Weather Wise

NICENESS is a hard quantity to measure when it comes to the weather. It is easy to quantify in terms of wind speed, temperature, pressure and so on, but is it possible to scientifically describe the weather in terms of human comfort?

Approximately-speaking, yes. In 1963, the climatologist Victor Olgyay came up with the idea of a "comfort chart", plotting limits for various activities by "inhabitants of temperate climatic zones" in relation to temperature and humidity.

Interestingly, humidity turns out to be as important, if not more so, than temperature in defining comfort. On Olgyay's chart, the line marking the high-temperature boundary of an "impossible environment" runs from more than 50C (122F) at 20 per cent humidity, to 33C (91F) when the humidity exceeds 90 per cent.

That is why dry desert heat is much more bearable than the stifling fug of an equatorial forest, even though the temperatures in the forest may be 20C below those in the desert. In bone dry air, provided you keep out of the sun, "work of moderate intensity" is possible at more than 43C (109F). At 95+ per cent humidity, even 24C (75F) is enough to make anyone down tools in a torrent of sweat.

As the mercury plummets, humidity has less of an effect on comfort, though a combination of high moisture content and low temperatures can be harmful. Just how harmful was discovered in a disastrous experiment in the 1880s, when a tuberculosis sanatorium was constructed underground in Mammoth Cave in Kentucky.

The idea was that the constant chill (10C) and high humidity in the cave would provide the ideal environment for the sickly patients. In fact, it turns out that the opposite is true - hot, dry air is required - and the patients all died.