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Most of us have heard a television meteorologist talk about the dew point and the relative humidity, yet few of us know what those quantities are. All air has some moisture in it, but how wet the air feels depends not just on the amount of moisture but also the temperature of the air. Imagine that you can put one cup of water in a roomful of air at 60 degrees Fahrenheit. If you cool the room down it will no longer be able to hold the same amount of water, so a fog may develop and some water may condense on the walls. We have all seen this on a cold winter's day in the bathroom when you run hot water in the shower or sink. The temperature at which fog or condensation occurs is the dew point temperature (because dew forms).

You can find the dew point temperature with a simple experiment. You will need

Fill the can ¾ full with room-temperature water. Place the thermometer in the water and measure its temperature. With the thermometer still in the water, slowly stir in some ice a little bit at a time. Do this slowly so you can watch the temperature fall on the thermometer. At some point water will start to condense on the outside of your can. The temperature at which this happens is the dew point temperature. Try it in a bathroom right after someone has taken a shower. Try it right next to a heating vent. You should be able to find different places and days when the dew point is very different. On a dry winter day it is possible that the dew point may be below freezing, and then this method of finding the dew point will not work because you can not make the water colder than its freezing point. However, it will still work in a humid bathroom.

Relative humidity is closely related to the dew point temperature. The relative humidity tells us how close the air is to being full of water (saturated). Remember that if the air temperature falls to the dew point the air cannot hold any more water (we get fog), so then the relative humidity is 100%. To find the relative humidity you need to know the dew point temperature and the air temperature. Look on the graph below and draw a straight line up from the air temperature and a straight line to the left from the dew point temperature. The place where these two lines meet gives the relative humidity. If the lines don't meet on one of the relative humidity lines estimate the value by guessing how far the meeting point is from the two nearest lines. For example if your meeting point is halfway between 50% and 60% then the relative humidity is about 55%. On the graph the temperatures are in °C. To get from °F to °C add 40 to your temperature in °F, then multiply by 5/9, and then subtract 40.

°C = ( °F + 40 ) 5/9 - 40

and the reverse:

°F = ( °C + 40 ) 9/5 - 40

The graph is from The Science and Wonders of the Atmosphere
by Stanley David Gedzelman, published by John Wiley & Sons.

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