You would imagine that dew is a very simple phenomenon of nature, easily understood and explained. Yet strangely enough, exactly what dew is have long been misunderstood, and whole books have been written on the subject!
Since the days of Aristotle until about 200 years ago, it was believed that dew “fell,” somewhat like rain. But dew doesn’t fall at all! The most familiar form of dew, seen on the leaves of plants, is now known not to be all dew! So you see, there have been many wrong ideas about dew.
In order to understand what dew is, we have to understand something about the air around us. All air holds a certain amount of moisture. Warm air can hold much more water vapor than cold air. When the air comes in contact with a cool surface, some of that air becomes condensed and thee moisture in it is deposited on the surface in tiny drops. This is dew.
The temperature of the cool surface, however, has to drop below a certain point before dew will form. That point is called “the dew point.” For example, if you place water in a grass or a polished metal container, dew may not collect on the surface. If you place some ice in the water, dew may still not collect until the surface of the glass or container is brought down to a certain point.
How does dew form in nature? First, there has to be moisture-laden warm air. This air must come into contact with a cool surface. Dew doesn’t form on the ground or sidewalk, because it still remains warm after having been heated by the sun. But it may form on grasses or plants which have become cool.
Then why did we say that the dew seen on plants is really not dew? The reason is that while a small part of the moisture seen on plants in the morning is dew, most of it-and in some cases all of it- has really come from the plant itself! The moisture comes out through the pores of the leaves. It is a continuation of the plant’s irrigation process for supplying the leaves with water from the soil. The action starts in the daytime, so that the surface of the leaf should be able to withstand the hot sun, and it simply continues into the night.
In some places in the world, enough dew is deposited every night for it to be collected in dew ponds and used as a water supply for cattle!
