Humidity is a measure of the percentage of water vapor in the air. “Dry air” will have a very low humidity value. Humidity is caused by evaporation of water in nearby bodies of water (lakes, rivers, oceans, etc.) which, in turn, is affected by the temperature. Warmer weather causes a higher rate of evaporation and thereby a higher humidity. Relative humidity is the percentage of humidity of a certain day based on the total possible. For example, if the total amount of water vapor possible is 20 and the measurement for the day is 10, then the relative humidity is 50%. Humidity is usually higher on the coast.