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
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.
and the reverse: