The HUMIDITYcast map shows the humidity forecast levels for the next 24 hours, at 6 hour intervals. The map for each interval is available within the drop-down menu. Each map shows the relative humidity forecast of a particular region, contoured every 10 percent. Relative humidity is defined as the amount of water vapor in a sample of air compared to the maximum amount of water vapor the air can hold at any specific temperature in a form of 0 to 100%. Humidity may also be expressed as absolute humidity and specific humidity.

Relative humidity is an important metric used in forecasting weather. Humidity indicates the likelihood of precipitation, dew, or fog. High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by preventing the evaporation of perspiration from the skin. This effect is calculated in a heat index table. Warm water vapor has more thermal energy than cool water vapor and therefore more of it evaporates into warm air than into cold air.