1. Moisture, Clouds, and Precipitation Chapter 19
Earth Science 101
Moisture, Clouds, and Precipitation Chapter 19
Instructor : Pete Kozich

2. Changes of state of water
Heat energy
Measured in calories – one calorie is the heat necessary to raise the temperature of one gram of water one degree Celsius
Latent heat
Stored or hidden heat
Not derived from temperature change, but from change of state
Very important in atmospheric processes

3. Changes of state of water
Three states of matter
Solid
Liquid
Gas
To change state, heat (energy) must be
Absorbed, or
Released

4. Changes of state of water
Processes that change the state of water
Evaporation
Liquid is changed to gas
600 calories per gram of water are added – called latent heat of vaporization
Condensation
Water vapor (gas) is changed to a liquid
Heat energy is released – called latent heat of condensation

5. Changes of state of water
Processes
Melting
Solid is changed to a liquid
80 calories per gram of water are added – called latent heat of melting
Freezing
Liquid is changed to a solid
Heat is released – called latent heat of fusion

6. Changes of state of water
Processes
Sublimation
Solid is changed directly to a gas (e.g., ice cubes shrinking in a freezer)
680 calories per gram of water are added
Deposition
Water vapor (gas) changed to a solid (e.g., frost in a freezer)
Heat is released

7. Changes of state of water
Figure 17.2

8. Moisture Amount of water vapor in the air
Saturated air is air that is filled with water vapor to capacity, therefore relative humidity is equal to 100%
Capacity is temperature dependent – warm air has a MUCH greater capacity
Water vapor has a partial pressure (called vapor pressure) as do other atmospheric gases. It has a low molecular weight, so is lighter.

9. ACTIVE FIGURE 19.1 Warm air can hold more water vapor than cold air can.
Fig. 19.1, p.471

10. Moisture Measuring moisture Mixing ratio Relative humidity
Mass of water vapor in a unit of air compared to the remaining mass of dry air
Often measured in grams per kilogram (grams of water vapor per kilogram of dry air)
Relative humidity
Ratio of the air's actual water vapor content compared with the amount of water vapor required for saturation at that temperature (and pressure), expressed as a percentage

11. Moisture Measuring moisture Relative humidity Expressed as a percent
Saturated air
Content equals capacity
Has a 100% relative humidity
Relative humidity can be changed in two ways
Add or subtract moisture to the air
Adding moisture raises the relative humidity (evaporation, sublimation)
Changing the air temperature
Lowering the temperature raises the relative humidity

12. Moisture Measuring moisture
Saturation vapor pressure (measured in mb or hPa)
Pressure exerted by water vapor molecules in
the air
Dewpoint temperature
Temperature to which a parcel of air would need to be cooled, at constant pressure, to reach saturation
Cooling the air to the dewpoint causes saturation and condensation if the air is not totally pure (the real atmosphere never is)
e.g., dew, fog, or cloud formation
Water vapor requires a surface to condense on

13. Relative humidity changes at constant temperature
Figure 17.4

14. Relative humidity changes at constant water-vapor content
Figure 17.5

15. Moisture Measuring moisture
Also used a chilled mirror in the past to measure dewpoint
Nowadays, done digitally
Prone to large error, hard to get accurate measurements when air is near saturation or very dry (digital is probably best)
Psychrometer - compares temperatures of wet-bulb thermometer and dry-bulb thermometer
If the air is saturated (100% relative humidity) then both thermometers read the same temperature
The greater the difference between the thermometer readings, the lower the relative humidity
Hair hygrometer – reads the humidity directly
Humidity increases, hair expands
Humidity decreases, hair contracts
Slow response to humidity, imprecise

16. A sling psychrometer
Figure 17.8

17. A hair hygrometer

18. Adiabatic heating/cooling
Adiabatic temperature changes
A process in which no heat is exchanged between an isolated air parcel and the surrounding environment
Adiabatic temperature changes occur when
Air is compressed
Motion of air molecules increases
Air will warm
Descending air is compressed due to increasing air pressure
Air expands
Air parcel does work on the surrounding air
Air will cool
Rising air will expand due to decreasing air pressure

