1. Comparing the Humidity and Dew Point of two different locations
Relative Humidity Lab
Comparing the
Humidity and Dew Point of two different locations

2. Measuring Relative Humidity
2 thermometers—note that they have the same starting temperature.
One thermometer is wrapped with a wet cloth at room temperature.
Fan them gently for 2 minutes.
Record the ending temperatures.

3. Why will this show anything about relative humidity?
Water on the cloth will evaporate.
Evaporation means that hydrogen bonds must be broken.
This requires energy. Where does the energy come from?
The thermometer and its surrounding air.
The amount of temperature decrease will be proportional to the rate of evaporation.
The rate of evaporation will depend on how close the air is to its capacity.

4. Why fan the thermometers?

5. Wet Bulb Thermometer in DRY Air

6. Wet Bulb Thermometer in DRY Air

7. Wet Bulb Thermometer in DRY Air—but NOT Fanned
If the air were not fanned, humidity would build up in the air around the wet bulb thermometer. This would cause the rate of evaporation to slow down, thus suggesting that the room has more moisture than it actually does.

8. How Does Wind Change Things?

9. How Does Wind Change Things?
--Now the thermometer is surrounded by dry air again—thus evaporation will continue at a faster rate.

10. What weather phenomenon does this explain?
Wind Chill—Air can ‘feel’ colder than the temperature since wind speeds evaporation which is a cooling process AND it carries away air that was warmed by your body temperature, and brings in cooler air.

11. What happens to a wet bulb when in a room with 100% RH?
No net evaporation can occur since the air is already saturated—thus there would be no cooling of the wet bulb.

12. Does High Relative Humidity always mean there is a large amount of moisture in air?NO
Since cold air has a low capacity, it doesn’t take much water vapor for it to become saturated.

13. How do you read the wet/dry bulb chart?
Suppose the thermometers both start at 26°C, but the wet bulb thermometer drops to 21°C after being fanned for 2 minutes. What is the relative humidity? Answer = 64%