1. NOTICE:
The information contained within this document is copyright © U.S. Coast Guard Auxiliary Association
No attempts to add, modify or delete information contained within this document shall be made. If errors are discovered, please contact the U.S. Coast Guard Auxiliary’s National Training Directorate.
Slide 1:

2. WEATHER Specialty Course AUXWEA
Chapter 5 – Clouds, Precipitation and Optical Effects

3. Section 1
Clouds

4. Overview Cloud particles consist of:
Water droplets.
Ice crystals.
Or a mixture of both.
Classification by type, based on appearance.
Classification based on altitude of bases.
Clouds of vertical development.
Some special cases.

5. What are Clouds?
Clouds are made up of tiny particles suspended in the atmosphere.
Water clouds are made up of small water droplets.
Ice clouds are made up of small ice crystals.
They form when water vapor condenses on particles called condensation nuclei when either:
The air temperature is reduced to the dew point, or
Water vapor is added until the dew point rises to equal the air temperature.
The average size of cloud particles is 0.02mm, smaller than the width of a human hair.

6. How are Clouds Classified?
By the height of their bases.
Low: less than 6,500 ft (2,000 m).
Middle: 6,500 to less than 20,000 ft.
High: 20,000 ft. (7,000 m) or above.
By their appearance.
Cumuliform (lumpy)
Stratiform (flat, uniform)
Cirriform (wispy)
A separate category is Clouds of Vertical Development.

7. Summary of Cloud Types (1 of 3)
Low Clouds (water droplets):
Cumulus (Cu). Cumuliform clouds of small to moderate vertical extent.
Stratus (St). Stratiform clouds that look grey from below.
Stratocumulus (Sc). Layers of lumpy, fuzzy clouds.
Nimbostratus (Ns). Dark, stratiform clouds with precipitation.
Middle Clouds (mostly water, some ice):
Altostratus (As). Similar to stratus. Often thin.
Altocumulus (Ac). Similar to stratocumulus. Sometimes in rows.

8. Summary of Cloud Types (2 of 3)
High Clouds (ice particles):
Cirrus (Ci). Wispy, thin, often with narrow streaks.
Cirrostratus (Cs).Thin, uniform layer or in clumps (e.g. anvil tops).
Cirrocumulus (Cc). Layer of uniform, or irregular, small cumulus.
Clouds of Vertical Development:
They are cumuliform.
More detail later.

9. Summary of Cloud Types (3 of 3)
Heights
Low
(<6,500 ft.)
Middle
(6,500-20,00 ft.)
High
(>20,000 ft.)
Clouds of Vertical Development
Cumuliform
Cumulus (Cu)
Stratocumulus (Sc)
Altocumulus (Ac)
Cirrocumulus (Cc)
Cumulus Congestus or Towering Cumulus
(Cu)
Also as (Tcu)
Cumulonimbus (Cb)
Stratiform
Stratus (St) Nimbostratus (Ns)
Altostratus (As)
Cirrostratus (Cs)
Cirriform
None
Cirrus (Ci)

10. Cumulus (Cu) Clouds
Ordinary cumulus are small, puffy clouds (“fair weather cumulus” or cumulus humilus).
They have little vertical development (see also clouds of vertical development).
They very rarely produce precipitation.
Courtesy of NOAA

11. Stratus (St) Clouds Stratus (St) clouds are flat and uniform.
They can be thin or thick.
When seen from below, they are grey—the thicker, the darker.
When seen from above, they are white (daytime).
They do not produce precipitation (see Ns).
Courtesy of NOAA

12. Stratocumulus (Sc) Clouds
Stratocumulus (Sc) have an appearance between stratus and cumulus.
They are usually layered like stratus.
They have a somewhat puffy shape.
The edges, unlike Cu, are fuzzy.
Courtesy of NOAA

13. Nimbostratus (Ns) Clouds
Nimbostratus (Ns) clouds are simply stratus clouds that have thickened and produce precipitation.
The are typically fairly dark as seen from below.
Shafts of rain can usually be seen below them.
Courtesy of NOAA

14. Altocumulus (Ac) Clouds
Altocumulus (Ac) clouds appear similar to Sc, with clumps of cumuliform pieces.
They can be arranged randomly, or,
In rows, like “streets”.
They do not precipitate.
The “streets” show the direction of upper winds.
The winds are perpendicular to the rows, like winds and water waves.
Courtesy of NOAA

15. Altostratus (As) Clouds
Altostratus (As) clouds look very much like stratus clouds.
They can be:
Thick (can’t see the sun) or
Thin (sun is partly visible).
When thin, there will be a corona around the sun or moon (see section 3 on optical phenomena).
Courtesy of NOAA

