NEWTON:Dew Point Apparatus
 
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Name: Michelle
Status: educator
Grade: 6-8
Location: NC
Country: USA
Date: Winter 2013-14


Question:
I would like to build a dew point station at our middle school so the children can see it on the way to breakfast in the morning. I have researched this and it seems no one has one -yet. I know I need a thermometer on it and some information about dew point, humidity and saturation. I would like to ask you what different types of materials should I include on the station to attract dew or will just one make the point?


Replies:
Hi Michelle,

Directly measuring the dew point actually isn't an easy thing to do. There are devices built to do this (e.g., a chilled mirror hygrometer) but they are expensive and aren't good teaching tools for kids.

The device you seem to be describing would seem to be at environmental temperature, so the best it could do is tell you whether you are above or below the dew point temperature (based on observation of condensation), but it won't tell you what that temperature actually is.

Mathematically, the easiest way to measure dew point is to just measure the relative humidity (RH) and temperature, which together determine the dew point temperature through a mathematical equation. But again, this isn't a very exciting experiment to do.

There is a good experiment that you can do, however. But it is hands-on and seems to be different than what you are envisioning. This is basically a simplified version of the chilled mirror hygrometer, and is called a dew point hygrometer. Here's what you do:

1) Find a metal container with a clean, shiny surface. You want a metal container because it has high heat conductance. A 12 oz soda can might work, if you cut the top off. Glass is a second choice. If you use glass, look for the thinnest glass you can find.

2) Fill the container with room temperature water. Put a thermometer in the can.

3) Add ice to the water and gently stir. The temperature will drop. As it does, keep a close eye on the surface of the container for condensation.

4) At some point you'll start seeing condensation form. When you do, glance at the thermometer -- this will be your dew point temperature, approximately.

This works only with dew point temperature above the freezing point of water, obviously :) But if you add salt to the water you may be able to get the temperature below 32 F.

I hope this helps. Regards, John C Strong


Unless you are REALLY interested in the “nuts and bolts” of construction and conversion of relative humidity to dew point, there are some pretty inexpensive desktop weather stations that can do the job for you. The conversion is already programmed into the device. Here are the issues: First, Relative Humidity is the amount of water vapor in the air at a GIVEN temperature (expressed as a pressure) relative to the vapor pressure of water at THAT GIVEN TEMPERATURE. Despite the common misconception, it has NOTHING to do with “how much water air will hold”. Air has nothing to do with it except for a tiny correction for the solubility of atmospheric gases in water – which itself is a function of temperature. In contrast the Dew Point is the temperature of air, containing a given amount of water vapor, must be reduced so that amount of water vapor can condense into liquid water. Or put another way, what is the reduced temperature that the Relative Humidity reaches 100%. These require a number of conversions: The vapor pressure of water as a function of temperature, conversions of degrees Kelvin to degrees F., and a couple of other things. These conversions are generally tabulated into tables so that they do not have to be calculated every time.

Relative Humidity and Dew Point are not measuring exactly the same thing, but the critical parameter is the vapor pressure of water. My advice? Invest in an inexpensive desktop weather station!

Vince Calder


Michelle,

One material is sufficient to show dew or frost.

To "attract" dew or frost and make the dew and frost easier to see, I would use a standard glass mirror, like the kind that ladies use for putting on their makeup in the bathroom. The mirror needs to point straight upward and it's a good idea to clean it frequently, using just clean tap water. You should locate the mirror about 3 feet above the ground so that the ground doesn't warm the mirror and so that air is free to flow around it.

David R. Cook Meteorologist / Team Lead Atmospheric and Climate Research Program Environmental Science Division


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