Why is a hygrometer important in weather forecasting?
An hygrometer tells you the relative humidity of the atmosphere. But, to be of any value, it has to be put outside. Here is why: Your home is probably heated. How much moisture the air can contain is proportional to the temperature. For example, at 15 C, a cubic meter of air can’t contain more than 13 grams of water. The relative humidity is then 100 percent. But when the same air comes into your home and is warmed up, it can contain more humidity and the reading of your hygrometer will show less than 100 percent. In any case, a hygrometer doesn’t tell much about the weather forecast by itself. If the relative humidity is high it can only mean that it is raining or there is fog; both observations can be done without the instrument. Dry continental air can move in and your hygrometer measures it. But early in the morning, just after sunrise, the ground may be very cold from the night and dew may form. The relative humidity is then 100 percent but as soon as the sun warms up the soil and
