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.
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