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.

Humidity

Humidity refers to the water vapor content of the atmosphere. Usually, this term does not include the water held as liquids and solids in clouds and precipitation. Instead, water vapor is the gaseous form of water. It is odorless and colorless, but significant amounts of humidity can impair visibility. Water vapor is one of the gases of the atmosphere and freely mixes and moves in the fluid flow we call wind. 

To persist in the form of a gas, water must achieve much higher energy content than the liquid or solid states. In the dynamic setting of the free atmosphere water readily changes state to and from its gaseous phase. When vapor changes phase to liquid, this process is called condensation and the water molecules organize themselves into chains because of loss of energy from the vapor. Conversely, when water evaporates and changes its phase from liquid to vapor, this necessitates the addition of substantial amounts of energy to break the chains of liquid molecules.

Less well known to most of us is that water vapor can lose enough energy at once to change directly from gas to the regular crystalline arrangement of ice molecules; this process is called deposition and the result can be witnessed as the accumulation of frost on an automobile on a clear winter night. In reverse, ice can directly change to water vapor by gaining enough energy; this process is called sublimation and can be subtly observed by noting the decrease in an ice or snow cover after windy conditions; even when temperatures are below 0°C, ice can sublimate into water vapor.

The energy inherent to water vapor that allows water vapor to remain as water vapor is known as latent heat. This name implies that the heat is “hidden” and not directly detectable by thermometer. At sea level, the conversion of ice to water vapor requires about 2,850 joules. This is energy lost from the water’s surroundings and converted into hidden, latent form. The latent heat remains with the water vapor until condensation or evaporation takes place. These processes may take place many thousands of kilometers from where the vapor was added to the air. The latent heat effect can be quite dramatic. When water changes phases from vapor to liquid or solid, the release of latent heat can warm the atmosphere. If this phase change is ongoing while air is rising latent heat release retards the adiabatic cooling, making the air rise to much greater heights than it would without this phase change. Latent heat provides the huge amounts of energy needed for thunderstorms and hurricanes.