1. Prentice Hall EARTH SCIENCE
Tarbuck Lutgens 

2. Moisture, Clouds, and Precipitation
Chapter 18
Moisture, Clouds, and Precipitation
Who is Stan Hatfield and Ken Pinzke

3. 18.1 Water in the Atmosphere
Water’s Changes of State
18.1 Water in the Atmosphere
 Precipitation is 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.

4. Water’s Changes of State
 Solid to Liquid
• The process of changing state, such as melting ice, requires that energy be transferred in the form of heat. Energy is absorbed.
• Latent heat is the energy absorbed or released during a change in state. Temperature can NOT change during a change of state!

5. Water’s Changes of State
 Liquid to Gas
Evaporation is the process of changing a liquid to a gas. Energy is absorbed, and the surroundings cool. So when you step out of the shower you feel cool because the water on your skin is evaporating.
Condensation is the process where a gas, like water vapor, releases heat and changes to a liquid, like water.

6. Dew on a Spider Web
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7. 18.1 Water in the Atmosphere
Water’s Changes of State
18.1 Water in the Atmosphere
 Solid to Gas
• Sublimation is the conversion of a solid directly to a gas without passing through the liquid state.
Dry ice = solid carbon dioxide sublimes without becoming a liquid first.
• Deposition is the conversion of a vapor directly to a solid.

8. Dry ice machine on stage

9. Changes of State
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10. 18.1 Water in the Atmosphere
Humidity
18.1 Water in the Atmosphere
 Humidity is a general term for the amount of water vapor in air.
 Saturation
• Air is saturated when it contains the maximum quantity of water vapor that it can hold at any given temperature and pressure.
• When saturated, warm air contains more water vapor than cold saturated air.

11. 18.1 Water in the Atmosphere
Humidity
18.1 Water in the Atmosphere
 Relative Humidity
• Relative humidity is a ratio of the air’s actual water-vapor content compared with the amount of water vapor air can hold at that temperature and pressure.
• To summarize, 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. Warm air holds more water vapor than cold air – so winter air is dry.

12. Check your understanding
So, on a cool summer morning, the relative humidity is _______ than in the warmer afternoon (if the amount of water vapor stays constant).

13. Relative Humidity Varies with Temperature
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14. 18.1 Water in the Atmosphere
Humidity
18.1 Water in the Atmosphere
 Dew Point
• Dew point is the temperature to which a parcel of air would need to be cooled to reach saturation.
High dew point = moist air
Low dew point = dry air
** Dew point can NEVER be greater than the temperature! Why not?
(only slightly in the atmosphere in the case of the updraft of a thunderstorm when hail forms)
If the temperature dips below the dew point, moisture will leave the air, forming dew. Moisture will condense on anything colder than the air.
thunderstorm updraft can be a supersaturated condition.

15.  Measuring Humidity
• A hygrometer is an instrument to measure relative humidity.
• A psychrometer is a hygrometer with dry- and wet-bulb thermometers. Evaporation of water from the wet bulb makes air temperature appear lower than the dry bulb’s measurement. The two temperatures are compared to determine the relative humidity.

16. Digital psychrometer

17. Sling Psychrometer
Air must continually pass over bulbs of each thermometer– one wet & one dry. Water evaporates from wet bulb and it cools. Dryer air will let wet bulb cool more. Compare temperatures to a table to get the relative humidity.
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18. Relative Humidity Table – pg 525

19. Why is Winter Air Dry? Water vapor content of air stays constant
Warm air holds more water vapor than cold air
In winter outside air is cold so it doesn’t hold a lot of moisture
When you pump that air into your house and warm it up the relative humidity drops to less than 10%
Causes static electricity, dry skin, sinus headaches, etc.

20. 18.1 Review
What is the most important gas for understanding atmospheric processes?
What happens to heat during a change of state? Temperature?
How does the temperature of air influence its ability to hold water?
List two ways relative humidity can be changed?
What does relative humidity describe about air?
If the dew point is low, is the air moist or dry?

