MEASURES OF ATMOSPHERIC MOISTURE

Absolute Humidity = (Mass of water vapor)/(Volume of air) [g/m3].  (The density of dry air at sea level under standard
      conditions is 1292.9 g/m3.)

Disadvantages:  Depends on moisture content and pressure.  Also, since the condition of saturation depends both upon atmospheric moisture content and temperature, knowing the absolute humidity and the pressure, without knowledge of the temperature, does not enable one to determine the probability of cloud formation or precipitation.

Specific Humidity = (Mass of water vapor)/(Mass of air) [g/kg].

Advantages:  Is unaffected by pressure.
Disadvantages:  Since the condition of saturation depends both upon atmospheric moisture content and temperature, knowing the specific humidity, without knowledge of the temperature, does not enable one to determine the probability of cloud formation or precipitation.

Mixing Ratio = (Mass of water vapor)/(Mass of dry air) [g/kg].

Advantages:  Is unaffected by pressure.
Disadvantages:  Since the condition of saturation depends both upon atmospheric moisture content and temperature, knowing the mixing ratio, without knowledge of the temperature, does not enable one to determine the probability of cloud formation or precipitation.

        Note that Mixing Ratio ≥ Specific Humidity, always.

Saturation = The condition of the atmosphere when it contains sufficient water vapor so that the condensation rate (which
      depends primarily upon water content of the atmosphere) equals the potential evaporation rate (which depends primarily upon
      temperature).  The higher the temperature, the higher the specific humidity at which saturation occurs.

(Actual) Vapor Pressure = Pressure exerted by atmospheric water vapor [mb].

Saturation Vapor Pressure = Actual vapor pressure which would cause air at the ambient temperature to be saturated
      [mb].  (Click here for a table of saturation vapor pressure versus temperature.)

Relative Humidity = (Actual Vapor Pressure)/(Saturation Vapor Pressure) [%]. Measures how close the air is to being
      saturated.

Advantages:  Since it expresses how close the atmosphere is to being saturated, it can be used to determine the probability of cloud formation or precipitation without a lot of additional information.  Is unaffected by pressure.
Disadvantage:  Depends on atmospheric moisture content and ambient temperature.

Wet-Bulb Temperature = The temperature measured by a thermometer with a wet bulb [°C or °F]. The higher the humidity,
      the closer this reading is to the actual temperature.  Evaporative cooling will not work below this temperature.

Dew Point = The temperature at which the relative humidity would reach 100% for the ambient vapor pressure (and hence the
      temperature at which dew would form) [°C or °F].  This measures the actual vapor content better than relative humidity since it
      does not depend on the ambient temperature.  The higher the humidity, the closer this reading is to the actual temperature.  If the
      air is unsaturated, (Dew Point) < (Wet-Bulb Temperature) < (Ambient Temperature).  If the air is saturated then both the "<"
      signs in the preceding relationship are replaced with "=".