RELATIVE HUMIDITY:

Relative humidity (RH) (expressed as a percent) also measures water vapor, but RELATIVE to the temperature of the air. In other words, it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer. What we "feel" outside is the actual amount of moisture (absolute humidity) in the air.

DEWPOINT:

Meteorologists routinely consider the "dewpoint" temperature (instead of, but analogous to absolute humidity) to evaluate moisture, especially in the spring and summer. The dewpoint temperature, which provides a measure of the actual amount of water vapor in the air, is the temperature to which the air must be cooled in order for that air to be saturated. Although weather conditions affect people differently, in general in the spring and summer, surface dewpoint temperatures in the 50s usually are comfortable to most people, in the 60s are somewhat uncomfortable (humid), and in the 70s are quite uncomfortable (very humid). In the Ohio Valley (including Kentucky), common dewpoints during the summer range from the middle 60s to middle 70s. Dewpoints as high as 80 or the lower 80s have been recorded, which is very oppressive but fortunately relatively rare. While dewpoint gives one a quick idea of moisture content in the air, relative humidity does not since the humidity is relative to the air temperature. In other words, relative humidity cannot be determined from knowing the dewpoint alone, the actual air temperature must also be known. If the air is totally saturated at a particular level (e.g., the surface), then the dewpoint temperature is the same as the actual air temperature, and the relative humidity is 100 percent.

RELATIONSHIP OF DEWPOINT AND RELATIVE HUMIDITY TO CLOUDS AND PRECIPITATION:

If the relative humidity is 100 percent (i.e., dewpoint temperature and actual air temperature are the same), this does NOT necessarily mean that precipitation will occur. It simply means that the maximum amount of moisture is in the air at the particular temperature the air is at. Saturation may result in fog (at the surface) and clouds aloft (which consist of tiny water droplets suspended in the air). However, for precipitation to occur, the air must be rising at a sufficient rate to enhance condensation of water vapor into liquid water droplets or ice crystals (depending on air temperature) and to promote growth of water droplets, supercooled droplets, and/or ice crystals in clouds. Droplets grow through a process called "collision-coalescence" whereby droplets of varying sizes collide and fuse together (coalesce). Ice crystal processes (including deposition and aggregation) also are important for particle growth. In thunderstorms, hail also can develop. Once the suspended precipitation particles grow to sufficient size, the air can no longer support their weight and precipitation falls from the clouds. In humid climates, thunderstorms often cause heavier rain than general wintertime rainfall since moisture content in the air typically is higher in the spring and summer, and since air usually rises at a much more rapid rate within developing thunderstorms than in general winter systems. "Cloud microphysics" is the study of droplet and ice crystal production and growth within clouds and their relationship to precipitation.

Source: National Weather Service Louisville, KY

http://www.crh.noaa.gov/lmk