Relative humidity is an indication of the amount of water actually in the atmosphere compared to the total water it could hold at a given temperature. Relative humidity remains relatively high throughout the year, averaging about 75 percent. Relative humidities show a diurnal pattern being generally highest in the cool, early morning and generally lowest in the warm afternoon hours. This decrease occurs because warm air is capable of holding more moisture than cool air.

The Keys generally exhibit a more stable relative humidity than the mainland. There is little seasonal or daily variation reflecting the dominance of the maritime influence on this aspect of climate as well. The surrounding ocean waters insure a moisture laden atmosphere.