In the first part of the experiment we took two of the glasses and observed one with ice water and one with water. Dew formed on the glass with ice in it.
Many folks mistakenly believe that dew falls from the sky, but this is not so. But, dew does reflect what is going on in the atmosphere. Dew forms when moist air comes into contact with cool surfaces. That's why dew only formed on the glass with the ice water.
Dew forms on the ground outside when during the night the surface of the earth cools. The air just above the ground also cools. Cool air can't hold as much moisture as warmer air. When the air directly over the ground cannot hold any more moisture, the droplets of dew condense onto blades of grass, or in our expirement, the outside of the glass of ice water.
Dew will form on some surfaces faster than others and rarely on others. When we compared different substances, we found that dew formed rather quickly on metal and paper but not so quickly if at all on the styrofoam cup. Can you guess why?
Some substances transfer heat or cold faster that others. Styrofoam holds temperature well, which is why it's so popular for hot drinks! Dew will not form until enough cold from the ice is transferred to the air around it.
Metal on the other hand, easily allows transfer of heat or cold. The air around the metal can is cooled quickly and dew forms easily on the outside of the can. This is why you will notice dew on cars before almost anywhere else.
In the last experiment we actually did two things. First we measured the temperature at which the dew formed. This temperature is called dew point. This is when the air over a cooled surface becomes saturated (cannot hold any more moisture) and begins to condensate on the surface.
When dew formed on one glass with ice water, we observed the other glass with ice water in front of the fan. This glass had little or no dew. Can you figure out why? Wind mixes the air around the glass. This greatly slows down the air around the glass from becoming cool enough for dew to form. That is why dew forms best on nights when there is little or no wind.
Copyright © 2002 Kathy A. Miles and Charles F. Peters II