1. How do we measure relative humidity?
Lesson 21
How do we measure relative humidity?

2. Have you ever tried to air dry clothes on a humid day?
It took a long time, didn’t it?

3. The air already contained many water molecules, so the water from the clothes evaporated very slowly.
Evaporation means to change to a gas.

4. How fast evaporation takes place depends on the humidity.
When the water does evaporate something happens. The surface that had been wet becomes cooler.

5. For example, when the sweat on your body evaporates, your skin cools down.

6. Now you know two important facts about evaporation.
-Water evaporates faster when the air is dry.
-When water evaporates, the temperature goes down.

7. These facts make it possible to measure relative humidity.
Relative humidity is measured with a hygrometer.
One kind of hygrometer is the wet and dry bulb thermometer.

8. The wet and dry bulb thermometer has two thermometers.
One of the thermometers has a wet cloth around the bulb. This is the wet bulb.

9. The thermometer without the wet cloth is the dry bulb.

10. -the dry bulb measures the normal air temperature.
-the temperature of the web bulb thermometer depends on how fast the water evaporates.

11. When the air is saturated, no evaporation takes place.
The temperature of the wet bulb thermometer is the same as the dry bulb thermometer

12. When air is not saturated, evaporation does take place.
The wet bulb cools down, and it has a lower temperature than the dry bulb.

13. If the wet bulb has a much lower temperature than the dry bulb, it means the air is not very humid.

14. To figure out the relative humidity of the air, you need to know two things:
-The dry bulb temperature
-The difference between the wet and dry bulb temperatures.

15. Once you know these two temperatures, you use a chart to find the relative humidity.
Relative humidity is stated in a percent.

17. Let’s try one.
The dry bulb temperature is 28°, and the wet bulb is 24°.
What is the difference between the two temperatures?

18. So, on the left side of the chart, we find the dry bulb temperature of 28°.
Along the top, we find the difference of 4.
Like a multiplication chart, we follow the two lines until they meet.

19. What is the relative humidity?

20. Please remember that scientists always measure in the metric system, and American weather people measure in the English system.

21. When you hear the relative humidity on TV, it was always found using degrees Fahrenheit.
Scientists will always measure in degrees Celsius.