2 Atmospheric Moisture and Cloud Formation

Water moves into the atmosphere

Water continually moves between Earth's surface and the atmosphere. Evaporation supplies water vapor, and changing temperature and air motion can cause that vapor to condense and form clouds.

Humidity and dew point

Humidity describes water vapor in the air. Relative humidity compares the water vapor currently present with the amount needed to saturate the air at its current temperature. It is expressed as a percentage, and air at $100\%$ relative humidity is saturated.

Because warmer air has a higher saturation vapor pressure than cooler air, relative humidity can change when air warms or cools even if its water-vapor content stays the same.

Dew point is the temperature to which air must cool, at constant pressure and with no change in water-vapor content, to become saturated. A smaller difference between air temperature and dew point generally means higher relative humidity. Dew point is useful because it indicates the amount of moisture in the air more directly than relative humidity does. For example, if air near the ground cools overnight to its dew point, water may condense as dew or fog.

Evaporation and condensation

Evaporation changes liquid water into water vapor. It occurs at the surface of water and moist ground, and also from plants through transpiration. Evaporation requires energy, commonly supplied by sunlight and the surrounding air. Water vapor then mixes into the atmosphere and can be carried far from where it evaporated.

Condensation changes water vapor into liquid water. It occurs when air becomes saturated and water vapor condenses onto tiny particles called condensation nuclei, such as sea salt, dust, or smoke. Condensation releases energy to the surrounding air. Below freezing, clouds may contain ice crystals as well as, or instead of, liquid droplets.

Air masses transport moisture

An air mass is a large body of air with relatively similar temperature and moisture characteristics. It takes on properties from the region where it forms: air that remains over an ocean tends to become moist, while air that forms over land tends to be drier. Maritime tropical air is generally warm and humid; continental polar air is generally cold and dry.

As air masses move, they transport these conditions to other regions. Where air masses meet, fronts can help force air upward and contribute to cloud formation.

How clouds form

Clouds form when moist air cools enough to reach saturation and water vapor condenses. A common way for air to cool is to rise. Higher in the atmosphere, pressure is lower, so rising air expands and cools. If it cools to its dew point, condensation can begin on condensation nuclei, producing vast numbers of tiny droplets or ice crystals. Together, these suspended particles make a visible cloud.

Air can be lifted in several ways:

• Convection: Sun-warmed air near the ground rises, often forming puffy clouds.

• Orographic lifting: Air is forced up over hills or mountains.

• Frontal lifting: One air mass is lifted as it meets another, often along a front.

• Convergence: Air flowing together near the surface is pushed upward.

Clouds change continuously. Droplets form when condensation exceeds evaporation, and shrink or disappear when evaporation exceeds condensation. Cloud formation does not by itself guarantee precipitation; the droplets or ice crystals must grow large enough to fall.