A SPELL of thundery refreshing rain in July may not be universally decried. The humorist Logan Pearsall Smith, for example, once famously remarked: "Thank heavens, the sun has gone in, and I don't have to go out and enjoy it." Less welcome, however, is the high humidity, which often accompanies such an interlude, and which may often have curious side effects.
Human hair, for instance, becomes shorter when the air is very dry, but lengthens when humidity increases. Those with a troublesome coiffure, therefore, if their hair is long and straight, may notice that dampness makes it limp, while naturally curly hair displays a tendency to frizz. Those, on the other hand, unfortunate enough to suffer from arthritis or similar complaints may feel increased pain when there is a sudden change in the humidity. Scar tissue, corns and other abnormal skin have rates of expansion and contraction different to normal skin, so a sudden rise in the humidity may cause nerve endings to give a response which registers as pain.
Wool, too, absorbs moisture and becomes heavier when the air is wet. The first to notice this, allegedly, was Cardinal Nicholas de Cusa in the 15th century: "If you place balls of dry wool in the pan of a large balance," he wrote, and if you then equalise the other pan by putting rocks in it, in a normal place and in average conditions (in loco et aere temperato, as he nicely puts it), you will see that when the air becomes more moist the wool becomes heavier, and when the air becomes drier the weight of the wool decreases. He who attributes this difference to the properties of the air draws the most accurate conclusion concerning future changes in the weather.
The link between rising humidity and imminent deterioration in the weather appears in many other contexts, and is probably the basis for the alleged meteorological perpicacity of many birds and animals. It is said, for example, that rain is on the way when cows lie down in their fields: a plausible explanation for this phenomenon is that the cows, who may be more sensitive to changes of this kind than we are, sense a dampness in the air, associate it with the likelihood of rain, and lie down so that the grass beneath them will be dry if rain does come.
And of course, if you live near the sea you can hang out strands of seaweed to help you detect when the humidity is high. In dry weather the seaweed shrivels and feels dry to the touch, but when the humidity is high, it swells and is noticeably damp.