For years relative humidity has been what we use to inform the TV audience, but it has many pit falls and to tell you the truth almost no one understands it, including many meteorologists. When thinking of humidity you always have to think in terms of energy. Of course this is expressed as temperature. There is only so much energy to go around and only a certain part of the thermal energy can go to do the work of evaporation. The rest goes to the other molecules in the air.
Relative humidity expresses how much of the available energy has been used.
Because the amount of energy increased as the sun warmed the atmosphere the percentage of the energy used decreased, i.e. the relative humidity, all the while there was no change in the amount of vapor in the air. Because the concept is confusing many meteorologists are trying to get use dew point temperature. The Dew Point Temperature Dew point temperature is a measure of humidity. If you take a parcel of air, and cool it, eventually you will remove enough energy to begin to get water vapor to condense. Remember the water vapor was originally liquid water and to get it to evaporate you had to add energy.