Slinging the Wx
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Relative humidity, or RH, is a measure of the water in the air. At one hundred percent humidity the air is completely saturated and fog forms. But, at a lower RH this moisture is invisible.
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We can actually measure that invisible moisture with an instrument called a sling psychrometer. It sounds complicated, but it's actually very simple. Just two thermometers tied together. One of the thermometers has a piece of wet cotton tied around the bulb. That's called the wet bulb thermometer. The other one has no cotton and we call that one, not surprisingly, the dry bulb thermometer. Then we go outside and sling them around in the shade.
The dry bulb gives us the air temperature, but something else happens to the wet bulb. Try this, put the back of your hand in front of your face. Now, lick it. Now, blow on it. It should feel colder because evaporation of the water takes heat to energize water molecules from liquid into gas. So, the wet bulb thermometer gives us a colder reading than the dry and the difference between the two is called the depression. With a psychrometric calculator we figure out the relative humidity as well as dew point. Both of which can tell us how much invisible water vapor is hanging out in the air.
To make your own psychometer, just take a thermometer with a bulb and find some clean cotton like a gauze pad to wrap around it and dip it in water. Then go have a sling.
For pictures of psychrometers and where to get humidity tables sign on to our website at www.weathernotebook.org. The Weather Notebook is a production of the Mount Washington Observatory. It's funded by Subaru of America and The National Science Foundation. Related Links
Make a sling psychrometer
http://www.miamisci.org/hurricane/psychrometer.html
Reference chart
http://asd-www.larc.nasa.gov/SCOOL/psychrometer.html