1. Humidity

2. Water
Most abundant
71% of surface
Hydrologic cycle

3. Phase changes -transfers of latent energy-
Gas (water vapor)
Liquid
Solid (ice)

4. Phase changes -transfers of latent energy-
Gas (water vapor)
evaporation
Liquid
Solid (ice)

5. Phase changes -transfers of latent energy-
Gas (water vapor)
LATENT HEAT OF
VAPORIZATION
+ 597 cal/gm
evaporation
Liquid
Solid (ice)

6. Phase changes -transfers of latent energy-
Gas (water vapor)
LATENT HEAT OF
VAPORIZATION
+ 597 cal/gm
evaporation
Liquid
Solid (ice)
melting

7. Phase changes -transfers of latent energy-
Gas (water vapor)
LATENT HEAT OF
VAPORIZATION
+ 597 cal/gm
evaporation
Liquid
Solid (ice)
melting
+ 80 cal/gm
LATENT HEAT OF FUSION

8. Phase changes -transfers of latent heat energy-
Gas (water vapor)
LATENT HEAT OF
VAPORIZATION
+ 597 cal/gm
sublimation
evaporation
Liquid
Solid (ice)
melting
+ 80 cal/gm
LATENT HEAT OF FUSION

9. Phase changes -transfers of latent heat energy-
Gas (water vapor)
LATENT HEAT OF
VAPORIZATION
597 80
100
+777 cal/gm
+ 597 cal/gm
sublimation
evaporation
Sublimation in winter: low temp, intense sunlight, winds;
100 to raise it to boiling point;
Heat required to change phase without temp change
Liquid
Solid (ice)
melting
+ 80 cal/gm
LATENT HEAT OF FUSION

10. Phase changes -transfers of latent heat energy-
Gas (water vapor)
+777 cal/gm
+ 597 cal/gm
sublimation
evaporation
deposition (-777)
condensation (- 597)
freezing (-80)
Liquid
Solid (ice)
melting
+ 80 cal/gm

11. Expressions of atmospheric humidity:
1. vapor pressure
2. relative humidity
3. dew point
4. mixing ratio

12. 1. Vapor Pressure
The total pressure of a mixture of gases equals the sum of the pressures exerted by the constituent gases.
Dalton’s Law
Water vapor is one of the gases in the atmosphere that contributes to air pressure.
Vapor pressure is the portion of the air pressure due to water vapor.

13. Saturation concept Constant two-way interchange at surface;
EVAPORATION: more molecules enter gas phase.
CONDENSATION: more enter liquid phase.
EQUILIBRIUM: equal amount become liquid
and gas.
At equilibrium, vapor pressure is SATURATION
VAPOR PRESSURE
vapor
liquid

14. Saturation vapor pressure
At higher temperatures, the amount
of water vapor in the atmosphere
(and therefore the saturation vapor
pressure) is higher.
Raising temperature increases
energy of molecules and more
readily escape surface as gas.
Initially evaporation prevails, but
eventually a new equilibrium is established.
Higher temp, higher water vapor conc,
higher equilibrium/saturation V.P.

15. Saturation vapor pressure
At higher temperatures, the amount
of water vapor in the atmosphere
(and therefore the saturation vapor
pressure) is higher.
Notice the difference between vapor
pressure and saturation vapor pressure
Vapor pressure (on graph axis) is partial pressure exerted by the actual amount of water vapor in the atmosphere
What if….
temperature is 30ºC and vapor
pressure is 20 mb?
Net evaporation or
net condensation?

16. Saturation vapor pressure
At higher temperatures, the amount
of water vapor in the atmosphere
(and therefore the saturation
vapor pressure) is higher.
if temperature drops to 20º ?

17. Saturation vapor pressure
At higher temperatures, the amount
of water vapor in the atmosphere
(and therefore the vapor pressure) is
higher.
There is less energy available for
evaporation ; condensation will occur,
then a new equilibrium is reached.

18. “Warm air can hold more water vapor than cold air”
“When temperature is higher, there’s more thermal energy for evaporation , so there is more water vapor in the air”
“Warm air expands and can hold more water vapor”
“There’s always plenty of room for water vapor molecules”. (Average intermolecular distance is 145 cm.)

19. Now look at inset graph:

20. Now look at inset graph:
Below freezing, sat. vapor pressure
over ice is different than saturation
vapor pressure over (supercooled)
water.

21. Now look at inset graph:
Below freezing, sat. vapor pressure
over ice is different than saturation
vapor pressure over (supercooled)
water.
Which is greater?
Does it take more water molecules
to saturate air over water or over ice?
Over water

22. Supercooled water is super cool!
Water can exist as liquid at temps below freezing if it doesn’t have a means to undergo nucleation.
Water droplets need a freezing nucleus (ice crystals)
Small droplets can supercool.
The vapor pressure for a curved surface (like a droplet) is higher than for a flat surface. If the radius is small enough (like a very small water droplet), the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally.

23. 2. Relative humidity
Amount of moisture in air relative to saturation amount, expressed as %.
actual amount water vapor X
saturation amount

24. If we have a parcel of air
at 10ºC and the vapor
pressure is 6 mb,
what is the relative
humidity?
6 X 100 = 50% 12

25. If temperature increases (and actual amount of water vapor stays same),
what happens to relative humidity?

27. 3. Dew Point Temperature
Temperature at which cooling air becomes saturated.
Dew
Clouds
Frost

33. Vapor Pressure= 10 mb Morning: 10°C ? Temperature Relative humidity
Dew point
10°C ?

34. Vapor Pressure= 10 mb Morning: 10°C 83% ? Temperature
Relative humidity
Dew point
10°C
83% ?

35. Vapor Pressure= 10 mb Morning: 10°C 83% 5ºC Temperature
Relative humidity
Dew point
10°C
83%
5ºC

36. Vapor Pressure= 10 mb Morning: 10ºC 83% 5ºC Afternoon: 30ºC ?
Temperature
Relative humidity
Dew point
10ºC
83%
5ºC
Afternoon:
30ºC ?

37. Vapor Pressure= 10 mb Morning: 10ºC 83% 5ºC Afternoon: 30ºC 24% ?
Temperature
Relative humidity
Dew point
10ºC
83%
5ºC
Afternoon:
30ºC
24% ?

38. Vapor Pressure= 10 mb Morning: 10ºC 83% 5ºC Afternoon: 30ºC 24%
Temperature
Relative humidity
Dew point
10ºC
83%
5ºC
Afternoon:
30ºC
24%