21. 18.2 Cloud Formation
- Clouds form when air is cooled to its dew point - Air must be saturated to be at the dew point
But these clouds form during warmest part of day – so how is that possible?
- Air Compression and Expansion
Think of pumping up bicycle tire - When you put air in the hose gets warm because compressing air increases motion of gas molecules & this causes temperature to rise
Tire losing air - When air is released it expands and cools
These are Adiabatic Temperature Changes =
Changes that happen even though heat isn’t added or subtracted

22. 18.2 Cloud Formation Air Compression and Expansion
Adiabatic Temperature Changes
(adiabatic = without loss or gain of heat)
• When air is allowed to expand, it cools, and when it is compressed, it warms.
 Expansion and Cooling
• Dry adiabatic rate is the rate of cooling or heating that applies only to unsaturated air.
• Wet adiabatic rate is the rate of adiabatic temperature change in saturated air.

23. Cloud Formation by Adiabatic Cooling
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24. 18.2 Cloud Formation Processes That Lift Air
 Four mechanisms that can cause air to rise are orographic lifting, frontal wedging, convergence, and localized convective lifting.
 Orographic Lifting (remember orogeny?)
• Orographic lifting occurs when mountains act as barriers to the flow of air, forcing the air to ascend.
• The air cools adiabatically; clouds and precipitation may result.

25. Orographic Lifting Elevated terrain acts as barrier
Causes air to rise and reach dew point
Clouds form and it rains on windward side
Other side of barrier is dry because all of precipitation fell out
Air descends and warms
up making it even drier
Rain Shadow Desert forms

26. 18.2 Cloud Formation Processes That Lift Air  Frontal Wedging
• A front is the boundary between two adjoining air masses having contrasting characteristics (warm/cold, etc.)

28. Orographic Lifting and Frontal Wedging
Frontal wedging - Warm air meets cold air and a front forms
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Warm air is less dense so it rises over cold air

29. 18.2 Cloud Formation Processes That Lift Air  Convergence
• Convergence is when air flows together and rises.
Leads to adiabatic cooling and possible cloud formation
This is why it rains many afternoons in Florida

30.  Localized Convective Lifting
Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy.
Rising parcels of air are called thermals
Birds use this to soar high in sky
People use it when they want to go hang gliding
If parcel rises above condensation level then clouds form and may produce rain

31. Convergence and Localized Convective Lifting
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32. 18.2 Cloud Formation Stability  Density Differences
• Stable air tends to remain in its original position, while unstable air tends to rise.
 Stability Measurements
• Air stability is determined by measuring the temperature of the atmosphere at various heights.
• The rate of change of air temperature with height is called the environmental lapse rate.

33. 18.2 Cloud Formation Stability  Degrees of Stability
• A temperature inversion occurs in a layer of limited depth in the atmosphere where the temperature increases rather than decreases with height. (warmer air over cooler air)

34.  Stability and Daily Weather When stable air is forced above the Earth’s surface, the clouds that form are widespread and have little vertical thickness compared to their horizontal dimension. (spread out wide but thin from top to bottom)

35. 18.2 Cloud Formation Condensation
 For any form of condensation to occur, the air must be saturated.
 Types of Surfaces
• Generally, there must be a surface for water vapor to condense on.
• Condensation nuclei are tiny bits of particulate matter that serve as surfaces on which water vapor condenses when condensation occurs in the air. (Like when they ‘seed’ clouds’)

36. 18.2 Review
Describe what happens to air temperature when work is done on the air to compress it.
What does stability mean in terms of air movement?
What are the four mechanisms that cause air to rise?
Describe conditions that cause condensation of liquid water in air.
What is a temperature inversion?
Hypothesize about other regions on Earth, other than the Florida peninsula, where convergence might cause cloud development and precipitation.

37. 18.3 Cloud Types and Precipitation
Types of Clouds
18.3 Cloud Types and Precipitation
 Clouds are classified on the basis of their form and height.
• Cirrus (cirrus = curl of hair) are clouds that are high, white, and thin.
• Cumulus (cumulus = a pile) are clouds that consist of rounded individual cloud masses.
• Stratus (stratus = a layer) are clouds best described as sheets or layers that cover much or all of the sky.

38. Cirrus Clouds
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39. Cloud Form #2 - Cumulus Rounded individual cloud mass
Usually have flat base and rise as domes or towers
Fluffy tops described as cauliflower head

40. Cloud Form #3 - Stratus Sheets or layers that cover the sky
No distinct units
Usually bring precipitation

41. 18.3 Cloud Types and Precipitation
Types of Clouds
18.3 Cloud Types and Precipitation
 High Clouds – Base above 6000 meters
• Cirrus clouds are high, white, and thin.
• Cirrostratus clouds are flat layers of clouds.
• Cirrocumulus clouds consist of fluffy masses.
 Middle Clouds – 6000 meters
• Altocumulus clouds are composed of rounded masses that differ from cirrocumulus clouds in that altocumulus clouds are larger and denser.
• Altostratus clouds create a uniform white to gray sheet covering the sky with the sun or moon visible as a bright spot.

