Frost develops on nights with clear skies, light wind, moist soil, and a low dew point depression.
The dew point is the temperature the environment needs to be for relative humidity to reach 100%. It is impossible for the temperature to be lower than the dew point. The air can only hold so much much moisture in the gas form known as water vapor.
Dew can form when the temperature equals the dew point. This is easier to achieve at night because the earth emits longwave radiation and the earth’s surface cools. If there are no clouds the radiation from the earth can escape from space. If there are clouds the radiation stays in the atmosphere and nightly temperatures will be bit warmer making it more difficult for the temperature to reach the dew point.
If the dew point is below 32 degrees it becomes the frost point. The air temperature is typically taken about six feet above the ground. Due to radiational cooling at the surface, air temperature can be above freezing, but the temperature right at the surface can drop below freezing allowing frost to form.
QUESTION: How can we get frost to form on a can?
HYPOTHESIS: If I get the temperature of the can to drop below freezing, then with a little water vapor I can get frost to form.
MATERIALS: can, ice, and salt (spoon optional)
EXPERIMENT: Empty and clean a can. I used a green bean can I had after dinner last night.
Add ice to fill the can at least half way up.
You can wait and see what happens here.
OBSERVATION: If you wait long enough you will notice a little condensation on the outside. You just created dew.
EXPERIMENT CONTINUE: Add salt to the ice and mix it up a little.
Watch what happens.
OBSERVATION 2: Frost forms on the outside of the can.
NEW QUESTION: Why did the salt help create frost?
CONCLUSION: In the winter cities use salt to lowers the melting point. This allows the snow and ice to melt even when the air temperature is below freezing.
Without the salt the air temperature around the can is not below freezing. When we lower the melting point of the ice, this allows the air around the can to drop below freezing. Frost can only form if the air temperature is below freezing.