1. Intro Objective 02/28/2012 Tell how dew point and humidity indicate the amount of water in the atmosphere. Benjamin Franklin posed this question for his students to figure out, can you? Whence comes the Dew that stands on the Outside of a Tankard that has cold Water in it in the Summer Time?

2. Measuring Water Vapor in the Air

3. Humidity Amount of water vapor in the air Higher the temperature = greater amount of water air can hold Forecasters describe humidity in two ways: – Absolute humidity – Relative humidity

4. Warm air holds more water than cold air

5. Measuring Water Vapor in the Air Absolute humidity Measure of mass of water vapor in a volume of air Example: – Absolute humidity of 2g/m³ = each cubic meter (m³) can hold 2grams of water vapor

6. Measuring Water Vapor in the Air Relative humidity A ratio of water vapor that is in the air compared to the maximum amount of water vapor the air can hold at a certain temperature Expressed as a percentage Example: – Relative humidity is 50% = air is 50% full of water vapor

7. Measuring Water Vapor in the Air Saturated When air is completely full of water vapor Relative humidity is 100%

8. Measuring Water Vapor in the Air Factors Affecting Relative Humidity Example: Amount of water vapor Temperature More water vapor in air… … higher the relative humidity As temperature increases… …relative humidity decreases

9. Measuring Water Vapor in the Air Measuring Relative Humidity Psychrometer – Consists of two thermometers Wet-bulb thermometer Dry-bulb thermometer Difference in temperature readings indicate amount of water vapor in the air

10. Measuring Water Vapor in the Air How a Wet-Bulb Thermometer Works Wet-bulb thermometer covered with a damp cloth If water evaporates quickly, relative humidity is low If water evaporates slowly, relative humidity is high

12. Dry-Bulb Temp = 40 Wet-Bulb Temp = 35 Difference? –5–5 Relative Humidity? – 61% Dry-Bulb Temp = 60 Wet-Bulb Temp = 56 Difference? –4–4 Relative Humidity? – 78%

13. Outro Objective 03/22/11 Describe how relative humidity is affected by temperature and levels of water vapor. How much humidity would a desert have? (high or low) Explain.