What is Relative Humidity?
Heat really needs no explanation. You may have difficulty defining it, but you’re certainly familiar with the experience of it. Relative humidity, on the other hand, does need explanation. Water is an extremely pervasive substance and can be found everywhere on the planet including in the air, where it’s held as vapour. The capacity of air to hold water-vapour varies according to the temperature of the air. The warmer the air, the more water-vapour it can hold. As the air cools down, its capacity to hold water will decrease. Relative humidity is a measure of the amount of the amount of water-vapour contained in air at a particular temperature. It is basically a comparison between: • the amount of water-vapour held in the air at any one time and at a particular temperature; and • the total amount of water-vapour which the air can hold at the same temperature, that is, the amount of water which will saturate the air at that temperature. Relative humidity is expressed as a percentage.
Relative humidity is defined as the ratio of the water vapor pressure to the saturation vapor pressure at the prevailing ambient temperature, expressed as a percentage. In simpler terms, it is the measure of the amount of water vapor in the air compared to how much it can possibly hold at that temperature. For example, air at 20° C. can hold, at a maximum, about 17 grams of moisture per cubic meter of air. Air holding this much moisture would be considered “saturated” and would have a measure of 100% relative humidity. If the 20° C. air becomes drier to the point where it only contains 8.5 grams of moisture per cubic meter, then the air is holding only one-half, or 50% of its total capacity. The measure of the air then would be 50% RH. The capacity of air to hold water is heavily dependent on the temperature of the air. The warmer the air, the more moisture it can hold. Air at 30° C. can hold about 30 grams of moisture per cubic meter or about 75% more than it can hold at 20° C. Air at
