Wet-Bulb Effect



When water evaporates, it cools. Thinking about the cooling at the molecular level, when water molecules evaporate, the faster ones, which can overcome the Van Der Waal's forces between the molecules, escape the drop or flake leaving the slower ones behind. So, both the total kinetic energy of the molecules, the heat content, and the average kinetic energy, the temperature are reduced. Nowhere in meteorology is this effect as important as in the study of humidity interacting with human bodies, cloud droplets, raindrops and snowflakes. The wet bulb temperature is the easiest way of illustrating the idea.

Measure the temperature of the air in the room.

Take a small piece of thin cotton cloth a piece of white cotton shoelace will do. Feel its sensible temperature. Measure its temperature by placing it on a thermometer bulb. Record its temperature. You may have to carefully cut away part of the backing so it is easy to slide it on. Now ventilate it with the wind, a fan or waving it back and forth. The temperature shouldn't change just because of the ventilation unless the air which is blowing over it has a lower temperature or the thermometer is in direct sunlight.

Then dip the thermometer with the piece of shoelace on it into lukewarm water, hold it into the wind or air stream from a fan, or wave it back and forth a few times, and feel it again. It should feel cooler. Read the thermometer again. The temperature should have dropped significantly. If you are using a fan, such as a muffin fan, continue until the water has evaporated and read the temperature again.

If you are using a fan, as preparation, it is a good idea to read the temperature before wetting, wet the bulb, read the temperature and then graph the temperature each 30 seconds or each minute until the bulb is dry. Since graphing is generally a learning objective, you can have the class do graphs of the temperature versus time. As the bulb dries out, the temperature should move asymptotically back towards room temperature.



Material: One thermometer

Small piece of cotton shoelace

Piece of cardboard or electric fan such as a muffin fan (Muffin fans can be easily

obtained from discarded, broken or surplus computers. Save the power supply as well as the fans run on 12 volts DC.)

Water, room temperature