text: Relative humidity is the moisture content of air relative to the temperature. Warmer air can hold more moisture (measured in grains) than cold air. If you have 70 degree air at 90 percent humidity and the temperature suddenly dropped to 50 degrees, the humidity level would rise to 100 percent. This is called the saturation or dew point. when the humidity is at 100 percent the air can hold no more moisture and it begins to condensate (fog, dew, rain) until the relative humidity drops below 100 percent. When you wake up and it is foggy (air is at dewpoint) and in a while the sun warms the air and the fog clears, this is an example of a drop in relative humidity.