For many centuries people kept warm during the winter by means of coal or wood fires, variously accommodated in braziers, grates and stoves.
More recently, central heating has made such open fires redundant but, although it provides the necessary warmth, it can also adversely affect another important aspect of our comfort. When we control the temperature, we inadvertently change the relative humidity.
Relative humidity is temperature dependent. It is a measure of the ratio between the amount of moisture actually present in a volume of air, and the amount of moisture that the same body of air is capable of holding at the same temperature - in other words, the amount of moisture it would contain if it were saturated.
As we know, the warmer the atmosphere the more moisture it can hold. Suppose, for example, the outside temperature is 10 degrees and the relative humidity is 80 per cent; at 10 degrees saturated air contains about nine grams of water vapour per cubic metre but since in this case the relative humidity is only 80 per cent, each cubic metre in fact contains only about seven grams.
Now if the heating inside a house raises the temperature to 20 degrees, since no moisture has been added to the air it still contains only seven grams of water vapour per cubic metre. But at 20 degrees each cubic metre is capable of absorbing 17 grams of water before becoming saturated - so at the higher temperature the relative humidity is only around 40 per cent.
Low humidity indoors is most likely to occur when the air coming into a house is dry to start with, and this in turn happens most frequently in the cold, showery north-westerly airflow behind a cold front. Moreover, somewhat paradoxically, it is a winter problem; in the summer, with windows and doors open for much of the time, the humidity indoors is normally very similar to that outside.
Whatever the circumstances, very dry air makes most people feel uncomfortable and often causes stinging eyes and irritated skin.
It also sometimes has more painful consequences. Water is a good conductor of electricity, so in a moist atmosphere it is relatively easy for static electricity accumulated as a result of friction to leak away into the air. But when the air is dry, the charge builds up, ready to follow the easiest direct path to earth when given half an opportunity - thereby giving a shock, accompanied by sparks and crackle, to susceptible persons who touch a piece of metal. The phenomenon was, quite literally, painfully obvious during the recent cold spell.