19. Adiabatic heating/cooling
Adiabatic temperature changes
Adiabatic rates
Dry adiabatic rate
Unsaturated air
Rising air expands and cools at 9.8˚C per 1 km (5.5˚F per 1000 feet)
Descending air is compressed and warms at 9.8˚C per km
Wet adiabatic rate
Commences at condensation level
Air has reached the dew point
Condensation is occurring and latent heat is being liberated
Heat released by the condensing water reduces the rate of cooling
Rate varies from 4˚C to 9.8˚C per km, less cooling for very warm, saturated air (lots of moisture and LH release)

20. Adiabatic cooling of rising air

21. Processes that lift air
Orographic lifting
Elevated terrain act as barriers
Result can be moist on one side and dry on the other
Frontal wedging
Cool air acts as a barrier to warm air, so it can be a boundary that aids lifting
Fronts are part of the storm systems called middle-latitude cyclones
Convergence where the air is flowing together and rising (low pressure generally)
Localized convective lifting
Localized convective lifting occurs where unequal surface heating causes pockets of air to rise because of their buoyancy

22. Processes that lift air

23. Stability of air Determines to a large degree
Type of clouds that develop
Intensity of the precipitation
Static stability determines vertical motion of air, unless the air is strongly forced upwards or downwards by some mechanism.

24. Stability of air Types of stability Absolutely stable air
A parcel resists vertical displacement
Cooler than surrounding air
Denser than surrounding air
Wants to stay put vertically or slowly descend
Absolute stability occurs when the environmental lapse rate is less than the moist adiabatic rate
If air is forced upwards, clouds may still form but they will be vertically thin (stratiform) and any precipitation would be light

25. Absolute stability
Figure 17.17

26. Stability of air Types of stability Absolute instability
Acts like a hot air balloon
Rising air
Warmer than surrounding air
Less dense than surrounding air
Continues to rise until it reaches an altitude with the same temperature
Environmental lapse rate is greater than the dry adiabatic rate
Usually occurs only in a shallow layer right above the ground where the air is dry, actually has little meteorological significance
Conditional instability
When the atmosphere is stable for an unsaturated parcel of air but unstable for a saturated parcel
Typically produces cumulus clouds (significant vertical development), although it can often also produce stratiform clouds as well

27. Absolute instability

28. Conditional instability

29. Condensation and cloud formation
Water vapor in the air changes to a liquid and forms dew, fog, or clouds
Water vapor requires a surface to condense on
Possible condensation surfaces on the ground can be the grass, a car window, etc.
Possible condensation surfaces in the atmosphere are tiny bits of particulate matter
Called condensation nuclei
Dust, smoke, etc
Ocean salt crystals
Anything that serves as hygroscopic ("water seeking") nuclei, which attract water

30. Condensation and cloud formation
Clouds
Made of millions of:
Minute water droplets, or
Tiny crystals of ice
Classification based on
Form (three basic forms)
Cirrus – high, white, vertically thin
Cumulus – clouds with significant vertical development
Stratus – sheets or layers that cover much of the sky in lower or middle portions of the troposphere

31. Cirrus clouds

32. Cumulus clouds

33. Altostratus clouds

34. Condensation and cloud formation
Clouds
Classification based on
Height and Water State of Matter
High clouds - above 6000 meters (cirrus or cirro-); ice
Types include cirrus, cirrostratus, cirrocumulus
Middle clouds – 2000 to 6000 meters (altus or alto-); ice and/or liquid
Types include altostratus and altocumulus
Low clouds – below 2000 meters (stratus or strato-); liquid
Types include stratus, stratocumulus, and nimbostratus (nimbus means "rainy"), light to moderate precipitation
Clouds of vertical development (cumulus or cumulo-), liquid (low); mixed (Cb and TCu)
From low to high altitudes are cumulus congestus and cumulonimbus
Often produce rain showers and thunderstorms

