Global water budget. Hydrologic cycle. Residence time. Latent Heat Transfer diagram. Saturation. Factors affecting rate of evaporation. Vapor pressure. Relative Humidity. Dew point. The adiabatic process. DAR, LCL, latent heat of condensation, SAR. Stable vs Unstable air. Clouds. Fog. Dew.

1. Physical Geography
Chapter 5
Clouds and Precipitation:
The Transfer of Latent Heat

2. The Global Water Budget
• Earth has a global water budget—if water
is lost in one place or in one form, it is
moved to another place or another form
• The total amount of water (in whatever
form) varies from place to place, but stays
constant over the planet as a whole

3. Where is all of Earth’s water found?
• Oceans = 97.2%
• Glaciers = 2.0%
• Underground sources
(aquifers, underground pools & groundwater) = 0.5%
• Lakes (half saline, half fresh) = 0.2%
• Pore spaces in soil (“soil water”) = 0.04%
• Atmospheric water, streams, living things = 0.01%

4. The Hydrologic Cycle
• The hydrologic cycle is the planetary circulation of
water within and between the different sources of water
on Earth—it is a closed system.
• The power source behind the hydrologic cycle is the
radiant energy of the sun.

5. Residence Time
• The amount of time a given amount of
water may remain in a particular segment
of the hydrologic cycle is its residence
time.
• Residence time can vary from hours
(evaporation followed by a
thundershower), to millions of years
(trapped in deep aquifers)

6. Residence Time
• As water changes its “residence,” it may
also change state.
• When water changes state it moves around
latent heat. The evaporation and
condensation phase changes are
especially significant...
How about a diagram???

7. Latent Heat Transfer
evaporation—latent heat absorbed
condensation—latent heat released
ice (solid)
water vapor (gas) water (liquid)

8. Saturation
 The saturation point is the point at which a given parcel
of air is holding the maximum amount of water vapor
that it can possibly hold at a given temperature and
pressure.
– Temperature is the key!
 If the air is not saturated, evaporation can continue, as
long as there is moisture available to be evaporated

9. Three Factors Influencing
the Rate of Evaporation
1. Temperature of the water
– The warmer the water, the faster the
molecules are moving and the more likely
they will be able to escape the surface
(evaporate)

10. Three Factors Influencing
the Rate of Evaporation
2. Temperature of the air
– Warm air can hold more water vapor
suspended in it
– Warm air transfers heat to the water and
speeds up water molecules to the point
where they can evaporate
– Cold air can hold less water as a vapor and
reaches its saturation point more quickly

11. Three Factors Influencing
the Rate of Evaporation
3. Degree of windiness
– Saturation is reached quickly right above the
water
– Wind blowing over a wet surface will reduce
saturation above that surface by moving
water vapor molecules away from the
surface. This leaves room for more
molecules to evaporate

12. Vapor Pressure
 Vapor pressure--the portion of total air
pressure made up of water vapor molecules
 Saturation vapor pressure--the pressure
exerted by the maximum amount of water
vapor a parcel of air can hold at a given
temperature.
12

14. Relative Humidity
 The amount of water vapor in the air at a given temperature,
compared with the maximum amount of water vapor which could
be in the air if it were saturated
RH = actual/maximum x 100 = ___ %
 RH = relative humidity
 Actual = the actual amount of water vapor in the air right now
 Maximum = the maximum amount of water vapor the air can hold at
the given temperature and pressure (in other words, saturation
point)

15. RH Example:
If the room you’re sitting in has 5 grams of
water vapor actually suspended in it, but the
maximum amount of water vapor that the air
could possibly hold is 10 grams, then:
RH = actual/maximum x 100 = ___ %
RH = 5g / 10g x 100 = 50%

16. Relative Humidity
 What happens when relative humidity
reaches 100%?
– Saturation
– Condensation
 Clouds or fog (if cooling continues)

17. Two Ways to Change
Relative Humidity
 Change the temperature of the air
– Temperature up, RH down
– Temperature down, RH up
 Add or subtract water vapor
– In the atmosphere, water is added through evaporation,
or lost through precipitation (rain, snow, etc.)

18. The Dew Point
 The dew point is the temperature at which
saturation is reached.

19. The Adiabatic Process

20. The Adiabatic Process
 The process by which rising air cools (as it
expands) and sinking air warms (as it is
compressed) in the atmosphere
 The physical principle involved:
– When a gas expands, it cools
– When a gas is compressed, it warms

21. The Adiabatic Process
 As an air mass rises through the atmosphere, it
moves into an area of lower density, allowing the
molecules the freedom to expand.
 As air expands, there are fewer collisions between
molecules and the air begins to cool.
 So rising air expands and cools down. If the air mass
cools enough to reach the dew point temperature,
condensation will occur and a cloud will form.

22. The Adiabatic Process
 On the other hand, a sinking air mass will move down
through the atmosphere into a region of increasingly
more molecules of air.
 The pressure of all of these molecules will compress
the air mass, forcing the molecules closer to one
another.
 This increases the number of molecular collisions,
speeding up the molecules, which translates into an
increase in temperature.
 So sinking air is compressed and warms up.

23. The DAR
 The rate at which unsaturated air will cool as
it rises is called the Dry Adiabatic lapse
Rate, or DAR (the air is not actually “dry”, it’s
just not saturated).
 Although this rate can vary based on several
atmospheric variables, a commonly-used
average value is:
10ºC/1000m (5.5ºF/1000ft)

24. The LCL
 The lifting condensation level (LCL) is the
elevation at which condensation occurs.
 As it rises, expands, and cools, the air’s
relative humidity increases (getting closer to
100%) until eventually the air parcel reaches
its dew point temperature.
 At that point, saturation has been reached
and a cloud begins to form.
 The elevation where this happens is the LCL.

25. You can “see” the LCL:
Look at the flat bottom of the cloud

26. A Quick Reminder!
The following five conditions all occur at
the same time:
 Saturation
 Condensation
 RH=100%
 Dew point temperature
 LCL (lifting condensation level)

27. The Latent Heat of Condensation
 Once condensation occurs, the water
molecules begin to give off the latent heat of
condensation. This heat becomes sensible
heat that can be measured.
 This heat interferes with the adiabatic cooling
that is going on, slowing down the cooling
process. So the air continues to get colder as
it rises, but it cools at a slower rate.

28. The SAR (or MAR)
 The rate at which a saturated parcel of air will
cool as it rises is called the Saturated
Adiabatic lapse Rate, or SAR (also called
the MAR, or moist adiabatic lapse rate)
 Again, the rate varies, but we’ll use an
average value of:
5ºC/1000m (3.3ºF/1000ft)
 As the air parcel continues to rise, it continues
to cool, though more slowly.

29.  Buoyancy
– The tendency of a substance to rise, especially in a fluid
 An air-filled balloon (or you!) in water
 A helium balloon in air
– Density is the key
 Equilibrium level
– Where both the rising and the still air are the same density
 The opposite of buoyancy is stability
– The substance does NOT want to rise
Stability vs. Buoyancy

30. Stable air

31. Unstable air

32. Conditionally unstable air

34. Condensation nuclei and cloud droplets

35. Classifying Clouds
Are you paying attention?

36. Extra Credit Section!!!

37. Cloud types…

38. Fog: A cloud on the ground

39. The Four Common Types of Fog

40. Dew: Condensation on Earth’s surface