1. Chapter 12 Section 3 How is relative humidity measured?
Objective:
Explain relative humidity and how it is measured.

2. Key Terms
relative humidity: amount of water vapor in the air compared with the amount of water vapor the air can hold at capacity
psychrometer: instrument used to find relative humidity

3. Relative Humidity
The relative humidity of air that is saturated, or filled to its capacity, is 100 percent.
The relative humidity of air changes:
as water vapor leaves the air and returns to the ocean or falls as precipitation or
when water evaporates and goes into the air.

4. Relative Humidity
If the amount of water vapor stays the same and the temperature drops, the relative humidity goes up.
Relative humidity goes down if the temperature goes up.

5. CALCULATE:
If air is filled to one-third its capacity, what is the relative humidity?

6. Measuring Relative Humidity
Simple psychrometer:
Made up of two thermometers
Used to find relative humidity
The bulb of one thermometer is covered with a damp piece of cloth. The other bulb is dry. Air is then passed over the psychrometer. This causes water on the cloth to evaporate. The dry thermometer measures air temperature. Water evaporating from the cloth cools the wet thermometer. The wet thermometer is cooled more when the relative humidity is low than when it is high. Relative humidity can be found by using the difference in temperature between dry- and wet-bulb thermometers and a chart .

7. Measuring Relative Humidity
Try it out:
Suppose the dry-bulb thermometer reads 25℃ and the wet one reads 18℃.

8. Measuring Relative Humidity
Another type of psychrometer is the sling psychrometer.
Can measure both relative humidity and dew point

9. 2. DEFINE:
What is a psychrometer?

10. Humidity and Comfort
As we perspire, moisture evaporates from our skin.
This cools us
The faster the moisture evaporates, the cooler we feel.
When the humidity is low, the rate of evaporation is faster.
This makes us feel cooler

11. 3. INFER:
What do you think an air conditioner does to the air temperature and humidity in a room?

12. 12-3 Classwork
The relative humidity of air filled to capacity at a given temperature is ___ percent.
The relative humidity goes up if the air temperature ___.
A ___ has two thermometers and is used to find relative humidity.
The relative humidity of air changes as ___ leaves the air.

13. 12-3 Classwork
5. CALCULATE: Find the relative humidity when the psychrometer readings are as follows:
wet bulb, 12℃; dry bulb, 20℃
wet bulb, 33℃; dry bulb, 35℃
wet bulb, 22℃; dry bulb, 30℃

14. 12-3 Classwork
6. INFER: In which city would people be the most comfortable? The least comfortable?