Understanding humidity

Understanding humidity

Humidity is the amount of water vapor in the air and can be described in different ways.

The term that you'll hear most often to describe the humidity is relative humidity. This also happens to be, in my opinion, the measurement of humidity that's hardest to understand.

Another common measurement of humidity is the dew point, which most of us also also have to work, at least a little, to understand.

Unless you are a weather forecaster, or a scientist or engineer involved in work involving the amount of water vapor in the air, you can easily go through life without understanding relative humidity. On the other hand, it is nice to know what a TV meteorologist means by dew point.

Why humidity is worth worrying about

The humidity combined with hot temperatures makes the heat more dangerous. 

Indoor, as well as outdoor humidity is important, and humidity that's too high or too low can be dangerous for both you and your house. (

Humidity is a key player in the weather

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, the Geological Survey estimates.

While this is a small share of Earth's water, our planet would be very different without it. If Earth's air didn't contain as much humidity as it does, our weather would be like that of Mars: No clouds (except dust), no rain, sleet or snow, no thunder and lighting, no fog. And, without all of this water in all of its forms, Earth's life, if there were any at all, but be as hard to find as life on Mars is to find even signs of. 

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%.

Humidity is important in climate change

Water vapor in the air, the humidity, plays an important part in global climate. Like carbon dioxide, water vapor is a greenhouse gas. Climate scientists have found that carbon dioxide human activities is adding to the air is causing the Earth's average climate to warm. 

This, in turn, is almost surely affecting other aspects of global climate, and could have even bigger effects in the future. 

As the climate warms, the humidity can increase. Since water vapor is a greenhouse gas, it should increase warming as it increases. Measuring how much global humidity is increasing and figuring out how it will increase, and the effects of such an increase are major challenges for climate scientists. 

Answers to questions about humidity

Definitions of humidity-related terms

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. 

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 varies with atmospheric pressure. When a given atmospheric pressure is steady, then the saturated 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