16. Cirrus (Ci) Clouds Cirrus (Ci) clouds are formed of ice particles.
Their appearance is thin and wispy.
Usually have thin filaments.
If these are curved, they are called “mares tails.”
They are often mixed with cirrostratus clouds.
Courtesy of NOAA

17. Cirrostratus (Cs) Clouds
Cirrostratus (Cs) clouds look very much like Altostratus clouds.
They are ice clouds.
Principle way to tell the difference is if the sun or moon is behind them.
As clouds show coronas.
Cs clouds show halos.
See Section 3
Courtesy of NOAA

18. Cirrocumulus (Cc) Clouds
Cirrocumulus (Cc) clouds are similar to Altocumulus clouds, but the puffs look smaller, since they are farther away.
Almost always ice.
Large patches are called “mackerel skies.”
They show the upper level is unstable.
Called “mackerel skies because they look like fish scales to mariners.
Courtesy of NOAA

19. Clouds of Vertical Development
Cumulus clouds whose tops grow above the low cloud region (<6,500 ft.) are called clouds of vertical development.
If the layer of unstable air is moderately thick, cumulus clouds can develop into towering cumulus, normally higher than they are wide.
If the layer is unstable throughout most of the troposphere, they grow into cumulonimbus (Cb).
Main difference is that Cb produce precipitation. They can produce violent weather, as discussed in the next chapter.

20. Cumulonimbus (Cb) Clouds
Cumulonimbus (Cb) are cumuliform clouds that have grown very tall.
They are almost always thunderstorms (produce lightning).
They can have rounded tops or flat, anvil-shaped tops of cirrostratus.
Much more in Chapter 6.
Courtesy of NOAA

21. Interesting Examples (1 of 3)
A very beautiful form of altostratus is the lens-shaped clouds that top mountains.
These “lenticularis” clouds form when there is a:
Deep layer of steady wind.
Stable atmosphere.
Sufficient moisture.
Mountain or range of mountains to provide uplift.
Courtesy of NOAA

22. Interesting Examples (2 of 3)
Cumulonimbus mammatus clouds are mature thunderstorms whose base is lumpy, rather than flat.
It is a sign that the cloud is about to start dissipating.
It is not, as was once thought, a sign of impending tornados.
Courtesy of NOAA

23. Interesting Examples (3 of 3)
One feature of thunder-storms that does portend the formation of a tornado is the wall cloud at the base.
It can often be seen as beginning the rotation that will lead to the tornado.
Courtesy of NOAA

24. Section 2
Precipitation

25. Overview Precipitation—How does it form? What are the basic types?
Ice on surfaces.
Station model—the final look.

26. Precipitation
Precipitation can be in the form of water drops or ice, just like clouds.
It can be rain, drizzle, snow, sleet, ice pellets, freezing rain, etc.
The average size of a rain drop is 2 mm (compare to cloud particle in the picture).
Drizzle is less than 0.5 mm.
Size of cloud particle compared to a typical raindrop
©1997, USA Today. Reprinted with permission

27. How Does Precipitation Form?
There are two accepted theories of how cloud particles become precipitation.
The first, applicable to warm tropical air, is called the collision- coalescence process. It requires many salt particles in the air.
The second, applicable to mid-latitude air, which is well below freezing at higher altitudes, is called the Bergeron process.
In the collision-coalescence process, water droplets are moved around in the cloud by winds and convection.
When they collide, they may coalesce into larger droplets.
The process repeats until the drops are large enough to fall.
They then grow larger by collision with cloud droplets.

28. The Bergeron Process
The Bergeron process relies on two facts of physics:
Pure water will not freeze until approximately -40°C.
With freezing nuclei, it can freeze well above -40°C.
There are not many freezing nuclei in the mid-latitudes at the heights where precipitation forms.
Water below 0°C is called super-cooled.
Vapor pressure of ice is less than that of water at a given temperature.
A few ice particles in the cloud will therefore grow at the expense of the water droplets.
Once the ice particles are large enough to fall, they begin collecting the super-cooled water droplets, which freeze instantly.

29. The Bergeron Process (cont.)
Cold cloud particles are a mixture of water and ice.
Between 0°C and -10°C, they are virtually all super-cooled water.
Between -10°C and -20°C, they are a mixture of water and ice.
Below -20°C, they are normally all ice particles.
Therefore, in the higher levels of the atmosphere, precipitation is in the form of ice pellets or snow.
Changes depend on the vertical temperature structure.
It can remain as frozen precipitation.
It can melt into rain or drizzle.
It can melt and then re-cool to sleet or super-cooled water.
The next slide shows these processes in a warm front.

