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Absolute Humidity
Absolute humidity is a direct measurement of the amount of water vapor in the air. It is calculated in grams of water vapor per cubic meter of air. Unlike relative humidity, the measurement is independent of the air's temperature. The moisture that you feel in the air is determined by the air's absolute humidity. The main sources of atmospheric moisture are water evaporation and plant transpiration, according to NOAA's Air Resources Laboratory.
Relative Humidity
Relative humidity refers to the percentage of moisture contained in the air compared to the maximum amount of water vapor that the air can hold at a given temperature. Because warm air can hold more water vapor than cold air, a measurement of 50 percent relative humidity at 80 degrees Fahrenheit means that the air contains more actual water vapor than it would with the same relative humidity at a lower temperature.
Humidity Changes
Altitude, weather systems and daily and seasonal temperature variations all affect relative humidity. The air at higher altitudes tends to contain less moisture than the warmer air below it, and water vapor at higher altitudes can cycle between water, vapor and solid phases without ever reaching the surface. When relative humidity is around 100 percent, clouds, fog or precipitation are likely to form. Because of the higher maximum moisture saturation of warm air, 100 percent relative humidity rarely occurs at very high temperatures.
Heat Index
At high temperatures, the human body cools itself by radiation, convection and evaporation of sweat. High humidity slows evaporation, making it harder for the body to cool itself effectively. The heat index is a calculation of how hot the air feels to humans as a result of relative humidity's impact on sweat evaporation. While hot, humid air may feel heavier than less humid air at the same temperature, the humid air is actually less dense, because water vapor is less dense than dry air.