How hot is hot?
Humidity plays its part

by Philip Eden

Temperature is not as simple a matter as we think. Today we are bombarded with temperatures: in radio and television forecasts, in the papers, digital displays attached to buildings. All these keep us up-to-date with current temperature readings or those predicted for the next few days. It seems that we all need to know how hot or cold it is going to be. Oddly, though, human beings are remarkably poor thermometers. In a recent exercise, a group of people were taken outside and asked to guess the shade t emperature. Only 42 per cent of the guesses were within 3°C of the actual temperature whereas 5 per cent were more than 10°C adrift.

One reason for these levels of inaccuracy is that we actually feel heat and cold by our physiological responses to our surroundings, in particular to the speed with which we gain or lose heat energy from the air around us. Physiological responses vary from individual to individual, so one person's "lovely warm day" may be another's "hot, sticky and uncomfortable" one. And they also var y from day to day in any one individual, according to health, tiredness, level of activity, and type of clothing.

Several external factors in addition to the air temperature also influence how hot we feel. These include the humidity level, the wind speed, the strength and directness of sunshine, and pollution levels. Thus our bodies tell us that it is "hotter" when a high temperature is accompanied by high humidity, strong sunshine, and no wind. Conversely we feel colder when a low temperature is accompanied by high humidity, no sunshine, and a strong gusty wind. High pollution affects our health and make us less abl e to cope with extremes.

Variations in humidity are particularly important. Even in moderate heat with the air temperature between 22 and 25C they have an impact. When the air is both hot and humid the weather feels sultry and oppressive. Our reactions slow down, we feel lazy, an d our efficiency at work suffers. This is particularly true of hard, physical, outdoor work, but applies also to people who work in factories, offices and shop without the benefit of air conditioning. We should also remember that high humidity levels also bring sticky, airless nights which in turn cause most of us to have poor or disturbed sleep, leading to tiredness and inefficiency.

By contrast, moderately dry air is stimulating, leading to high work efficiency. The temperature usually drops briskly after dark, we get a better night's sleep, and we are more likely to feel fully refreshed the following morning. But when the humidity d rops to a very low level and the air becomes excessively dry, the effects are again negative. People become dehydrated without realising it, and this leads to irritability and lack of concentration.