Dew and Frost
Condensation takes place when the air is cooled below its dew point. In nature, the formation of dew, or frost if the temperature is low enough, occurs usually when the air is cooled by contact with objects on the earth which in turn have been cooled by radiational heat loss at night.
You can cause the formation of dew and frost by filling a small Styrofoam cup with a slush made of crushed ice, water, and a tablespoon of salt. Make the slush up a few minutes before you start taking data.
Tape a thermometer bulb to the outside of the tin can to obtain a rough measure of the dew point temperature. Fill the can with the slush, and read the temperature immediately as the dew, tiny water droplets, form on the outside of the can. If the temperature of the environment is below roughly 35 degrees Fahrenheit, frost crystals may form as they do on the freezer compartment of a refrigerator.
Empty the can being careful not to wipe the dew off and read the temperature again, as soon as the dew disappears. The average between the two temperature readings is the approximate dew point.
Shiny can (small)
Tablespoon of salt.