Moisture in the Air

Water Cycle

Water cycles from one reservoir to another through the processes of evaporation, condensation and precipitation. Energy transfers are associated with each of these processes. When water absorbs energy from the Sun, a phase change may occur through the process of evaporation. Changes in phase do not change the particles but do change how they are arranged. In the troposphere, temperature decreases with altitude. As warm, moist air rises, the water particles transfer energy to the surrounding air, move slower and closer together and the water vapor turns back to water droplets (process of condensation). The slower the movement of the water vapor particles, the more particles that join together and form droplets of liquid water.

Bill Nye the Science Guy - Water Cycle

Relative Humidity

Relative humidity is a measurement comparing how much moisture is actually in the air compared to what it can hold. For example, if at the temperature is 80°F, the humidity is 13 g/m3, but can hold up to 25 gm/m3, the relative humidity is about 50%, or half the amount it can actually hold. In this scenario, at 25 g/m3, the air has a relative humidity of 100% and is considered at its saturation point, and can not hold any more water vapor in the air.


Dew Point

The dew point temperature is the temperature at which water droplets form from saturated air. The dew point temperature is dependent upon the amount of water vapor in the air. The greater the difference between air temperature and its dew point temperature, the least likely it is for water vapor to condense. A parcel of air with more water vapor will have a smaller difference between temperature and dew point temperature. When the temperature and dew point are the same, fog or clouds will form.

Dew is formed when the air near the ground cools to the dew point. If the dew point temperature is below freezing, frost will form. When this occurs, the relative humidity is 100%.

finding Dew Pt & Relative Humidity

Cloud Formation

Condensation is the process by which water vapor changes into liquid water droplets and is necessary for clouds to form. When the air rises and cools, the amount of water vapor needed for saturation decreases and the relative humidity will increase. When the relative humidity reaches 100 %, the air is saturated and water vapor will soon begin to condense into tiny water droplets around microscopic matter in the air (called condensation nuclei), such as dust and salt. Billions of these water droplets form a cloud.