A. Moisture parameters.

Consider a parcel of air.  If m_v is the mass of water vapor and m_d is the mass of dry air, then the water vapor mixing ratio is given by:

w is usually a few g oer kg of air.

Question: How does the water vapor mixing ratio relate to the ratio of pressures?  e = (w/(w+e))p.

Question: How does virtual temperature relate to temperature and mixing ratio?  T_v = T(1 + 0.61 w)
 

B. Saturation vapor pressure.

When the evaporation of water molecules from a flat surface of pure water equals the condensation of water vapor on the liquid, then the pressure of the water vapor is called the saturation vapor pressure, e_s.
 

C.  Saturation mixing ratios.

Because pressures are related to mixing ratios, we can relate the saturation vapor pressure to saturation mixing ratio:

Because e_s depends only on temperature, the saturation mixing ratio depends only on temperature and pressure.
 

D.  Relative humidity.

The relative humidity is defined as:  RH = 100 w/w_s .

The dew point (temperature), T_d, is defined as the temperature to which air must be cooled so that it is saturated with respect to water (in other words, dew forms.)
This process is assumed to occur at constant pressure.

The frost point (temperature) is the temperature to which air must be cooled so that it is saturated with respect to ice, at constant pressure.
 

E. Lifting condensation level.

What happens as moist air rises?  The pressure changes; the partial pressure of water vapor changes; the temperature changes.  These changes occur in a way such that the temperature change eventually causes the water vapor mixing ratio to become equal to the saturation mixing ratio.  The level at which this occurs is called the lifting condensation level.
 

F.  Wet bulb temperature.
 
The wet bulb temperature is the lowest temperature to which moist air can be cooled by evaporative cooling alone.  We assume constant pressure and no heating.

Assume that we have a wet bulb on a thermometer.  As we swing the sling psychrometer around, the water evaporates from the sock on the thermometer bulb in an attempt to reach equilibrium between the water vapor and the liquid water surface.  In the process of evaporation, the air moistens and cools, due to the energy needed to evaporate the water from the bulb.  Equlibrium is reached at the wet bulb temperature, T_w.

From a combination of the wet bulb and dry bubl temperatures, we can find the dew point temperature.