When the weather is hot and sunny, its pleasantness or otherwise depends very much on the humidity. If the atmosphere is "dry" on a warm day, we can work or play with zest and energy, but if the air is both hot and "humid" we find it sticky, heavy and oppressive.
Every activity of the body - even the very process of living - generates heat. In normal Irish temperatures, excess heat from the body drains off easily into the cooler atmosphere.
But on a hot day this process is less efficient, and additional measures are required: when the brain senses overheating it calls into action the millions of sweat glands which perforate the outer layers of our skin, and we begin to perspire.
Sweating by itself, however, cannot cool the body; cooling occurs only when the excess water is removed by evaporation. And while evaporation is a very powerful cooling process, it depends for its effectiveness on the ability of the air to absorb the moisture - a factor which is in turn determined by its humidity.
Humid air has so much moisture in it already that it is reluctant to accept more. Dry air, by contrast, soaks up all available moisture, and the body's cooling device works very effectively.
Meteorologists over the centuries have devised many clever ways of measuring the humidity of the air around us. Probably the simplest method is to employ a material which changes its physical characteristics as it absorbs or loses moisture. Human hair has this property - it expands or contracts - and it is often used to construct instruments to provide a continuous record of the relative humidity.
Other methods make use of the phenomenon that the electrical conductivity of some "hygroscopic", or water-absorbing, substances varies with the amount of moisture they contain.
Lithium chloride, for instance, is such a substance, so that if a known voltage is applied across a disc with a thin coating of lithium chloride, and the resultant current measured, its magnitude allows us to calculate the humidity of the surrounding air.
But the most accurate humidity measurements are achieved using a "psychrometer". This comprises two thermometers side by side, one of which - the "wet bulb" - has its bulb enclosed in a "glove" of muslin dampened by distilled water.
The dryer the surrounding air, the more evaporation that will occur, and evaporation from the "wet bulb" results in a drop in its temperature. The difference between the readings of the two thermometers, therefore, is a measure of the moisture content of the air.