ATOC 4720 class 13 n Water vapor in

ATOC 4720: class 13 n Water vapor in the air: moisture parameters

Water vapor Where Vapor pressure

Moisture parameters a) Mixing ratio Units: g/kg: A few to 20 Conserved if no phase changes.

Example question 1: If w=5. 5 g/kg, and P=1026. 8 mb, calculate the vapor pressure e. Example 2: Calculate the virtual temperature of moist air at 30 C which has a mixing ratio of 20 g/kg.

b) Saturation vapor pressure The vapor pressure when the air is Saturated is called the saturation vapor pressure over a plane surface of pure water ( ). Similarly, saturation vapor pressure over a plane ice Surface.

Function of only T: Important forming clouds.

c) Saturation mixing ratio Approximately,

d) Relative humidity

e) Dew point The dew point Td is the temperature to which air must be cooled at constant pressure in order for it to become saturated with respect to a plane surface of water. The frost point is the temperature to which air must be cooled at constant pressure in order for it to become saturated with respect to a plane surface of ice.

Since pressure changes small near The earth’s surface, dew point is a good Indicator for moisture content. RH is not.

f) Lifting condensation level The lifting condensation level is defined as the level to which a parcel of moist air can be lifted adiabatically before it becomes saturated with respect to a plane surface of water.

Lifting condensation level

g) Wet-bulb temperature The wet-bulb temperature Tw is the temperature to which a parcel of air is cooled by evaporating water into it at constant pressure until the air is saturated with respect to a plane surface of liquid water.