I recently returned from Bahrain, in the Middle East. I’ve been there quite a few times and, in the open, it was hot and sticky (like toffee, rather than a stick). The British (and also chocolate) are not used to such conditions. I came to thinking what the actual temperature really felt like.
How warm we feel depends on lots of factors – air temperature, wind strength, clothing, exercise, humidity (the measure of how much moisture is in the air). Some of these are obvious – given the same air temperature, heat feels more bearable when it is windy (wind chill), the humidity is low and we are not performing some strenuous exercise. “Dry heat” seems more bearable than “wet heat”.
Generally, as we all know, all other things being equal, the higher the humidity, the warmer it feels. That’s because it makes sweating (one of the body’s heat loss mechanisms) less effective. Humidity isn’t that important at ordinary comfortable temperatures (< 26°C according to ISO 7730). It’s only when you get outside the comfort range that it has a more marked effect.
Absolute humidity can be defined in several ways but essentially it’s how much water vapour is in the air. This changes with air temperature. Often, for convenience, we tend to use “relative humidity” instead, which is a simple ratio of the water content compared to how much the air could hold if it were totally saturated with water vapour (and could hold no more). It’s usually expressed as a percentage, which is always nice because finance people understand that kind of thing.
Because scientists, like the Police, love investigating things, several people have come up with handy tables to look up what the temperature effectively feels like to the average human as the humidity varies. Some have even tried to develop equations so the result can easily be calculated. The equations are not exact; the numerical methods used lead to a rather inelegant multi-variable equation, of which there are several variants. People have done this with all sorts of factors, like the wind velocity too.
But, in Bahrain, there was no wind, relative humidity was 80% and the temperature was 38°C in the shade. Using the handy online Heat Index calculator incorporating these equations, gives me a calculated effective temperature of 71°C. The actual figure may not be that accurate, but, either way, that’s going to feel pretty hot and not bearable for long without air conditioning. It’s quite interesting that it’s not until the relative humidity drops to around 25%, that the effective temperature reflects the actual temperature. Below this humidity, the effective temperature starts to feel cooler than the actual temperature. Now I’m back in the UK, I have no such problems….