Doing the Dew 






Does Dew really fall?

Why does dew form mostly on calm, clear nights?

Why might a car be covered with dew while a concrete driveway have none at all.

And, is frost merely frozen dew?


Dew and frost are minor weather events in our lives and often we hardly notice

them. In our busy lives we tend to only notice weather events which affect our

travel, making roads slippery, getting us wet or causing school closing. It is the

country dwelling folk who are more likely to comment on dew and frost.


You've seen dew many times, even if you live in a high rise apartment building. Just take a can of soda out of the refrigerator and you'll see dew start to form in minutes. It's really quite easy to "do the dew." Nature has its on way of producing dew though.


There is always some amount of water vapor in the air even though we cannot

see it. The amount of water vapor the air can hold depends mainly on the

temperature of the air. Warm air can hold more water vapor than cold air. Air at 80 degrees F can hold about 22 times more water than air at 0 degrees F. The air is saturated when it cannot hold additional water. As the Sun sets, the air begins to cool. If it cools enough, it will reach dew point. Dew point is the

temperature to which the air must cool to in order to be saturated.


During the day, the Sun heats up the ground and objects such as cars and

buildings. After nightfall, the objects begin to cool. As the objects cool, they chill the air just above them and on calm, clear nights, can make the air above them reach dew point. When this happens, dew will condense out of the air onto surfaces such as grass, plants and other objects.


So why does dew form most often on clear windless nights? Winds move the

layers of air and disperse the cooling effect. Cloud cover tend to keep the heat

from escaping and so the temperature does not fall as much and dew point is

less likely to be reached.


.Now what about the scenario where a car is covered with dew while the

driveway has none? Some surfaces cool faster than others. Some objects hold

heat well enough they may not get below dew point temperature. Metal loses

heat quickly and this is why a car might be covered with dew within an hour

after sunset while the concrete driveway it sits on might be dry throughout the

night. Grass and plants are usually the first to collect dew because they lose

water vapor themselves and so the air surrounding the plant becomes saturated more quickly than air a few feet above it.


Contrary to popular belief, frost is not frozen dew. White frost, also called hoar

frost, forms when the dew point of the air is below freezing. The frost forms

when water changes directly from a gaseous to a solid state without passing

through the liquid state. This process is called deposition. Frost will often create delicate lacy patterns on windows in late fall.


And finally, the answer to "does dew fall?" Dew and frost are not forms of

precipitation because they do not fall from clouds. Rather, they develop in place

on exposed surfaces.


Though we usually only think about frost this time of year, dew is actually far

more common. Each time you come in from the cold and your eyeglasses fog

up, dew has formed, but not fallen!


Copyright © 2000 Kathy A. Miles and Charles F. Peters II