Using dew-point temperature as an index of moisture overcomes most of the difficulties of using relative humidity, because dew-point temperature does not change as much through the course of a “normal” day as relative humidity does. Dew-point depression is used in some agricultural and forestry applications and tells us how much temperatures might drop overnight, since air temperatures at night generally do not cool much below the afternoon's dew point.Ħ. Thus, large dew-point depressions suggest dry air and small dew-point depressions suggest humid air so that only a minor drop in temperature will cause saturation. A large dew-point depression indicates that a large temperature drop is required to reach saturation. The difference between current air temperature and the dew-point temperature is known as the dew-point depression.

In contrast, air that is very dry (like the 5% to 10% relative humidities common in the Mojave Desert in California) has to cool significantly to reach its dew point, and on most nights does not.ĥ. If air is very humid, for example with a relative humidity of 80% to 90%, it may need to cool only a few degrees to reach its dew point. To generalize, humid air at a given temperature has a higher dew-point temperature than does less humid air at the same temperature. Air mass 2 has less water vapor than air mass 1, and as a result has a lower dew-point temperature (T2).Ĥ. In order to reach saturation (at point S2), however, it has to cool more than air mass 1. If the air cools, as indicated by the green arrows pointing to the left, it also approaches the saturation curve. A second air mass, at position 2, starts at the same temperature (T0) but has less water vapor in it (lower vapor pressure) than does air mass 1. The temperature at which saturation occurs is the dew-point temperature and can be read from the graph by drawing a vertical line down from S1 to the temperature axis (T1).ģ. It cools until it reaches saturation at position S1, at which point the air has a relative humidity of 100%. As this air cools, its position on the graph moves horizontally to the left (temperature is changing but vapor pressure is not), as indicated by the reddish arrows. We begin with an air mass at a certain temperature (T0) and vapor pressure, marked as position 1, well within the unsaturated field of the graph. The curved blue line marks conditions where air is saturated in water vapor it is labeled as es, where the s indicates saturation.Ģ. To better visualize why cooling can cause the formation of dew, we return to a familiar graph, one that plots vapor pressure (e) versus temperature. As we know from our experience with cans or glasses of an icy beverage, drops of dew can also form on the outside of the container, especially if the surrounding air has high humidity. As a result, the water begins to condense as drops on the inside of the container, forming dew. At some temperature, the cooled air reaches saturation and can no longer allow so much vapor to exist in it. Recall that cold air has lower water-vapor capacity than warm air, so as the air cools it moves closer to saturation. If we cool that same container with ice cubes, the temperature of the water and air will decrease. The container is warm enough that all the available water vapor can exist in the air without condensing - that is, the air is unsaturated. It contains liquid water and water vapor (represented as blue dots). To explore the formation of dew, examine the sealed glass container on the left. In any case, drops of dew or crystals of ice disappear rapidly once the Sun rises and air temperatures begin to climb. If temperatures are at or below freezing, earlier formed drops of dew can freeze or ice can form directly from the air ( deposition). It typically forms at night, in response to the air cooling. How Is the Dew Point Expressed?ĭew is expressed as drops of liquid water that condense out of the atmosphere and onto plants, rocks, walls, or any solid surface. When you wake in the morning and everything outside is covered with moisture (dew), it signifies very humid air and that night-time temperatures cooled to the dew point. If the air temperature is at the dew point, the air is so saturated with water vapor that vapor begins to condense as drops of liquid water, such as drops in clouds or rain, or as water drops (dew) on solid surfaces. ANOTHER USEFUL MEASURE OF HUMIDITY is the dew point, the temperature to which a volume of air must be cooled to become saturated with water vapor.

Dew point map