Which is worse, hot or humid? Summers combine the two in the worst way. Humidity as reported by a weather forecaster refers to a percent ratio between the amount of water vapor in the atmosphere and the highest possible vapor concentration at that temperature. If the atmosphere is saturated then the relative humidity is 100 percent.
At saturation a dynamic equilibrium exists between the rate of condensation and the rate of evaporation. For a liquid to evaporate, its molecules must vibrate with enough force to break the bonds holding them together. These vibrations can't happen without energy, so a liquid provided with more energy (in the form of heat) evaporates more quickly.
Rate of evaporation depends on both the heat available to the liquid and the strength of the intermolecular forces between the molecules. Evaporation can be faster on sunny days not just because of the extra heat, but also because sunny days are often drier and so have lower relative humidity.