1. Chapter 18 – Moisture, Precipitation, & Clouds
Define all vocabulary for Chapter 18
Read each night to review material covered in class and to gain an understanding of material to be covered next class meeting
Good Luck on the test to come 

2. Moisture, Clouds, and Precipitation
Chapter 18
Moisture, Clouds, and Precipitation
Key Concepts
Which gas is most important for understanding atmospheric processes?
What happens during a change of state?
How do warm and cold air compare in their ability to hold water vapor?
What is relative humidity?
What can change the relative humidity of air?
Who is Stan Hatfield and Ken Pinzke
Vocabulary
Precipitation, latent heat, evaporation, condensation, sublimation, deposition, humidity, saturated, relative humidity, dew point, hygrometer

3. 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.
Water vapor is the source of all condensation and precipitation.
Water vapor is 0-4% of the atmosphere

4. Water’s Changes of State
3 states of matter are solid, liquid, and gas
The process of changing state required energy to be gained or lost. It is transferred in the form of heat.
Latent heat is energy added to matter that does not produce a temperature change.

5. Solid - Liquid Solid to Liquid
Melting: when ice gains energy the molecules move faster until they become a liquid
Freezing: when liquid water loses energy the molecules slow down until they become a solid

6. Liquid - Gas Liquid to Gas
Evaporation: when liquid water gains energy the molecules move even faster until they become a gas
Condensing: when water vapor loses energy the molecules move slower until they condense into a liquid

7. BIOLOGY: The Water Cycle
The water cycle is essential to life on land. Water evaporates from the oceans and from the leaves of plants. Some of the water vapor condenses and falls as precipitation over the land back to the oceans, it is drunk by animals and absorbed by plants through their roots.

8. BIOLOGY: The Water Cycle
What would happen if water evaporated and condensed at temperatures not found on Earth’s surface?
What effect would this have on life on Earth?

9. Solid - Gas
Solid to Gas
Sublimation is the change from a solid directly to a gas without a liquid state.
Example: Dry ice
Deposition is the change from a gas to a solid without a liquid state.

10. 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
 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.
• 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.

12. Relative Humidity Varies with Temperature
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13. 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.
 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.

14. Dew on a Spider Web
Figure 4
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15. Sling Psychrometer Figure 5
This psychrometer is used to measure both relative humidity and dew point.
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16. 18.2 Cloud Formation Key Concepts Vocabulary
What happens to air when it is compressed or allowed to expand?
List four mechanisms that can cause air to rise.
Contrast movements of stable and unstable air.
What conditions in air favor condensation of water?
Vocabulary
Dry adiabatic rate, wet adiabatic rate, orographic lifting, front, temperature inversion, condensation nuclei.

17. 18.2 Cloud Formation  Adiabatic Temperature Changes
• 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.

18. Unsaturated air cools as it rises, at the constant rate of 10°C for every 1000 meters of ascent.
Descending air encounters higher pressures, compresses, and is heated 10°C for every 1000 meters it moves downward.
If a parcel of air rises high enough, it will eventually cool to its dew point. (condensation begins)
As the air rises, latent heat of condensation will be released.
The released latent heat works against the adiabatic cooling process.
The wet adiabatic rate varies from 5–9°C per 1000 meters.

19. Cloud Formation by Adiabatic Cooling
Figure 7
Rising air cools at the dry adiabatic rate until the air reaches the dew point and condensation (cloud formation) begins. As air continues to rise the latent heat released by condensation reduces the rate of cooling.
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20. 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
• 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.

21. 18.2 Cloud Formation  Frontal Wedging
• A front is the boundary between two adjoining air masses having contrasting characteristics.

22. Orographic Lifting and Frontal Wedging
Figure 8
A Orographic Lifting
Frontal Wedging
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Why does the warm air mass move upward over the cold air mass?

23. 18.2 Cloud Formation  Convergence  Localized Convective Lifting
• Convergence is when air flows together and rises.
 Localized Convective Lifting
• Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy.

24. Convergence and Localized Convective Lifting
Con’t Figure 8
Convergence Localized Convective Lifting
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25. 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.

26. 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. A temperature inversion is the most stable condition.
 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.

27. 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.

28. 18.3 Cloud Types and Precipitation
Key Concepts
How are clouds classified?
How are clouds and fogs similar and different?
What must happen in order for precipitation to form?
What controls the type of precipitation that reaches Earth’s surface?
Vocabulary
Cirrus, cumulus, stratus, Bergeron process, supercolled water, supersaturated air, collision-coalescence process

29. Cloud Chart A cloud is classified based on ________ & _________.
High Clouds
Middle Clouds
Low Clouds
Multi-level Clouds
Altitude
Type, description, and precipitation

30. 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.
The three basic forms are:
-cirrus
- cumulus
- stratus

31. Cirrus (cirrus = a curl of hair) They are high, white, and thin.
They can occur as patches or as delicate veil-like sheets or extended wispy fibers that often have a feathery appearance.

32. Cumulus (cumulus = a pile)
clouds consist of rounded individual cloud masses.
They have a flat base and the appearance of rising domes or towers. (cauliflower structure)

33. Stratus (stratum = a layer)
clouds are best described as sheets or layers that cover much or all of the sky.
There are no distinct individual cloud units.

34. 18.3 Cloud Types and Precipitation
Types of Clouds
18.3 Cloud Types and Precipitation
 High Clouds (above 6000m)
• Cirrus clouds are high, white, and thin.
• Cirrostratus clouds are flat layers of clouds.
• Cirrocumulus clouds consist of fluffy masses.
All high clouds are thin and white and are often made up of ice crystals.
These clouds are not considered precipitation makers. (they may warn of approaching stormy weather)

35. 18.3 Cloud Types and Precipitation
Types of Clouds
18.3 Cloud Types and Precipitation
 Middle Clouds (from about 2000 to 6000 meters)
• Altocumulus clouds are composed of rounded masses that are larger and denser cirrocumulus clouds.
• Altostratus clouds create a uniform white to gray sheet covering the sky with the sun or moon visible as a bright spot.
Infrequent light snow or drizzle may accompany these clouds.

36. 18.3 Cloud Types and Precipitation
Types of Clouds
18.3 Cloud Types and Precipitation
 Low Clouds (below 2000m)
• Stratus clouds are best described as sheets or layers that cover much or all of the sky. (May produce light precipitation.
• Stratocumulus clouds have a scalloped bottom that appears as long parallel rolls or broken rounded patches.
• Nimbostratus clouds are the main precipitation makers.

37. Clouds are classified according to form and height.
Cloud Classification
Clouds are classified according to form and height.
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38. 18.3 Cloud Types and Precipitation
Types of Clouds
18.3 Cloud Types and Precipitation
 Clouds of Vertical Development
• Some clouds do not fit into any one of the three height categories mentioned. Such clouds have their bases in the low height range but often extend upward into the middle or high altitudes.
They all are associated with unstable air.
Once upward movement is triggered, acceleration is powerful, and clouds with great vertical range form.
The end result often is a cumulonimbus cloud that may produce rain showers or a thunderstorm.

39. 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.

40. 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.

41. The Bergeron Process
Ice crystals grow at the expense of cloud droplets until they are large enough to fall. The size of these particles has been greatly exaggerated.
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42. 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.

43. 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.

44. 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.

45. 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.

46. Largest Recorded Hailstone
The largest recorded hailstone fell over Kansas in 1970
and weighed 766 grams.
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47. Precipitation Chart
Type
Description
Rain
Snow
Sleet
Glaze
Hail