The relative humidity is often included in the day’s weather report, if we are in the know, we do not expect refreshing showers when the relative humidity is 10, 20 or even 50 per cent. We know that the gaseous water vapor in the air begins to gel into drops of liquid when the relative humidity soars toward 100 per cent.
Absolute humidity tells the vapor content of the air, which may be six grains of vapor gas in every cubic foot of air, But this does not help us predict the weather. For warm air can hold more vapor than cool air and the vapor does not gel into liquid until the air is saturated to its dew point. We get this information from relative humidity which is based on the vapor content and the temperature of the air. When the day's air reaches a relative humidity of 100 per cent, some of the vapor may turn to rain.
It is a complicated job to find the relative humidity of the air, but the meteorologist has several instruments to help him. He finds the temperature and the vapor content of the air and then consults charts which tell exactly how much more vapor this air can hold.
One instrument is called the hydrometer. Water is stirred vigorously in a shiny metal cup until the metal and the water reach the same temperature as the surrounding air. Then ice is added and stirred with the water and soon vapor from the air condenses in misty drops on the outside of the cup. The temperature of the air is compared with the cooler temperature of the cup where it is chilled to dew point. This ratio shows relative humidity.
With another instrument, the meteorologist can pass a certain volume of air through chemicals which absorb moisture.
The vapor extracted from the air adds weight to the chemicals and this figure, along with temperature, is computed to give the relative humidity of the air.
Another such instrument is the psychrometer, which is basically a pair of whirling thermometers. The bulb of one is wrapped in a wet cloth which gives a cooler temperature as its moisture evaporates. Evaporation is faster when the air is dry, which means that the temperature of the wet bulb can give the vapor content. Relative humidity is computed by comparing this with the dry bulb figures which give air temperature.
These or other instruments can be used to find the amount of vapor in air at a certain temperature, The weatherman's charts for competing the ratio of relative humidity from these figures are very precise. Air at 61 degrees Fahrenheit which contains 4.85 grams of vapor per cubic meter has a relative humidity of 36 per cent. If 3.x.2 grams of vapor are added to the volume without changing the temperature, humidity reaches 53 per cent. This same air reaches 100 per cent humidity when it contains 13.65 grams of vapor per cubic meter.