Clouds S.K. Satheesh Centre for Atmospheric & Oceanic Sciences Indian Institute of Science

1. Clouds S.K. Satheesh Centre for Atmospheric & Oceanic Sciences Indian Institute of Science Bangalore.

2. General Features Cloud Condensation Nuclei Cloud Droplets and Rain Droples Detection of Clouds using Satellites

3. What is a cloud? There are lots of drops of water floating together in the air. We can’t see each drop, but we see them together as a cloud.

4. How do clouds form? • First there is a body of water. This could be a lake, ocean, or river. • Sun warms up the water from the lake or ocean, and turns it into water vapor.

5. Water Vapor • Water vapor rises into the air. It then forms tiny drops of water.

6. Clouds • These tiny drops join together to make clouds.

7. Rain • The drops get so big that they can no longer stay in the air. Then they fall down to earth as Rain. • Rain comes down to earth again, and the cycle starts all over. • This is calledHYDROLOGICAL CYCLE.

8. Now What? • Why some clouds are white and puffy? • Why some are producing rain while others are not? • Why some are dark?

9. There are many different types of clouds. Clouds are classified by their shape, altitude and characteristics.

10. Clouds are visible aggregates of minute water droplets or tiny crystals of ice. Clouds are classified on the basis of their appearance and height. There are three basic cloud types. Cirrus, Stratus, and Cumulus. Cirrus clouds are high, white and thin. The cumulus consists of individual cloud masses with large vertical extent. Stratus clouds are sheets or layers that cover much or all of the sky.

11. Cloud Classification Clouds Cirrus Cumulus Stratus

12. Cloud Types Height Type High clouds: Cirrus above 6 kms Cirrocumulus Cirrostratus Medium Clouds: Altostratus 2 - 6 kms Altocumulus Low Clouds: Stratocumulus below 2 kms Stratus Nimbostratus Clouds with Cumulus vertical extent Cumulonimbus

13. Because of low temperature and small quantities of water vapour available at high altitudes, all of the high clouds are thin and white and are made of ice crystals. Since most of water vapour is available at lower altitudes, middle and low clouds are denser and darker. Layered clouds generally indicates that the air is stable. This is because clouds would not grow vertical in stable air. Some clouds do not fit into any of the height categories. Such clouds have their bases in the low height range and often extend upward into the middle or high altitudes. These are referred to as clouds of vertical development. When upward air movement is intense, clouds of large vertical extent are formed. As the cumulus grow vertically, its top exceed the low altitude range and is called cumulus congestus. When it becomes even larger and rain begins, it is called cumulonimbus.

14. Cloud Classifications

15. Cloud Classifications

16. Cloud Classifications Types of Cumulus Clouds Cumulus Altocumulus Cumulonimbus Cumulus Congestus

17. Cirrus • Thin, delicate, ice-crystal clouds in the form of thin elements that allow sunlight to pass through them

18. Cirrus Clouds cirrus Fair weather cumulus

19. Cirrostratus • White ice crystal clouds that appear in the form of extensive sheets that may cover the whole sky. • Sometimes produce halos around the Sun

20. Cirrostratus Cloud Pictures

21. Cirrocumulus • High level cloud • Appears as small rounded puffs arranged in rows or sheets

22. Cirrocumulus Cloud Pictures

23. Stratus • Form in sheets or layers in the lower parts of the atmosphere. • Sometimes cover the sky completely. • May produce drizzle. • Fog is classified as stratus clouds

24. Stratus Cloud Pictures

26. Nimbostratus • Rain producing cloud • Varies in thickness and layers • Mostly occurring in a widespread sheet

27. Nimbostratus Cloud Pictures

28. Altostratus • Forms in the middle levels of the atmosphere • Always a sign of significant amounts of moisture in those layers • Ranging from a thin, white veil of cloud to a dense gray mantle

29. Altostratus Cloud Pictures

30. Cumulus Congestus • Cumulus clouds with great vertical extent with tops that resemble cauliflower • Their heights exceed the dimensions of their bases

31. Cumulus Congestus Pictures

32. Stratocumulus • Forms in layers • Usually has a ragged upper surface while base is flat • Stratocumulus clouds are the most common type of cloud

33. Stratocumulus Cloud Pictures

34. Altocumulus • Cumulus in the middle levels of the atmosphere associated with the lifting of a large air mass

35. Altocumulus Cloud Pictures

36. Fair Weather Cumulus

37. Cumulonimbus • Relatively large • Producing most of the damage associated with a thunderstorm • Can produce large hail, flash floods, severe wind gusts, wall clouds and tornadoes

38. Cumulonimbus Thunderstorm Clouds

42. Water can occur in 3 states: • Gas - water vapour (invisible) • Liquid - water droplets (visible) • Solid - ice crystals, hail, snow

43. How clouds are formed? • Clouds are formed of tiny droplets of water or ice. • Clouds form when water vapour cools and condenses. • The temperature at which condensation occurs is called dew point. • Condensation also requires nuclei (small particles) such as aerosols.

44. What happens when air is cooled? • Relative humidity increases. • Dew point is reached (100% relative humidity). • Water vapour (invisible) condenses to form water droplets (visible). • Sometimes the water vapour may change straight into solid state: called sublimation.

45. Necessary Conditions for Condensation: Condensation Occurs when water vapour condense to form liquid. The result of this process may be fog or clouds. For any form of condensation to occur, the air must be saturated. Saturation occurs by two ways. The first is when air is cooled below the dew point, which most commonly happens. The second is when water vapour is added to the air. Moreover there should be a surface on which water vapour may condense. When dew occurs, objects near the ground serve this purpose. When condensation occurs in the air above the ground, tiny bits of particulate matter known as condensation nuclei serve as surfaces for condensation of water vapour. The importance of these nuclei is that if they are absent, a relative humidity of about 400% is needed to produce clouds.

46. How air is cooled? During cloud formation and often in the formation of fog, the air is cooled to its dew point. Near the ground the heat is exchanged readily between ground and air above. This accounts for the cooling involved in the formation of some types of fog. However as air is a poor conductor of heat, this exchanges are not possible above few thousand meters. Thus some other mechanism must operate during cloud formation. When air moves upward it passes through regions of successively lower pressure. As a result, an ascending air parcel expands and cools adiabatically. Unsaturated air cools at a constant rate of 1C per every 100 meters of ascent.

47. Conversely, descending air comes under increasingly higher pressures, compresses and is heated 1C for every 100 meters of descent. This rate of cooling or heating only applies to an unsaturated air and is known as dry adiabatic rate. If the air ascents long enough, it will cool sufficiently to get saturated and to condense. From this point the latent heat stored in the water vapour will be released. Although the air will continue to cool continuously after condensation begins also, the released latent heat works against the adiabatic process, thereby reducing the rate at which air cools. This slower rate of cooling is called wet adiabatic rate.

48. -8C 5000 -3C 4000 2C Condensation level Altitude (m) 3000 12C 2000 1000 22C Surface 32C

49. Stability of Atmosphere: Stable: When lapse rate is less than wet adiabatic rate (~6K per km). Unstable: When lapse rate is less than wet adiabatic rate (~9.8K per km). Conditionally Stable: When lapse rate is between wet and dry rates

50. How Air Ascents? Stable air will not ascent by its own. There are mainly three mechanisms to trigger vertical movement. (1) Convergence (2) Orographic lifting (3) Frontal lifting