Humidity and Dew Point


Humidity refers to the amount of moisture in the air. You might be surprised to know that the air in our atmosphere has water in it. Lots of water!

Water in the atmosphere exists in 3 main states.


Air near the ground or ocean usually has more water in it than the colder air up high.

Measuring the Amount of Water Vapor in the Air:

There are several different ways to measure and to express the amount of water in the air. Lets imagine we have a blob of air with moisture in it. The temperature of our blob of air is 80 degrees. So, how do we tell how much moisture is in it ????

Relative Humidity:

The relative humidity tells how much water the air is holding compared to how much it could hold at a certain temperature. If our blob of air has a relative humidity of 50% then that means it is holding half of the amount of water a blob of air 80 degrees could hold. The relative humidity can change if the moisture changes or if the temperature changes.

Dew Point:

The dew point is a much better indicator of moisture in the air and is preferred by most meteorologists.

What is the Dew Point?

The dew point is the temperature at which the air will be holding all the moisture it can if cooled. Or...another way of putting it. The dew point is the temperature at which the relative humidity reaches 100%.


Lets talk about our blob of 80 degree air. If we start cooling our blob of 80 degree will eventually reach a temperature at which it can no longer hold the water vapor in it. Lets say that in this case our blob of air forms a cloud when we cool it to 50 degrees. Then 50 degrees in the dew point of our blob of air! Note that the dew point does not depend on the temperature like Relative Humidity!


List of Materials:

  • a thin metal cup (plastic will do in a not use styrofoam
  • a good thermometer
  • ice water
  • a large dropper

Do the following:

Outside on a warm day in the spring or summer. If the dew point is below 32 degrees you will have to use antifreeze or another liquid that has been chilled to near zero in a freezer. Be careful! Antifreeze is poisonous..only use it with an adult nearby.

Put a couple of inches of warm (80 F) water in a cup. Measure the temperature of the water, now add a few droppers of cold water and stir while measuring the temperature of the water. Keep adding a small amount of cold water to the metal cup until you see a thin film of water (water vapor)form on the outside of the cup. Make a note of the temperature of the water as soon as you see the water vapor form on the outside of the cup. The temperature you measured is the DEW POINT!

What Happened?

As you added colder water to the warm water in the cup, the temperature of the water in the cup kept dropping. The sides of the cup got colder as the water inside got colder. The cup also chilled the air right next to it! When the temperature of the metal cup reached the dew point... The air just next to the cup was cooled by the dew point as well and water began to fall out of the air and stick to the cup!

Special Note:

If you live in the western USA or at a high elevation, the dew point may be less than 32 degrees F, and you will see no condensation unless you put a very cold liquid, like salt water or antifreeze that is chilled to near zero in the cup.


This experiment was taken from Dan's Wild Wild Weather Page located at is the U.S. government's official web portal to all federal, state and local government web resources and services.