1. CLOUDS PHYSICS: BASICS

2. What is a cloud?
An aggregate of very small water droplets, ice crystals, or a mix of both, with its base above the earth’s surface

3. WHAT IS A CLOUD?
It is assembly of tiny droplets numbering several 100 per cc
Radii about 10 μm
Whole population grows

4. WHY DO CLOUDS FORM ? Water is present everywhere in troposphere
Mixture of dry air and water vapour, subjected to upward motion, cools and expands as a result of decrease in pressure in upper atmosphere
Relative humidity increases
The rising air becomes fully saturated
Further cooling results in condensation of water vapour and cloud formation

5. WHY DO CLOUDS FORM ? Involves Four Processes and Four Sources
Source Process
Sun Heating
Moisture Evaporation
Instability Lifting/Convection
Nuclei Condensation

6. CONDENSATION NUCLEI
Condensation of water vapour may occur even before the relative humidity reaches 100%
In such cases, condensation is facilitated by minute particles such as dust, smoke, sea salt, etc., present in atmosphere
These particles have an affinity for water vapour (hygroscopic) and help faster condensation

7. CONDENSATION NUCLEI Most active condensation nuclei are:
Particles of sea salt
Combustion products containing sulphuric and nitrous acids
Such nuclei are numerous in atmosphere

8. SUPER COOLED WATER DROPLETS
Water droplets in atmosphere may be cooled below 0° C without freezing if remain undisturbed. Such droplets are called as super cooled water droplets
Cloud droplets commonly occur in super-cooled state at temperatures below –20°C and even sometimes – 40°C
Freeze only if they are disturbed or if they come in contact with foreign solid material

9. TRIGGERING MECHANISMS
The upward motion that generate clouds can be produced by
Convection
Convergence
Topography
Frontal Boundaries

10. TRIGGERING MECHANISMS

11. CONVECTION Atmospheric motions in the vertical direction
As the earth is heated by the sun, bubbles of hot air (called thermals) rise upward from the warm surface
Thermals cools as it rises and if conditions are favourable, will develop in to a cloud

12. CONVECTION
When a deep stable layer exists just above the cloud base, continued vertical growth is restricted
If a deep unstable layer (cold air aloft) is present, continued vertical growth is likely
Convective clouds are much more vertically developed than clouds generated by convergence lifting

13. CONVECTION
Stability must be considered: Stable air = Layer cloud Unstable air = Heap cloud

14. CONVERGENCE
Convergence is an atmospheric condition that exists when there is a horizontal net inflow of air into a region
When air converges along the earth's surface, it is forced to rise since it cannot go downward
Large scale convergence can lift a layer of air hundreds of kilometers across

15. TOPOGRAPHY
When air is confronted by a mountain, it is lifted up and over the mountain cools as it rises.
If the air cools to its saturation point, the water vapor condenses and a cloud forms
These types of clouds are called "orographic clouds”
The exact type of cloud depends upon the moisture content and stability of the air

16. FRONTAL BOUNDARIES
In the case of a cold front, a colder, denser air mass lifts the warm, moist air ahead of it
Due to the steep slope of a cold front, vigorous rising motion is often produced, leading to the development of showers and occasionally severe thunderstorms
In the case of a warm front, the warm, less dense air rises up and over the colder air ahead of the front
Warm fronts have a gentler slope and generally move more slowly than cold fronts, so the rising motion along warm fronts is much more gradual

17. CLASSIFICATION: BY LOOKS
Stratus - layer Cumulus - heaped (Convective) Nimbus - rain bearing Cirrus - fibrous Alto - middle-level

18. CLASSIFICATION: BY HEIGHT
High - Above 20,000 ft Km - 18 Km
Medium - 6,500 ft - 20,000 ft Km - 6 Km
Low - Surface - 6,500 ft Surface – 2 Km

19. CLASSIFICATION: BY TYPE
(GENERA)
High - Ci Cirrus Cs Cirrostratus CC Cirrocumulus
Medium - As Altostratus Ac Altocumulus
Low - St Stratus Sc Stratocumulus Cu Cumulus Cb Cumulonimbus
NS = Nimbostratus which can be regarded as medium or low

20. THE TEN TYPES
High Cirrus (Ci) Cirrocumulus (Cc) Cirrostratus (Cs) Medium Altocumulus (Ac) Altostratus (As) Nimbostratus (Ns) Low Cumulus (Cu) Cumulonimbus (Cb) Stratocumulus (Sc) Stratus (St)

