Humidity is the amount of water vapor in a given volume of air. Knowing exactly how much water is in the air in specific regions is essential for accurate weather forecasts.

When TV meteorologists talk about humidity, they are usually referring to relative humidity, or how much water vapor is in the air compared with how much the air could hold at the current temperature. That difference is expressed as a percentage. For example, a relative humidity of 70 percent means the air is at 70 percent of its water-holding capacity for the present temperature. Cold air cannot hold as much water vapor as warm air can. Thus, as temperature falls, with no change in the amount of water in the air, the relative humidity rises. As temperature rises, with no change in the amount of water in the air, relative humidity falls.

Another way of stating humidity is with a measurement called the dew point. The dew point is the temperature at which water will start to condense out of the air into liquid water as dew; it can also be called a saturation point. For many people, a dew point below 13°C (55°F) feels dry, but beginning above 18°C (65°F) often feels muggy.

The highest dew point ever recorded was 35°C (95°F) in Saudi Arabia. In comparison, areas of the United States that are very humid, like the southern state of Mississippi and Washington, D.C., often have dew points of around 27°C (80°F). Areas close to warm bodies of water typically have higher humidity because warm water can evaporate into water vapor more easily than cold water, which adds more water into the air.

When the dew point temperature and air temperature are the same, the relative humidity is 100 percent. At 100 percent relative humidity, the air is completely saturated with water vapor. If the air cools a little, such as rising to a higher altitude, the water vapor can condense as fog, clouds, and other forms of liquid water. Once droplets of liquid water form and get too heavy to stay in the air, they fall as precipitation — like rain, snow, or ice.

During the summer months, high humidity is not just uncomfortable, it can make people feel unwell. That is why a heat index is often given on summer days as a public service. The heat index takes relative humidity as well as temperature into account to explain how hot the day will feel. Knowing the heat index can help people avoid heat-related ailments.