|Humidity refers to the moisture content of the atmosphere. Moisture in the atmosphere comes from the evaporation of large bodies of water. Humidity measurements include absolute humidity (the mass of water vapor per unit volume of natural air); relative humidity (the ratio of the actual water-vapor content of the air to its total capacity at the given temperature), specific humidity (the mass of water vapor per unit mass of natural air), and the mixing ratio (the mass of water vapor per unit mass of dry air). When the term "humidity" is used by itself, it usually is referring to relative humidity.
When no more water can evaporate into the atmosphere, the air is saturated (full of water molecules) and is said to be at 100% relative humidity. The rate of evaporation decreases as the moisture content of the air increases and approaches saturation. In addition, the saturation point (moisture-holding capacity of the air) increases rapidly as the temperature of the air rises. Thus cold air, while its moisture content is necessarily quite low (low absolute humidity), may be almost saturated with respect to the maximum amount of water vapor it is capable of holding (high relative humidity). Cold air with high relative humidity "feels" colder than dry air of the same temperature because high humidity in cold weather increases the conduction of heat from the body. Conversely, hot air attended by high relative humidity "feels" warmer than it actually is because of an increased conduction of heat to the body combined with a lessening of the cooling effect afforded by evaporation. On the other hand, a low relative humidity “modifies” the effect of temperature extremes on the human body. Humidity decreases with altitude. Proximity to large bodies of water and the prevalence of moisture-bearing winds favor high humidity.
Another way meteorologists measure the amount of water vapor in the atmosphere is by using the dew point temperature. The dew point temperature is the point at which the air will become saturated. Cooler air can hold less water vapor than warmer air. Therefore, as the air cools, the saturation level increases. Fog or low clouds are a good indicator of saturated atmosphere.