Relative humidity was of concern during the recent heatwave. The definition of relative humidity is the amount of water vapor present relative to the amount needed to reach the saturation point in the air. However, that amount of water varies, depending on the temperature. A relative humidity of 70 percent requires a lot more water vapor when the temperature is hot than when it’s cold. That matters because very high or very low humidity values make people uncomfortable. And, we change the relative humidity with our heating and cooling systems. In winter, we take in cold air and heat it up, thus lowering its relative humidity. If moisture isn’t returned with humidifiers, the drier air sets up static electricity, dry skin and coughs. In contrast, during the summer a common method of increasing comfort is to reduce the relative humidity using dehumidifiers.
Mary Knapp, Weather Data Library