21. Altitude of cloud base above ground (km)
THE TEN TYPES
Name
Altitude of cloud base above ground (km)
High clouds
Cirrus (Ci)
Cirrostratus (Cs)
Cirrocumulus (Cc)
6-18
Middle clouds
Altostratus (As)
Altocumulus (Ac)
2-6
Low clouds
Stratocumulus (Sc)
Stratus (St)
Nimbostratus (Ns)
0-2
0-4
Clouds with vertical development
Cumulus (Cu)
Cumulonimbus (Cb)
0-3

22. THE TEN TYPES 1 2 3 4 5 6 10 7 9 8

23. STRATUS Lowest cloud Grey cloud layer with a fairly uniform base
Give rise to drizzle, rain
If the sun is visible through the cloud, its outline is clearly discernible
Does not produce any halo phenomena

24. STRATOCUMULUS CLOUDS
A layer or patch composed of globular masses or rolls
Soft, white or grey, or both white and grey with dark parts, non – fibrous
Most of the regularly arranged small elements have an apparent width of > 5°

25. SUNRAYS THROUGH STRATOCUMULUS

26. STRATOCUMULUS IN CLEAR SKY

27. TOWERING CUMULUS Clouds with vertical development
Upper surface is dome shaped and exhibit rounded edges resembling cauliflower while the
base is horizontal
Sunlight parts of these clouds are mostly white and bases are relatively dark
Sometimes it is ragged

28. FAIR WEATHER CUMULUS

29. Cumuli form Clouds

30. Sunrays through Cu Clouds

31. Towering Cumulus becoming a CB

32. Cumulonimbus
These are heavy masses of the clouds with great vertical development
Cumuliform summits rise in the form of mountain or tower
Upper parts having a fibrous texture and often spreading out in the form of anvil

33. CB Approaching Over Head

34. Altocumulus spreading out of Cumulus
a layer of patches composed of flattened globular masses, rolls, etc
White or grey, or both white and grey
Generally with shading, which may or may not be merged
Most of the regularly arranged small elements have an apparent width of between 1° and 5°

35. Altocumulus spreading out of Cumulus

36. Altocumulus Clouds

37. Altocumulus Clouds

38. Altostratus It is a fibrous veil more or less grey or bluish in colour
Totally or partially covering the sky
Sun or moon vaguely seen through it with a faint gleam as seen through ground glass

39. Nimbostratus A thick rainy layer of dark grey clouds
Nearly uniform, feebly illuminated seemingly from inside
Mostly dark grey in colour
Severe weather producing nature
Not usually seen in the tropics

40. Cirrus as filaments
Detached clouds in the form of white, delicate filaments or patches or narrow bands
Clouds have fibrous (hair like) appearance, or a sheen or both
Transparent in nature
Does not produce weather

41. Sheet of Dense Cirrus

42. Cirrocumulus
Composed of small white flakes or very small globular masses without shades
Arranged in groups or lines or more often in ripples resembling those of sand on the sea shore
Most of the element have an apparent width of less than 1°

43. Cirrostratus
Transparent, whitish cloud veil of fibrous or smooth appearance
Totally or partly covering the sky

44. Cirrostratus as a thin sheet

45. MEASUREMENT Cloud Amounts - are measured in Oktas
Cloud Heights - are given as the height of the base of the cloud above ground level

46. HEIGHT MEASUREMENT Ceiling balloon method (for day time obs)
Laser Range Finder
Ceilometer

47. PRECIPITATION

48. PRECIPITATION
Precipitation is any form of water particles, liquid or solid that fall from sky and reach ground
Does not include clouds, fog, dew, virga
Precipitation can be long lasting and steady, or brief and intense
Rain, sleet, freezing rain and hail are all forms of precipitation
It is measured using instruments called Rain gauges in units of millimeters

49. PRINCIPLE OF PRECIPITATION
Water is present in some degree everywhere in the troposphere
When a mixture of dry air and water vapour, is subjected to upward motion it cools as the result of the expansion
RH increases.
A stage is reached when the rising air becomes fully saturated
Any further cooling would result in condensation of water droplets

50. PRINCIPLE OF PRECIPITATION

51. CONDENSATION NUCLEI Nuclei Size (Radius in µm) Aitken Radius <0.1
Large 0.1<Radius<1.0
Giant Radius>1.0
Large and giant nuclei lay their first claim to the available water
Large nuclei are more than giant nuclei
Larger the number of condensation nuclei, smaller will
be the drop size
Drops falls towards ground when they no longer supported by force of buoyancy and updrafts
1 micron = 10-6 meter = mm

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