Where does dew come from? (Dew Point, Condensation)

Summary:When the air becomes saturated with water vapor and the temperature drops below a certain point, condensation will form. But, what is that point?

I still remember a lab experiment I did in eighth grade science class. To demonstrate the concept of dew point, we each paired up and were given a stainless steel cup, some water, some ice, a thermometer, and a small spoon. While one of us added tiny bits of ice to the water in the cup, the other recorded the temperature. This continued until moisture formed on the outside of the cup at which point – voila! – dew point!
Interesting enough, but I have to admit, apart from the fun-factor of it all the actual point of the experiment was kind of wasted on my 13 year old brain. But these late spring storms have had me thinking more and more about rain, condensation, humidity, and of course, dew point.

Most of us associate dew with the moisture we encounter on our cars or lawns on early spring mornings. This forms in exactly the same manner that it did in the stainless steel cup experiment of yore, but how does that translate into real-life meteorology for you and me? Clearly, my geeky little brain had some research to do.

Technically speaking, the glossary of the American Meteorological Association defines dew point (also known as “dewpoint temperature”) as “The temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur. When this temperature is below 0C (or freezing), it is sometimes called the frost point [other scientific stuff deleted].” So in layman’s terms, when the air becomes saturated with water vapor and the temperature drops below a certain point – the dew point – condensation will form. If it’s warm it the condensation will be in liquid form and if it’s cold it will frost (in which case it’s referred to as the “frost point”). If the individual condensation droplets are large enough then they will form dew on say, blades or grass or the hood of your car. If the condensation droplets are tiny, they will evident themselves as mist or fog.

So what can dew point tell us about precipitation? Well, in order for condensation to form the temperature of the air must be at or below the dew point, but that doesn’t necessarily mean rain. The average size of a cloud droplet is tiny (0.02mm) in comparison to the size of a rain droplet (2mm), so in order to get rain droplets to form something else needs to become part of the process. In order for it to rain (or snow) the air temperature needs to be at or below dew point and condensation nuclei – tiny particles around which droplets can form – need to be present. Condensation nuclei can be lots of things: dust, ash from erupted volcanoes, factory smoke, smoke from forest fires, or ocean salt – just like an oyster needs a grain of sand around which to form a pearl, a rain droplet needs to form around condensation nuclei.

If you and your kids (or just you – we won’t tell) are interested in recreating the dew point experiment from my junior high days check out: http://www.scholastic.com/schoolage/activities/3up/dewpoint.htm where you’ll find a list of everything you’ll need and directions on how to find out the dew point wherever you perform your experiment in about five minutes.


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