Intro

Objective 02/28/2012Tell 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?

Measuring Water Vapor in the Air

Measuring Water Vapor in the Air

• 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

Warm air holds more water than cold air

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

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

Measuring Water Vapor in the Air

• Saturated • When air is completely full of water vapor

• Relative humidity is 100%

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

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

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

Outro

Objective 03/22/11Describe how relative humidity is affected by temperature and levels of water vapor.

How much humidity would a desert have? (high or low)Explain.