Home Meteorology

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.
What you need:
• A large metal can or pot
• A thermometer that can read temperatures down to 35° or 40° Fahrenheit
• Some ice
Try this:

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1. Fill the can 3⁄4 full with room temperature water.
2. Place the thermometer in the water and measure its temperature.
3. 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.
4. 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.
5. 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.
What’s going on?
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 on the next page 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