26. What is the dew point? How is air humidity expressed?
Air humidity is usually expressed using the concept of relative humidity. This refers to the ratio of the amount of water vapor present in the air at a given temperature to the maximum amount of water vapor that air can hold at that temperature. Relative humidity decreases as the air temperature rises. It is a ratio, expressed as a percentage. When relative humidity reaches 100%, it means the air is fully saturated with water vapor. Generally, relative humidity fluctuates between 50% and 90%.
When unsaturated air (i.e., air with humidity below 100%) cools, its relative humidity increases. Once the air reaches saturation (100% relative humidity), water vapor begins to condense. The temperature at which this happens is called the dew point.