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Physical Geography
Chapter 5
Clouds and Precipitation:
The Transfer of Latent Heat
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 ano...
Where is all of Earth’s water found?
• Oceans = 97.2%
• Glaciers = 2.0%
• Underground sources
(aquifers, underground pools...
The Hydrologic Cycle
• The hydrologic cycle is the planetary circulation of
water within and between the different sources...
Residence Time
• The amount of time a given amount of
water may remain in a particular segment
of the hydrologic cycle is ...
Residence Time
• As water changes its “residence,” it may
also change state.
• When water changes state it moves around
la...
Latent Heat Transfer
evaporation—latent heat absorbed
condensation—latent heat released
ice (solid)
water vapor (gas) wate...
Saturation
 The saturation point is the point at which a given parcel
of air is holding the maximum amount of water vapor...
Three Factors Influencing
the Rate of Evaporation
1. Temperature of the water
– The warmer the water, the faster the
molec...
Three Factors Influencing
the Rate of Evaporation
2. Temperature of the air
– Warm air can hold more water vapor
suspended...
Three Factors Influencing
the Rate of Evaporation
3. Degree of windiness
– Saturation is reached quickly right above the
w...
Vapor Pressure
 Vapor pressure--the portion of total air
pressure made up of water vapor molecules
 Saturation vapor pre...
Relative Humidity
 The amount of water vapor in the air at a given temperature,
compared with the maximum amount of water...
RH Example:
If the room you’re sitting in has 5 grams of
water vapor actually suspended in it, but the
maximum amount of w...
Relative Humidity
 What happens when relative humidity
reaches 100%?
– Saturation
– Condensation
 Clouds or fog (if cool...
Two Ways to Change
Relative Humidity
 Change the temperature of the air
– Temperature up, RH down
– Temperature down, RH ...
The Dew Point
 The dew point is the temperature at which
saturation is reached.
The Adiabatic Process
The Adiabatic Process
 The process by which rising air cools (as it
expands) and sinking air warms (as it is
compressed) ...
The Adiabatic Process
 As an air mass rises through the atmosphere, it
moves into an area of lower density, allowing the
...
The Adiabatic Process
 On the other hand, a sinking air mass will move down
through the atmosphere into a region of incre...
The DAR
 The rate at which unsaturated air will cool as
it rises is called the Dry Adiabatic lapse
Rate, or DAR (the air ...
The LCL
 The lifting condensation level (LCL) is the
elevation at which condensation occurs.
 As it rises, expands, and ...
You can “see” the LCL:
Look at the flat bottom of the cloud
A Quick Reminder!
The following five conditions all occur at
the same time:
 Saturation
 Condensation
 RH=100%
 Dew po...
The Latent Heat of Condensation
 Once condensation occurs, the water
molecules begin to give off the latent heat of
conde...
The SAR (or MAR)
 The rate at which a saturated parcel of air will
cool as it rises is called the Saturated
Adiabatic lap...
 Buoyancy
– The tendency of a substance to rise, especially in a fluid
 An air-filled balloon (or you!) in water
 A hel...
Stable air
Unstable air
Conditionally unstable air
Condensation nuclei and cloud droplets
Classifying Clouds
Are you paying attention?
Extra Credit Section!!!
Cloud types…
Fog: A cloud on the ground
The Four Common Types of Fog
Dew: Condensation on Earth’s surface
Physical Geography Lecture 07 - Clouds and Transfer of Latent Heat 102616
Physical Geography Lecture 07 - Clouds and Transfer of Latent Heat 102616
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Physical Geography Lecture 07 - Clouds and Transfer of Latent Heat 102616

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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.

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Physical Geography Lecture 07 - Clouds and Transfer of Latent Heat 102616

  1. 1. Physical Geography Chapter 5 Clouds and Precipitation: The Transfer of Latent Heat
  2. 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. 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. 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. 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. 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. 7. Latent Heat Transfer evaporation—latent heat absorbed condensation—latent heat released ice (solid) water vapor (gas) water (liquid)
  8. 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. 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. 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. 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. 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
  13. 13. 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)
  14. 14. 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%
  15. 15. Relative Humidity  What happens when relative humidity reaches 100%? – Saturation – Condensation  Clouds or fog (if cooling continues)
  16. 16. 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.)
  17. 17. The Dew Point  The dew point is the temperature at which saturation is reached.
  18. 18. The Adiabatic Process
  19. 19. 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
  20. 20. 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.
  21. 21. 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.
  22. 22. 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)
  23. 23. 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.
  24. 24. You can “see” the LCL: Look at the flat bottom of the cloud
  25. 25. A Quick Reminder! The following five conditions all occur at the same time:  Saturation  Condensation  RH=100%  Dew point temperature  LCL (lifting condensation level)
  26. 26. 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.
  27. 27. 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.
  28. 28.  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
  29. 29. Stable air
  30. 30. Unstable air
  31. 31. Conditionally unstable air
  32. 32. Condensation nuclei and cloud droplets
  33. 33. Classifying Clouds Are you paying attention?
  34. 34. Extra Credit Section!!!
  35. 35. Cloud types…
  36. 36. Fog: A cloud on the ground
  37. 37. The Four Common Types of Fog
  38. 38. Dew: Condensation on Earth’s surface

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