What is the difference between dewpoint and relative humidity?

Dewpoint (Td) and relative humidity (%RH) are two different ways of expressing the amount of water vapor in the air.

Dewpoint is defined as the temperature to which the air must be cooled, at constant pressure, for water vapor to begin to condense to water.  In other words, when the dewpoint temperature has been reached, the air is fully saturated with water vapor.  

Relative humidity is defined as: 

%RH = (p_w / p_ws) x 100

Where:

p_w = partial pressure of the water vapor

p_ws = saturation vapor pressure of water vapor at a given temperature

Relative humidity is a percentage describing the ratio of the amount of water vapor in the air to the maximum amount of water vapor the air can hold at a given temperature.  For example, a relative humidity of 70% indicates that the air contains 70% of the maximum amount of water vapor it is capable of holding at that particular temperature. 

When RH = 100%, the air is fully saturated and the air temperature = dewpoint temperature.

Relative humidity will vary with changes in temperature and pressure.  Dewpoint is independent of temperature and will only vary with pressure.