30. Forms of Precipitation
©1997, USA Today. Reprinted with permission

31. Precipitation Rates
Drizzle drops are 0.5 mm or less (by definition), and the rate of fall is measured by the visibility:
Light: Visibility greater then 1 km (5/8 mile).
Moderate: Visibility between 1 and 0.5 km.
Heavy: Visibility 0.5 km (5/16 mile) or less.
Rain drops average 2 mm, and the rate of fall is measured in inches per hour:
Light: 0.1 inches per hour or less.
Moderate: 0.1 to 0.3 inches per hour.
Heavy: Greater than 0.3 inches per hour.
For snow, bounds are the same as drizzle.

32. Other types of Precipitation
Snow pellets are snow partially melted and re-frozen. They are brittle and can be crushed with the fingers.
Snow grains are the frozen equivalent of drizzle.
Ice crystals (“diamond dust”) are tiny single crystals.
Sleet (ice pellets) are re-frozen rain or ice unchanged after formation (never melted).
Freezing rain/drizzle is ice or snow melted and re-cooled.
Freezing rain forms ice sheets (glaze ice) on cold objects.
Freezing drizzle usually forms rough ice (rime ice).
Hail is formed in cumulonimbus clouds (next chapter).

33. Final Station Model
This completes the station model (for our purposes).
To the left of the circle: Visibility of 1 mile with steady light rain.
Below: Nimbostratus clouds with bases 200 to 599 ft (coded as 2).
Above: Middle cloud is thick altostratus.
The very top: High cloud is cirrus.
Center: Total cloud cover is 6/8.
What is the weather situation? 67
965 49 1
-23\ 2
The weather situation is most likely an approaching warm front.
To review, the temperature is 67 F, the dew point 49 F, the pressure mb, the pressure tendency minus 2.3 mb in 3 hours, and the winds from the SW at 15 knots.
Present weather symbols (to the left), cloud symbols (above and below the station circle), and low cloud heights are coded. See the SSG Appendix B, page B-6, for the complete set of symbols.

34. Section 3
Optical Phenomena

35. Overview Types of meteors. How light interacts with matter.
Absorption.
Reflection.
Scattering.
Refraction.
Halos and coronas.
Sun dogs and sun pillars.
Rainbows.

36. Meteors Things in the atmosphere are called meteors.
Hydrometeors are things like clouds and precipitation.
Lithometeors are things like dust, aerosols, and soot.
(“Shooting stars” are meteors only while in the atmosphere).
Photometeors are not objects, but are interactions of light with the molecules of the air and with other meteors.
Photometeors, such as those below, tell us something about the state of the atmosphere.
Halos and coronas.
Sun pillars and sun dogs.
Rainbows.

37. Interaction of Light with Matter
When light from the sun or moon enters the atmosphere, it can interact in four distinct ways:
It can be absorbed, changing its electromagnetic energy into thermal energy (heat).
It can be reflected, which changes only the propagation direction without affecting the light in any other way.
It can be scattered (form of diffraction), dispersing the energy in all directions, like river water striking a rock.
It can be refracted, changing direction and dispersing the colors—i.e., bending depends on wavelength.

38. Scattering Scattering disperses the light spatially in all directions.
More energy is scattered near the forward direction in most cases.
The amount scattered decreases with angle from the forward direction
The pattern depends on the ratio of the wavelength to the size of the particle.
Air molecules scatter blue light more than red.
Particle Sizes
Direction
of propagation
Very Small
(air molecules)
Small
Large

39. Refraction
Refraction bends light in one direction or a small range of directions.
Long wavelengths (red) bend less than short (blue)
The degree of bending is influenced by:
The optical properties of the material.
The angle of incidence.
An example is a prism.
©2008, US Power Squadrons. Reprinted with permission

40. Why is the Sky Blue?
Air molecules scatter blue light more strongly than other colors.
The blue light is scattered many times, so it reaches our eyes from all directions.
A very blue sky is a sign of clean air (no smog or haze).
Courtesy of NOAA

41. Why are Clouds Red at Sunset?
A low sun angle means the light travels through more atmosphere.
Even more blue light (and most green) is removed from the ray.
Only red light remains to illuminate the cloud.
This effect is even more spectacular after a volcanic eruption.
©2008, US Power Squadrons. Reprinted with permission
Courtesy of NOAA

42. Halos
Halos around the sun or moon are caused by refraction in hexagonal ice crystals.
The angle from the sun is 22° on all sides.
Faint color rings can sometimes be seen—less often in lunar halos.
Halos in the west can indicate an approaching front.
©2008, US Power Squadrons. Reprinted with permission
Courtesy of NOAA

43. Coronas
Coronas are caused by the scattering of sunlight or moonlight by water drops in thin stratiform (usually As) clouds.
Because light is scattered more in the forward direction, halos are brighter near the source.
They can also signal an approaching front.
Courtesy of NOAA

