USEFULNESS: Relative humidity is the ratio of the amount of moisture in the air to the maximum amount of moisture the air could have. It is defined as the vapor pressure (or mixing ratio) divided by the saturation vapor pressure (or saturation mixing ratio). The RH is NOT the dewpoint number divided by the temperature number (i.e. if the temperature is 60 F and the dewpoint is 30 F, you would not take (30/60)*100% = 50% to get the RH). Since the maximum amount of moisture in the air is temperature dependent, the relative humidity is also temperature dependent. For a particular temperature, relative humidity tells how close the air is to saturation. Evaporation is most intense at high temperatures and with low humidities. Relative humidity has been described as "the drying power of the air". Evaporation from lakes, soil, vegetation and the ground will increase as RH decreases. At high relative humidities, evaporation is slowed. At warm temperatures and high humidities, cooling of the human body by evaporation is reduced, thus making the person feel uncomfortably warm. The evaporative cooling potential depends on how much moisture can be evaporated into the air. At high RH's, evaporational cooling is small. Relative humidity plotted on an upper level chart can give a forecaster an idea of whether the air is rising or sinking. High relative humidities are associated with rising air or moisture advection (convergence) from upstream.

USELESSNESS: Relative humidity can not tell the following directly: The amount of moisture in the air, how uncomfortable it feels outside, and the "total" evaporation potential. The relative humidity without temperature information is nearly meaningless. The relative humidity says nothing about the actual amount of moisture in the air (dewpoint and mixing ratio give the forecaster a much better idea how much moisture is in the air). A relative humidity of 80% will feel uncomfortable at 95 degrees Fahrenheit but will not feel as uncomfortable at 70 degrees. Dewpoint is a better indicator of how uncomfortable it will feel. Dewpoints above 60 degrees are uncomfortable to some people and above 70 are uncomfortable to most people. Much more moisture can be evaporated into air with a RH of 60% and a temperature of 90 degrees than air with a RH of 60% and a temperature of 60 degrees. The relative humidity varies DRAMATICALLY with the time of day. On a typical day, relative humidity is high in the morning (since temperatures are cooler) and lower in the late afternoon (since temperatures are warmer). Relative humidity can vary significantly even when the amount of moisture in the air remains nearly constant.