Q1.
But how do we measure such numbers?
Answer
With an instrument called a hygrometer. It reads out the relative humidity as a percentage, on this scale:
| Relative humidity | What it means |
|---|---|
| 0% | Air with absolutely no water vapour — impossible in natural conditions |
| 20% – 40% | Dry weather |
| 60% – 80% | Humid weather |
| 100% | Air saturated with water vapour — it can hold no more |
There are several types of hygrometer, depending on the principle each one is based on. A simple classical one uses two thermometers side by side, one with a wet cloth around its bulb; the drier the air, the faster the water evaporates from the cloth, the more that thermometer cools, and the bigger the gap between the two readings. Modern weather stations use electronic sensors instead.
Why anyone bothers: Humidity measurement is of great importance in industry — food processing, for instance, needs closely controlled humidity so that products neither dry out nor spoil. Museums monitor humidity too, because they must keep a dry environment to preserve their exhibits. And every daily weather bulletin quotes it, because humidity decides how the temperature actually feels.