Humidity

The distribution and movement of water vapor in the atmosphere play an important role in determining the weather of any region. Humidity is the amount of water vapor in the atmosphere at a given location on Earth’s surface. Two ways of expressing the water vapor content of the atmosphere are relative humidity and dew point. 

Relative humidity Consider a flask containing water. Some water molecules evaporate, leaving the liquid and becoming part of the water vapor in the flask. At the same time, other water molecules condense, returning from the vapor to become part of the liquid. Just as the amount of water vapor in the flask might vary, so does the amount of water vapor in the atmosphere. Water on Earth’s surface evaporates and enters the atmosphere and condenses to form clouds and precipitation.

In the example of the flask, if the rate of evaporation is greater than the rate of condensation, the amount of water vapor in the flask increases. Saturation occurs when the amount of water vapor in a volume of air has reached the maximum amount. 

The amount of water vapor in a volume of air relative to the amount of water vapor needed for that volume of air to reach saturation is called relative humidity. Relative humidity is expressed as a percentage. When a certain volume of air is saturated, its relative humidity is 100 percent. If you hear a weather forecaster say that the relative humidity is 50 percent, it means that the air contains 50 percent of the water vapor needed for the air to be saturated.

Dew point Another common way of describing the moisture content of air is the dew point. The dew point is the temperature to which air must be cooled at constant pressure to reach saturation. The dew point is often called the condensation temperature because it is the temperature at which water vapor in air condenses into water called dew. If the dew point is nearly the same as the air temperature, then the relative humidity is high.

Latent heat As water vapor in the air condenses, thermal energy is released. Where does this energy

come from? To change liquid water to water vapor, thermal energy is added to the water by heating it. The water vapor then contains more thermal energy than the liquid water. This is the energy that is released when condensation occurs. The extra thermal energy contained in water vapor compared to liquid water is called latent heat.