Higher temperatures make higher levels of humidity possible. This is because warm air is capable of holding more moisture than cold air. Warm air has a stronger bond to water molecules, making the air feel drier and allowing more water to bond with air.
In weather forecasting, humidity that is expressed as a percentage is called relative humidity. Relative humidity measures how much of the air's current moisture capacity is used. A relative humidity of 50 percent means that the air holds 50 percent of the moisture possible at the current temperature. This percentage changes with temperature, since colder air holds less moisture.
A relative humidity of 100 percent means that the air is completely saturated with moisture. When humidity reaches this level, clouds form. 100 percent humidity is not necessary on the ground for rain to fall; it is only required in the atmosphere at the site of cloud formation. High humidity makes the air feel hotter because less sweat evaporates from human skin in saturated air.
One related measurement is the dew point. The dew point is the temperature at which the current amount of moisture in the air reaches 100 percent humidity. When the temperature drops to the dew point, water vapor forms clouds or condenses on surfaces.