44. Sun Pillars and Sun Dogs
These are also caused by ice crystals.
Near sunrise and sunset.
Sun pillars, above and below the sun, are reflected from flat, hexagonal crystals.
Sun dogs (mock suns) are on either side of the sun and are refracted by hexagonal crystals (like halos).
They are sometimes seen together.
Courtesy of NOAA

45. Rainbows
Rainbows are caused by refraction and reflection in water drops.
Primary bows—reflected once.
Secondary are reflected twice. The colors are reversed and dimmer.
Must have the sun behind you, shining on rain ahead of you.
(Rare tertiary bows are seen in the opposite direction).
©1997, USA Today. Reprinted with permission
Courtesy of NOAA

46. Summary (1 of 3)
Clouds are collections of very small water or ice particles.
Must have condensation nuclei and saturated air.
About 0.02 mm (i.e. 20 microns) in size.
Clouds are organized into three basic types:
Cumuliform—puffy (Cu, Sc, Ac, Cc, Cb).
Stratiform—flat and uniform (St, Ns, As, Cs).
Cirriform—wispy (Ci).
Clouds are also categorized by height of their bases.
Low—up to 2000 meters (6,500 ft.).
Middle—prefix Alto, from 2,000 to 7,000 meters (20,000 ft.).
High—prefix Cirro, above 7,000 meters.
Prefix Nimbo or suffix nimbus—precipitating (Ns, Cb).

47. Summary (2 of 3) Precipitation can be water or ice.
Rain, drizzle, freezing rain, etc.
Snow, sleet, hail, etc.
It forms by one of two processes:
Collision-coalescence operates in warm air (e.g. tropics).
Bergeron process operates in air well below freezing.
Cloud droplets are super-cooled.
The few ice particles present grow at the expense of the droplets.
Water at 0° C to -10° C; Mix at -10° C to -20° C; Ice below -20° C.
Ice or snow can change by melting (rain, drizzle), or melting and cooling (freezing rain), or re-freezing (sleet).

48. Summary (3 of 3)
Light interacts with molecules and meteors (particles) in the atmosphere by absorption, reflection, scattering, and refraction.
Scattering is a form of diffraction and spreads energy in all directions.
Refraction bends the light and disperses the wavelengths (colors).
The sky is blue because blue light is scattered more strongly.
Sunsets are red because the blue and green are scattered away.
Halos are caused by scattering in water clouds.
Coronas are caused by refraction in ice clouds.
Rainbows are caused by refraction/reflection in rain drops.
Primary rainbows have one internal reflection.
Secondary rainbows have two reflections and the colors are reversed.

49. Chapter 5 Questions QUESTION ANSWER
“-nimbus” added to a cloud name means:
Clouds with a horizontal, layered appearance are:
Clouds with a wispy appearance are:
Clouds with a lumpy appearance are:
Middle clouds are those that have bases between what altitudes?
It is precipitating.
Stratiform.
Cirriform.
Slide 49: Some Chapter Questions (1 of 2)
ANIMATIONS
Five questions are displayed.
[CLICK] to display each answer in sequence. Font change used to simulate human handwriting.
After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer).
* * * * * * * * * * (end comments).
Cumuliform.
6,500 ft and 20,000 feet (2 to 7 km).

50. Chapter 5 Questions QUESTION ANSWER
What type of cloud would you associate with the phrase “mare’s tails”?
What type of cloud would you associate with the phrase “mackerel sky”?
Cirrus clouds consist of:
A wintertime deposit that can affect the stability of vessels is:
Clouds that form “caps” over mountains are called:
Cirrus (Ci).
Cirrocumulus (Cc).
Ice crystals.
Slide 50: Some Chapter Questions (1 of 2)
ANIMATIONS
Five questions are displayed.
[CLICK] to display each answer in sequence. Font change used to simulate human handwriting.
After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer).
* * * * * * * * * * (end comments).
Glaze ice.
Lenticularis (or cap clouds).

51. Chapter 5 Questions QUESTION ANSWER
The reason the clear sky is blue is due to what fact?
Clouds in the west at sunset are red due to what fact?
Halos are caused by what?
Rainbows require:
A cyclone is an example of what size of meteorological scale?
The atmosphere preferentially scatters blue light.
The atmosphere scatters red light the least.
Ice crystals.
Slide 51: Some Chapter Questions (1 of 2)
ANIMATIONS
Five questions are displayed.
[CLICK] to display each answer in sequence. Font change used to simulate human handwriting.
After last answer displayed, one more [CLICK] to next screen (allows instructor pause for last answer).
* * * * * * * * * * (end comments).
Rain drops and direct sunlight.
Mesoscale.

52. End of Chapter 5 Are there any questions?
Chapter 6 covers Severe Weather.