, an ivy, or a gourd. This last, as it happens, would have been by far the most effective, since it has broad thin leaves which transpire moisture so profusely that the temperature in their immediate vicinity, we are told, may be 7C below that of the surrounding air.
Transpiration is one of the many tricks of trees for coping with extremes of weather. Like ourselves, in very hot conditions they run the risk of over heating in the sun, and also like us, they cool themselves by the equivalent of sweating by opening and closing tiny pores, or stomata, on their leaves. When the pores are open the water inside the leaf evaporates, providing a cooling effect similar to that which occurs when perspiration evaporates from skin.
The bark of a tree performs a similar function, acting as a layer of insulation to protect the trunk and branches from the thermal radiation of the sun. A thick "craggy" bark, like that of an ancient oak, is the most effective because of the little hills and valleys thus created: the former provide shade for much of the surface area, so that the sun shines directly into each fissure for only a little time each day.
Others increase their chances of survival by the colour of their bark. The white bark of the silver birch, for example, controls the temperature of the interior by reflecting sunlight. The common beech, on the other hand, has a thin smooth bark with none of the above advantages, and may be subject to overheating if growing on its own in hot weather, a beech inside a stand, shielded from the sun by other, better insulated, trees, is better able to withstand the stress.