35. Classification of clouds according to height and form
Figure 17.20

36. Classification of clouds according to height and form (continued)
Figure 17.20

37. Condensation and cloud formation
Unusual clouds
Lenticular cloud
Round, lens-shaped cloud formed from mountain wave effects
Virga
Rain falls from a cloud and evaporates if there is a dry layer below the cloud
Noctilucent and nacreous clouds above the troposphere
Mammatus
Occurs in mature cumulonimbus clouds
Downdrafts in anvil
Shelf cloud
Wall cloud
Protruding cloud on the updraft base, often where tornadoes form
Funnel cloud
Cloud extending toward the ground from a wall cloud that does not contact the surface

38. Virga and Lenticular Clouds

39. Mammatus and Shelf Clouds

40. A Wall Cloud

41. Fog Considered an atmospheric hazard
Cloud with its base at or near the ground
Most fogs form because of
Radiation cooling, or
Movement of warm, moist air over a cold surface or cool air over warm water

42. Fog Types of fog Fogs caused by cooling Advection fog
Warm, moist air moves over a cool surface
Air near the surface cools, water vapor condenses and fog forms
Radiation fog
Earth's surface cools more rapidly at night than the air a few hundred feet above
Forms during cool, clear, calm nights
Upslope fog
Humid air moves up a hill or mountain
Adiabatic cooling occurs

43. Advection and Upslope Fog

44. Fog Types of fog Evaporation fogs Steam fog
Cool air moves over warm water and moisture is added to the air
Water has a steaming appearance
Frontal fog, or precipitation fog
Forms during frontal wedging when warm air is lifted over colder air
Rain evaporates to form fog

45. Steam Fog

46. Precipitation Cloud droplets Formation of precipitation
Less than 20 micrometers (0.02 millimeter) in diameter (human hair is 75 μm in diameter)
Fall incredibly slow
Raindrops large enough to reach the ground contain about a million times more water than a cloud droplet
Cloud droplets must coalesce (come together) to form precipitation
Formation of precipitation
Bergeron process (ice crystal process)
Temperature in the cloud is below freezing
Ice crystals collect water vapor; supercooled water droplets (liquid water droplets between 0° and -40° C) evaporate to replace the water vapor collected by the ice crystals
Large snowflakes form and fall to the ground or melt during descent and fall as rain

47. Particle sizes involved in condensation and precipitation
Figure 17.24

48. The Bergeron process
Figure 17.25

49. Precipitation Formation of precipitation
Collision-coalescence process (warm rain process)
Based on the simple fact that different size drops fall at different speeds (terminal velocity)
Warm clouds (temperatures not below freezing)
Large hygroscopic condensation nuclei allow large droplets form
Larger droplets collide with other droplets during their descent and coalesce to grow
Common in the tropics

50. The collision- coalescence process
Figure 17.26

51. Precipitation Forms of precipitation Rain, drizzle, and mist
Rain – droplets have at least a 0.5 mm diameter
Drizzle – droplets have less than a 0.5 mm diameter
Mist – droplets range from .005 to .05 mm in diameter
Snow – ice crystals, or aggregates of ice crystals
Dry snow – colder temperatures, less water content in the air, more snow per inch of liquid water
Wet snow – warmer temperatures, more water content
Sleet
Wintertime phenomenon
Small particles of ice
Raindrops freeze when falling through a layer of sub-freezing temperatures

52. Precipitation Forms of precipitation Glaze, or freezing rain
Supercooled raindrops land on an object at the surface that is below freezing
The freezing layer, in the atmosphere, near the ground is not thick enough to freeze the rain drops in mid air, therefore the droplets become supercooled
Rime (frozen dew)
Forms on cold surfaces (seen on windshields)
Freezing of
Supercooled fog, or
Cloud droplets
Graupel
Rime collects on snow crystals to produce irregular masses of “soft ice”
Normally flatten upon impact

53. Precipitation Forms of precipitation Hail Hard rounded pellets
Concentric shells
Most diameters range from 1 (pea) to 5 cm (golf ball)
Formation
Occurs in large cumulonimbus clouds with violent up- and downdrafts
Layers of rain are caught in up- and downdrafts in the cloud, grows when lifted to subfreezing temperatures in cloud, may make several cycles in the cloud
Pellets fall to the ground when they become too heavy for the updraft

54. End of Chapter 19