Absolute humidity is the amount of water vapor in the air. Relative humidity measures the amount of water vapor in the air relative to the amount of water the air can hold. The lower the percentage, the drier the air. Very arid areas have single digit relative humidity, while rain forests rarely have lower than 90% humidity. Relative humidity can never go above 100%, because at that point the water precipitates out of the air and it becomes foggy or rains. The temperature at which water precipitates out of the air is called the dew point. This temperature varies because it is based on the absolute amount of water vapor in the air, not the amount relative to how much the air can hold at that temperature.
Because warmer air can hold more water, relative humidity changes with temperature. This is the reason for condensation on cold windows and on the outside of cold glasses. The air touching the cold surface becomes too cold to hold the same amount of water it held at the warmer temperature.
Water vapor enters the air primarily through evaporation or transpiration, although some is also released through sublimation. Evaporation is the release of water vapor into the atmosphere as a result of changing state from liquid to gas. The larger the surface area of the body of water, the greater the evaporation. Warmer water releases more water vapor, and warmer or drier air can absorb more water vapor. The high rate of evaporation in tropical places and during summer accounts for the common afternoon thunderstorms, which occur when the air can hold no more water vapor. Wind can also increase the rate of evaporation by replacing saturated air with drier air, which is how a fan works. Very high rates of evaporation over warm, windy ocean water can cause hurricanes.
Sublimation is the change of state from solid to gas, from ice directly to water vapor, and occurs when it is both cold and very dry. This is why ice cubes in freezers shrink. In places such as Antarctica, sublimation is the primary way that water vapor enters the atmosphere.
Transpiration is the release of water vapor into the atmosphere by living things. This is what causes a mirror to fog when breathed on, and makes the inside of a house more humid in the winter than the outside air. Because water is both a start and end product of metabolism, living things take in liquid water and breathe it out as water vapor. This process transfers ground water into the atmosphere more rapidly than evaporation can.
The greater the amount of water vapor in the air, the less the temperature will change between day and night. This is because water vapor acts like a heat sink, which evens out the sun's energy. As water evaporates, it cools down its surroundings. The continual release of water vapor into the air by trees can cool down their surroundings by 10 degrees.
In high humidity, liquid water evaporates much more slowly. The resulting dampness encourages mold and mildew and peels off paints and glues. This is why stamps used in tropical regions use a different glue which is resistant to high humidity.
In low humidity, surfaces quickly build up static charges, which can damage electronics and cause uncomfortable shocks. The skin and lung membranes quickly dry out, reducing the efficiency of air exchange. Skin cracks, and the nose and lungs feel dry. This also strips away two of the body's main defenses against disease, which is why winter is flu season.
Condensation results where higher humidity air meets a cold surface. The dripping water is very bad for electronics and wood, and also damages other substances by encouraging rust and mold.
Humidity is measured using a hygrometer, while measuring relative humidity also requires a thermometer. House humidity in temperate climates can be regulated using a humidifier in winter and a dehumidifier in summer. A fan helps improve air circulation, which should cut down on condensation problems.