1. Humidity

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

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

4. Phase changes -transfers of latent energy-
Gas (water vapor)
LATENT HEAT OF
VAPORIZATION
+ 597 cal/gm
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)
melting

6. 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

7. 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

8. 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

9. 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

10. 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 a that contributes to air pressure.
Vapor pressure is the portion of the air pressure due to water vapor.

11. 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

12. 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.

13. 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?

14. 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 ?

15. 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.

16. “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.)

17. Now look at inset graph:

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

19. 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
This will be important when we study
how precipitation happens

20. 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.
vapor pressure for curved surface (like a droplet) is higher than for flat surface. If radius is small enough (like a very small water droplet), amount of supersaturation needed for condensation is so large, that it does not happen naturally.

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

22. 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

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

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

32. What is FOG? Similar to clouds but forms at surface.
To cause surface layer of air to saturate,
what has to happen?
Surface air has to cool to dew point OR
Water Vapor has to be added to the air.

33. Cool the surface air: RADIATION FOG
Surface cools by terrestrial radiative loss
of heat during night; if air cools below dew
point, condensation occurs.
Favored by cloudless skies, light wind to
mix lower layer.

34. Cool the surface: VALLEY FOG
Cold, dense air
sinks into valley.

35. Cool the surface: ADVECTION FOG
Cold current
Air moves horizontally over cold surface; cools to dew point

36. Advection fog over snow

37. Cooling the surface: UPSLOPE FOG
Air blowing towards mountains, forced to rise and cool.

39. Adding water vapor to the air: PRECIPITATION FOG
Happens as raindrops evaporate while falling.

41. Add water vapor to air: STEAM FOG
Steam fog over Lake Superior, X-mas 2017: warmer moist air over lake
mixes with colder air.

42. a closer look at dew point temperature :
Even “clear” air contains microscopic water droplets,
BUT evaporation > condensation so drops don’t survive long enough to clump and grow into cloud droplets.

43. Why do droplets
have to grow to
become cloud droplets?

44. Why do we never hear of relative humidities = 100% ?
(even if it is raining)
Because RH is measured at surface; not at level of clouds.
For same reason there can be clouds in sky and a reported RH < 100%

45. Dry line is a humidity front between dry and wet air

47. Spread between temp and dew point on station models is indication of humidity