When a weather report gives a percentage value for humidity, this is a relative humidity. This value compares the amount of water in the air relative to the maximum water the air could contain at that temperature. A humidity rating of 75 percent when it's 90 degrees Fahrenheit outside represents a lot more water than a humidity of 75 percent when the temperature is 40 degrees Fahrenheit.
Absolute humidity, a less commonly used measurement, describes how much water is in the air by weight. At 30 degrees Celsius or 86 degrees Fahrenheit, absolute humidity ranges from 0 grams of water per cubic meter of air to 30 grams per cubic meter at saturation.
The dew point, another measurement given in weather forecasts, is the temperature to which humid air must be cooled for condensation to occur. If humidity is at 100 percent, for example, the dew point and the actual temperature are the same. On very dry days, the dew point will be much lower than the actual temperature; it would have to get much, much colder before the air could no longer hold the moisture.
