How is relative humidity measured? What is the dew point? Unit 12-3 and Unit 12-4 Notes How is relative humidity measured? What is the dew point?

12-3 Relative Humidity The actual amount of water vapor in the air compared to its capacity is relative humidity. Rel. Humidity of air = 100% at capacity. Rel. humidity = changes as water vapor comes into and out of the air.

12-3 Measuring Relative Humidity Psychrometer- measures relative humidity. Finding relative humidity: Get the difference between the dry and wet bulb amounts (subtract). Use the chart- a. dry bulb reading, b. the difference, and c. the chart to find relative humidity in %.

High humidity = uncomfortable. Low humidity= comfortable.

12-3 Lesson Summary Relative humidity- amt. of water vapor in the air compared to the amt. of water vapor the air can hold at capacity. Changes in temp. or the amt. of water vapor in the air changes the relative humidity.

A psychrometer is an instrument used to find relative humidity. 12-3 Lesson Summary A psychrometer is an instrument used to find relative humidity.

12-3 Class Work Complete “Checking Concepts” and “Thinking Critically” on p. 283.

12-4 Condensation The water vapor in the air changes from a gas to a liquid, as the temp. drops = condensation.

The temp. at which condensation takes place is called the dew point. As air cools, it can hold less and less water vapor.

12-4 Dew Point If the temp. drops low enough, the air becomes saturated, its relative humidity reaches 100%, and then water vapor condenses, and turns to dew….mainly at night.

12-4 Dew Point Dew point is the temp. at which saturation of the air occurs and condensation takes place..usually at night.

Freezing point of water = 0 degrees Celsius or 32 degrees Fahrenheit. 12-4 Frost Freezing point of water = 0 degrees Celsius or 32 degrees Fahrenheit.

Frost If the air temp. drops below the freezing point of water, water vapor will come out of the air and change directly to ice at dew point….this is frost.

12-4 Lesson Summary Condensation = changing of a gas to a liquid. When saturated air is cooled, condensation happens. The temp. at which water vapor condenses = dew point.

When saturated air is cooled close to the ground, dew forms. 12-4 Lesson Summary When saturated air is cooled close to the ground, dew forms. Frost forms when the dew point is below the freezing point of water.

12-4 Class Work Complete “Checking Concepts” and “Thinking Critically” on p. 285.