25th December 2005

Dew

Source: Excerpt from The Book " Weather "

Distribution: Worldwide, most common in coastal and tropical areas. Height: On ground and on surface of grass, leaves and other objects. Cause: Condensation occuring in a thin layer of air immediately above the ground. Associated Weather: None Often, after a cold, cloud free night, we wake to find the ground and other surfaces wet and glistening in the sunlight. The cause of this is a form of condensation called dew. This occurs when the temperature of the ground, or any other surface, drops low enough to cause condensation in the air immediately above it. This results in the formation of water droplets on the surface. Virtually the same process can give rise to Fog, and it is often difficult to predict when dew and fog will occur together. It is possible to have dew without fog, but it is not possible to have fog without dew. The ideal conditions for dew are a still, clear night, high humidity in the air next to the ground, and low humidity in the air above. The absence of cloud allows the ground to radiate much of the heat it has absorbed during the day and cool sufficiently for condensation to occur. The layer of moist air at ground level ensures that condensation will take place only on, or near, the surface of the ground. With fog, a deeper layer of moist air is required. In the case of dew, liquid forms because water droplets merge more readily on solid surfaces. In the air, the droplets tend to bounce off each other. Dew is often associated with cold environments, but it also occurs in hot and humid regions. In desert areas, dew formation is a vital source of water for many plants and animals.

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Acknowledgement due: John W. Zillman, William J. Burroughs,
Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot and Richard Whitaker.