Atmosphere Moisture & Precipitation

Humidity: The amount of water vapor present in the air. Humidity varies with location. The amount of water vapor that air can hold varies with temperature.

Cold Air: Cannot hold very much water vapor. (e.g. -10�c = 2 g/m³)
Warm Air: Can hold a larger about of water vapor. (e.g. 25�c = 23 g/m³)

Relative Humidity: The proportion (%) of water vapor present, relative tot he maximum possible for a given air temperature.

e.g. Air with a temperature of 15�c contains 6.3 g/m3 of water vapor. The maximum water vapor that 15�c can hold in 12.6 g/m3. Therefore: Relative Humidity = (6.3) / (12.6) x 100 = 50%

Saturated Air: Air becomes saturated at 100% Relative humidity. At that point, it cannot hold any more vapor.

Dew Point: The temperature to which air must cool in order to reach a point of Saturation.

(Remember, when air is cooled, the max amount of vapor it can hold drops.)

Clouds Begin to form after air reaches 100% Relative humidity. Any extra water vapor added will condense into water/ice crystals, and form precipitation.

Precipitation

Precipitation occurs when mist air rises above the dew point for a particular atmosphere (e.g. Temperature & Pressure). This can happen in 3 ways:

1. Convection Precipitation
   

   - Convection currents occur when the sun heats up the surface, which warms the atmosphere above it.
   - Strong heating occurs during the summer and in Canada we see convection precipitation during the afternoon.
   - Strong winds, lightning and hail accompany this precipitation when extreme heating occurs. (Fig. 7.9, Pg 49, Earth Dynamics)
   

2. Orographic Precipitation