CLOUDS PHYSICS: BASICS

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 • SourceProcess • 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 6 Km - 18 Km • Medium- 6,500 ft - 20,000 ft 2 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 mediumorlow

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

21. THE TEN TYPES

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