42. 18.3 Cloud Types and Precipitation
Types of Clouds
18.3 Cloud Types and Precipitation
 Low Clouds - Form below 2000 meters
• Stratus clouds are best described as sheets or layers that cover much or all of the sky.
• Stratocumulus clouds have a scalloped bottom that appears as long parallel rolls or broken rounded patches.
• Nimbostratus clouds are the main precipitation makers.

43. Cloud Classification
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44. 18.3 Cloud Types and Precipitation
 Clouds of Vertical Development
• Some clouds do not fit into any of the other categories. They may start
in one layer and extend into
another.
Form in unstable air
Cumulus are usually fair
weather but grow out of control
under right circumstances
– become cumulonimbus
Produce rain or
thunderstorms

45. 18.3 Cloud Types and Precipitation
Fog
18.3 Cloud Types and Precipitation
 Fog is defined as a cloud with its base at or very near the ground.
 Fog Caused by Cooling
• As the air cools, it becomes denser and drains into low areas such as river valleys, where thick fog accumulations may occur.
 Fog Caused by Evaporation
• When cool air moves over warm water, enough moisture may evaporate from the water surface to produce saturation.

46. Fog caused by cooling
Warm moist air over Pacific moves over cold California Current then carried onshore by prevailing winds
Cool, clear, calm nights air near surface of Earth cools when Earth loses heat gained during day and reaches its dew point
As the air cools
it becomes dense
and sits in low areas

47. ‘Steam fog’ caused by evaporation

48. 18.3 Cloud Types and Precipitation
How Precipitation Forms
18.3 Cloud Types and Precipitation
 For precipitation to form, cloud droplets must grow in volume by roughly one million times.
 Cold Cloud Precipitation
• The Bergeron process is a theory that relates the formation of precipitation to supercooled clouds, freezing nuclei, and the different saturation levels of ice and liquid water.

49. The Bergeron Process
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50. 18.3 Cloud Types and Precipitation
How Precipitation Forms
18.3 Cloud Types and Precipitation
 Cold Cloud Precipitation
• Supercooled water is the condition of water droplets that remain in the liquid state at temperatures well below 0oC.
• Supersaturated air is the condition of air that is more concentrated than is normally possible under given temperature and pressure conditions.

51. 18.3 Cloud Types and Precipitation
How Precipitation Forms
18.3 Cloud Types and Precipitation
 Warm Cloud Precipitation
• The collision-coalescence process is a theory of raindrop formation in warm clouds (above 0oC) in which large cloud droplets collide and join together with smaller droplets to form a raindrop.

52. 18.3 Cloud Types and Precipitation
Forms of Precipitation
18.3 Cloud Types and Precipitation
 The type of precipitation that reaches Earth’s surface depends on the temperature profile in the lower few kilometers of the atmosphere.
 Rain and Snow
• In meteorology, the term rain means drops of water that fall from a cloud and have a diameter of at least 0.5 mm.
• At very low temperatures (when the moisture content of air is low) light fluffy snow made up of individual six-sided ice crystals forms.

53. 18.3 Cloud Types and Precipitation
Forms of Precipitation
18.3 Cloud Types and Precipitation
 Rain and Snow
• Sleet is the fall of clear-to-translucent ice.
• Hail is produced in cumulonimbus clouds.
• Hailstones begin as small ice pellets that grow by collecting supercooled water droplets as they fall through a cloud.

54. Liquid Forms of Precipitation
Rain
Size: 0.5-5mm diameter
Drizzle
Size: <0.5mm diameter
Falls very slow

55. Solid Forms of Precipitation
Snow
Ice particles
Usually falls as flakes
small flakes at low temps
larger flakes closer to
freezing
depends on moisture
content in air

56. Solid Forms of Precipitation
Sleet
ice pellets
rain falls through
layer of freezing air

57. Solid Forms of Precipitation
Freezing Rain
Glaze Ice
Rain doesn’t freeze
until it strikes surface
near ground

58. Largest Recorded Hailstone
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