The capacity of air to hold water vapor is based on vapor pressure of water. Conversely, if you wish to dry something (perhaps your hair), it is more effective to blow hot air over it rather than cold air, because, among other things, hot air can hold more water vapor. At the dew point temperature, relative humidity is 100%, and fog may result from the condensation of water droplets if they are small enough to stay in suspension. For example, relative humidity rises in the evening, as air temperature declines, sometimes reaching the dew point. The amount of water vapor the air can hold depends on its temperature. At its maximum, denoted as saturation, the relative humidity is 100%, and evaporation is inhibited. Relative humidity tells us how much water vapor is in the air compared with the maximum possible. When we say humidity, we really mean relative humidity. Low humidity, on the other hand, can cause discomfort from excessive drying of mucous membranes and can lead to an increased risk of respiratory infections. Because evaporation is inhibited by high humidity, we feel hotter at a given temperature when the humidity is high. We keep cool in hot weather by evaporating sweat from our skin and water from our breathing passages. The expression “it’s not the heat, it’s the humidity” makes a valid point. At the dew point, the air can no longer hold all of the water vapor it held at higher temperatures, and some of the water condenses to form droplets. Dew drops like these, on a banana leaf photographed just after sunrise, form when the air temperature drops to or below the dew point.