Humidity is the amount of water vapor in the air and can be described in different ways, including "relative humidity," which is the term used most often in weather information meant for the public. Relative humidity is the amount of water vapor in the air compared with the amount of vapor needed to make the air saturated at the air's current temperature.
The U.S. Geological Survey estimates that the Earth has about 326 million cubic miles of water. This includes all of the water in the oceans, underground and locked up as ice. Only about 3,100 cubic miles of this water is in the air, mostly as water vapor, but also as clouds or precipitation, at any one time.
Why humidity can be less than 100% when it's raining
Humidity is a measure of the amount of water vapor in the air, not the total amount of vapor and liquid. For clouds to form, and rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is coming from. This normally happens when the air rises and cools. Often, rain will be falling from clouds where the humidity is 100% into air with a lower humidity. Some water from the rain evaporates into the air it's falling through, increasing the humidity, but usually not enough to bring the humidity up to 100%.
Absolute humidity: The mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter
Actual vapor pressure: The partial pressure exerted by the water vapor present in a parcel. Water in a gaseous state (i.e. water vapor) exerts a pressure just like the atmospheric air. Vapor pressure is also measured in millibars.
Condensation: The phase change of a gas to a liquid. In the atmosphere, the change of water vapor to liquid water.
Dewpoint: the temperature air would have to be cooled to in order for saturation to occur. The dewpoint temperature assumes there is no change in air pressure or moisture content of the air.
Dry bulb temperature: The actual air temperature. See wet bulb temperature below.
Freezing: The phase change of liquid water into ice.
Evaporation: The phase change of liquid water into water vapor.
Melting: The phase change of ice into liquid water.
Mixing ratio: The mass of water vapor in a parcel divided by the mass of the dry air in the parcel (not including water vapor)
Relative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways. One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent. See What Relative humidity means below.
Saturation of air: The condition under which the amount of water vapor in the air is the maximum possible at the existing temperature and pressure. Condensation or sublimation will begin if the temperature falls or water vapor is added to the air.
Saturation vapor pressure: The maximum partial pressure that water vapor molecules would exert if the air were saturated with vapor at a given temperature. Saturation vapor pressure is directly proportional to the temperature.
Specific humidity: The mass of water vapor in a parcel divided by the total mass of the air in the parcel (including water vapor)
Sublimation: In U.S. meteorology, the phase change of water vapor in the air directly into ice or the chance of ice directly into water vapor. Chemists, and sometimes meteorologists, refer to the vapor to solid phase change as "deposition."
Wet bulb temperature: The lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity
The warmer air is, the more water vapor it can "hold." Dew point is a measure of how much water vapor is actually in the air. Relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at the temperature it happens to be when you measure it. To see how this works, let's use the chart below.
Air temperature in degrees C Water vapor air can hold at this temperature. 30 degrees 30 grams per cubic meter of air 20 degrees 17 grams per cubic meter of air 10 degrees 9 grams per cubic meter of air
These numbers, which apply to air at sea level pressure, are based on measurements over the years. They are basic physical facts.
Now, let's see how dew point and relative humidity work. Imagine, that at 3 p.m. you measure the air's temperature at 30 degrees and you measure its humidity at 9 grams per cubic meter of air. What would happen if this air cooled to 10 degrees with no water vapor being added or taken away? As it cools to 10 degrees the air becomes saturated; that is, it can't hold any more water vapor than 9 grams per cubic meter. Cool the air even a tiny bit more and its water vapor will begin condensing to form clouds, fog or dew - depending on whether the air is high above the ground, just above the ground, or right at the ground. Back at 3 p.m., when we made the measurements, we could say that the air's dew point is 10 degrees C. That is, if this particular air were cooled to 10 degrees at ground level, its humidity would begin condensing to form dew.
How about relative humidity? At 3 p.m. the air has 9 grams of water vapor per cubic meter of air. We divide 9 by 30 and multiply by 100 to get a relative humidity of 30% In other words, the air actually has 30% of the water vapor it could hold at its current temperature. Cool the air to 20 degrees. Now we divide 9, the vapor actually in the air, by 17, the vapor it could hold at its new temperature, and multiply by 100 to get a relative humidity of 53% (rounded off). Finally, when the air cools to 10 degrees, we divide 9 by 9 and multiply by 100 to get a relative humidity of 100% - the air now has all the vapor it can hold at its new temperature.