Friday, October 5, 2007

More About Dew

A CoCoRaHS observer left me a comment yesterday asking if dew contributes much to the water table over the long run? And Does it decrease evaporation so that water already IN the soil is not touched?

The answers are both no.

Dew is not precipitation -- meaning it doesn't fall from the clouds. It is a process that happens at the surface. What you are seeing is the process of a gas (water vapor) condensing into a liquid (dew or if it is cold enough -- frost)

Dew forms on surfaces that do not receive heat conducted from the soils of the earth. (roof tops, car tops, grass, plants, etc.)

SO while it does benefit the surfaces it forms on (meaning it is a brief little drink of water for plants and grass) -- the key words are brief and little.

The dew or frost forms during the coldest part of the night (just prior to and at sunrise) and goes away almost as fast as it forms. As soon as the sun gets above the horizon and begins to warm the air temperature a bit, dew and frost will go back into a gas (water vapor).

So it is a fast and short-lived process, and there really isn't enough of it to make a difference in the water table, and because it is just on the surface, it comes and goes so fast that it doesn't really slow down the normal processes of evaporation.

Now having said that, if you are seeing a lot of frost and dew -- you are probably in a climate that is somewhat moist overall -- so evaporation is going to be slower than in a drier climate.

Let me explain...

As a kid growing up in Arkansas, dew or frost seemed to be a part of almost every single day. And it would stay on the ground through a good chunk of the morning -- say until 10 am. But Arkansas is a very moist climate overall.

Here in Colorado, we don't see dew or frost all that often. And when we do, it evaporates within minutes of the sun rising. We are a very arid climate overall.

It all ties back to the climate. The less moisture (or water vapor) in the air, the less the dew or frost potential will be.

You can gauge how much water vapor is in the air by either looking at the humidity or dewpoint value on your local weather report.

If that isn't available, something else you can look at for a gauge is simply the daily high and low for a location.

The greater the difference in your temperatures between day and night -- the drier the air. The closer the daily high and low temperature are, the more moist the air is.

Dry air cools and warms much much faster than moist air.

That is why places here in the west have such a large spread in daily temperature. That is called the diurnal range. Denver often sees a daily temperature spread of 35 to 50 degrees. Yesterday our high was 83 and the low was 39. That is a 44 degree spread. A great example of our very dry climate.

Back east -- let's say Nashville, Tennessee -- their high was 85 and the low was 71. A difference of only 14 degrees. A great example of a humid and wetter climate.

With nothing more than a daily high and low temperature for a city, you can tell a lot about the weather and climate. The closer the high and low, the more moisture there is. There was probably morning dew and filtered sunshine throughout the day due to passing clouds in Nashville yesterday. Maybe even some patchy overnight fog!

In Denver, just looking at that temperature spread -- you can pretty much bet there were clear skies, maybe a few high passing clouds, and plenty of sun! It was darn dry.


  1. Chris,
    I have observed open sky overhead with mostly cloudy in one direction and partly cloudy in the other direction. There is a definite division. I have felt this is a high pressure area overhead with different developing cloud covers. Possibly due to jet stream or wind. I am only thinking in layman terms.

  2. Great post Chris....very informative!