from wikipedia
Humidity is a term for the amount of
water vapor in the
air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture, called the
Absolute humidity.
[1] In everyday usage, it commonly refers to
relative humidity, expressed as a percent in weather forecasts and on household
humidistats; it is so called because it measures the current absolute humidity
relative to the maximum.
Specific humidity is a
ratio of the water vapor content of the mixture to the total air content (on a mass basis). The water vapor content of the mixture can be measured either as mass per volume or as a partial pressure, depending on the usage.
http://en.wikipedia.org/wiki/Humidity
dew point is the
temperature to which a given parcel of humid
air must be cooled, at constant
barometric pressure, for
water vapor to
condense into liquid water. The condensed water is called
dew when it forms on a solid surface. The dew point is a saturation temperature. The dew point is associated with relative
humidity. A high
relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity will decrease.
http://en.wikipedia.org/wiki/Dew_point