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ATMOSPHERIC WATER
UNITS FOR EXPRESSING HUMIDITY
humidity - amount of water vapor in air; can be expressed as:
a) absolute humidity - amount of water vapor in air expressed as grams water per cubic
                                         meter of air (g/m3)
b) specific humidity - amount of water vapor in air expressed as grams water per kilogram
                                         of  air (g/kg)
c) relative humidity - amount of water vapor in air expressed as a percentage of the amount
                                        of water vapor that could be held at saturation.
d) vapor pressure - amount of water vapor in air expressed as the downward pressure
                                      exerted by the water vapor present in the atmosphere. (1-55 mm Hg).

RELATIONSHIP BETWEEN HUMIDITY AND TEMPERATURE
dew point - the temperature where relative humidity equals 100%. 
condensation - conversion of water vapor to liquid or ice; condensation occurs when the 
                         temperature drops below the current dew point.
precipitation - loss of atmospheric water; occurs when condensation is extensive enough
                           that the water or ice coalesce to form droplets/crystals that fall from the
                           atmosphere due to the force of gravity.
Cold air can hold less water vapor than warm air, so as temperature decreases the absolute humidity or specific humidity that can be held at saturation decreases.  Relative humidity is a percent of saturation.  So as temperature decreases the absolute or specific humidity at saturation decreases, but the amount of water vapor present stays the same, thus relative humidity increases (conversely, as temperature increases relative humidity decreases). If the temperature continues to decrease (such as cooling over night or as air rises in the atmosphere),  a temperature is reached where the relative humidity reaches 100% - this temperature is the dew point.  If the temperature drops below the current dew point, the air becomes over saturated and condensation will occur.  If this occurs in the upper atmosphere, a cloud forms or  precipitation occurs.  If this occurs close to the earth's surface, fog or dew forms. 

TYPES OF CONDENSATION
1) dew - condensation of water onto solid surfaces
2) fog - condensation of water into small droplets that stay suspended in air close to the
            earth's surface.
3) cloud - condensation of water into small droplets that stay suspended in air high in the
               atmosphere.

TYPES OF PRECIPITATION
I) condensation occurs above freezing (when dew point is above 32 oF)
    a) drizzle - water droplets less than 0.5 mm
    b) rain - water droplets greater than 0.5 mm.
2) condensation occurs below freezing (when dew point is below 32 oF)
    a) snow- water condenses below freezing directly into small, loose ice crystals.
3) condensation occurs above freezing, followed by freezing
    a) sleet - liquid rain droplets fall through a layer of freezing air and then freeze.
    b) hail- liquid rain droplets fall through a layer of freezing air and then freeze; air currents
                 carry the frozen droplets back up into the upper atmosphere; they pick-up more
                 water,  then freeze upon falling through the layer of freezing air; each time the
                 ice crystal  circulates through the warm and freezing layer it gets larger until it 
                 finally falls to earth.