1. Warm Up 3/13/08 What are the three states of matter?
a. Gas, liquid, solid b. Gas, liquid, vapor
c. Vapor, liquid, solid d. Gas, vapor, solid
What is the change of state from a gas to a liquid called?
True or False: Heat is released when water evaporates.
Answers: 1) a. 2) condensation 3) False

2. Water in the Atmosphere
Chapter 18, Section 1

3. Water in the Atmosphere
Precipitation – any form of water that falls from a cloud
When it comes to understanding atmospheric processes, water vapor is the most important gas in the atmosphere
Water vapor makes up only a small fraction of the gases in the atmosphere, ~0-4 percent by volume

4. Precipitation

5. Concept Check
What is the range in volume percent of water in the atmosphere?
Atmosphere is nearly 0 to about 4 percent water by volume.

6. Water’s Changes of State: Solid to Liquid
The process of changing state requires that energy is transferred in the form of heat
When heat is transferred to a glass of ice water, the temperature remains a constant 0ºC until the ice is melted
Latent Heat – does not produce a temperature change; stored energy
Latent heat plays a crucial role in many atmospheric processes; it is the major source of energy for thunderstorms, tornadoes, and hurricanes

7. Solid to Liquid

8. Water’s Changes of State: Liquid to Gas
Evaporation – the process of changing a liquid to a gas
The energy absorbed by the water molecules during evaporation gives them the motion needed to escape the surface of the liquid and become a gas (latent heat of evaporation)
“evaporation is a cooling process” means that it takes incredible energy to evaporate water, usually it comes from whatever object it is evaporating from (i.e. you!)
Condensation – the process where water vapor changes to the liquid state
For condensation to occur, water molecules must release their stored heat energy, latent heat of condensation, equal to what is absorbed during evaporation

9. Evaporation

10. Water’s Changes of State: Solid to Gas
Sublimation – the conversion of a solid directly to a gas, without passing through the liquid state
This is the process you see when you watch dry ice, the “smoke” is actually the solid carbon dioxide becoming a gas
Deposition – the conversion of a vapor directly to a solid
This happens when water vapor is deposited as frost on cold objects

11. Sublimation

12. Changes of State

13. Concept Check What happens to heat during a change of state?
Heat is absorbed or released, depending on the change of state.

14. Saturation
As water begins to evaporate from the water surface, a small increase in pressure can be detected in the air above
This increase is a result of the motion of the water molecules that were added to the air through evaporation
As more and more molecules escape from the water surface, the pressure in the air increases steadily
Saturated – the number of vapor molecules returning to the surface of a body of water equals the amount leaving
When saturated, warm air contains more water vapor than saturated cold air

15. Saturation

16. Concept Check
How does the temperature of air influence its ability to hold water?
When saturated, warm air contains more water vapor than cold air.

17. Relative Humidity
Relative Humidity – ratio of the air’s actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure
Relative humidity indicates how near the air is to saturation, rather than the actual quantity of water vapor in the air
Relative humidity can be changed by adding or removing water vapor from the air
When the water-vapor content of air remains constant, lowering air temperature causes an increase in relative humidity, and raising air temperature causes a decrease in relative humidity

18. Changing Relative Humidity by Changing Temperature

19. Changing Relative Humidity by Changing Specific Humidity

20. Variations in Temperature and Humidity in Washington D.C.

21. Concept Check List two ways that relative humidity can be changed.
Relative humidity can be changed by adding or removing water vapor or by changing temperature.

22. Dew Point
Dew Point – the temperature to which a parcel of air would need to be cooled to reach saturation
If the same air was cooled further, the air’s excess water vapor would condense, typically as dew, fog, or clouds
For every 10ºC increase in temperature, the amount of water vapor needed for saturation doubles
High dew-point temperatures indicate moist air, and low dew-point temperatures indicate dry air

23. Dew Point Temperature

24. Measuring Humidity Hygrometer – device to measure relative humidity
One type of hygrometer, the psychrometer, consists of two identical thermometers mounted side to side
The amount of cooling that takes place in the psychrometer is directly proportional to the dryness of the air

25. Psychrometer

26. Assignment Read Chapter 18 (pg. 504 – 522)
Do Chapter 18 Assessment #1-30 (pg )