1. Lecture DS1 Condensation of water as droplets in the atmosphere: How do clouds form? David Stevenson, Crew 314, dstevens@staffmail.ed.ac.uk

2. Ahrens ‘Meteorology Today’ 9 th Ed. Chapter 5 p 113 ‘Condensation nuclei’ Chapter 7 p166-168 ‘Precipitation processes’ For more details of the physics, see Wallace & Hobbs, Chapter 6

3. z T What happens at the lifting condensation level? How does saturated water vapour convert to liquid water droplets? Lifting condensation level Lift an ‘air parcel’ from the surface - initially cools at the ‘dry adiabatic lapse rate’ (red line) - reaches the ‘lifting condensation level’ - then cools at the ‘saturated adiabatic lapse rate’ (blue line)

4. How do water droplets form? Two theoretical routes: 1.Homogenous nucleation Pure water drops form (overcome surface tension effects) and then grow by further condensation 2.Heterogeneous nucleation Water condenses on existing particles (aerosols – e.g. sea-salt, other chemical compounds, both natural and man-made), which then grow Which route is correct? (or both/neither?)

5. Homogeneous Nucleation Chance collisions of H 2 O molecules – more likely as vapour pressure increases Embryonic water droplet – large enough to remain intact 1. When a droplet forms, latent heat of condensation is released. 2. However, energy is required to overcome the surface tension needed to hold a drop together. If energy from (1) > (2) then drop forms & grows; if (2) > (1) drop shrinks

6. Condensation in the atmosphere Saturation or more properly, equilibrium, vapour pressure is established when the number of water vapour molecules entering and leaving a water surface is about the same. We defined this in terms of a plane water surface. SVP is different over a curved water surface since it is easier for molecules to leave the water drop since there are fewer H bonds binding them to each other.

7. More like a plane surface Easier for molecules to escape to vapour phase, i.e. harder to form smaller and smaller liquid droplets

8. vapour pressure temperature unsaturated saturated equilibrium about the same number of molecules are re-entering water surface as are escaping = equilibrium. For a curved surface of a droplet, get higher vapour pressure for same temperature So air needs to be super-saturated (with respect to a plane water surface) to be in equilibrium with a droplet

9. Convenient diameter sizes to remember: condensation nucleus 0.2  m, droplet 20  m, raindrop 2000  m. Typical raindrop 2000  m Typical cloud droplet 20  m condensation nuclei 0.2  m The important point to note is that small cloud droplets have a greater curvature and hence have a greater rate of evaporation.  to stop them evaporating away, smaller droplets require an even greater vapour pressure (i.e. higher supersaturation).

10. So what level of supersaturation do you need to produce a condensation nuclei (0.2  m) of pure water by homogeneous nucleation?

11. Homogeneous Nucleation Chance collisions of H 2 O molecules – more likely as vapour pressure increases Embryonic water droplet – large enough to remain intact 1. When a droplet forms, latent heat of condensation is released. 2. However, energy is required to overcome the surface tension needed to hold a drop together. If energy from (1) > (2) then drop forms & grows; if (2) > (1) drop shrinks

12. Droplet always shrinks System will always tend to minimise  E Droplet shrinks Droplet grows Unstable equilibrium radius  E = Change in energy when a droplet of radius R forms Air at less than 100% humidity cannot keep pure water drops: they evaporate and shrink Supersaturated air can grow droplets if they are large enough to overcome surface tension

13. States of equilibrium StableUnstableNeutralConditional instability

14. The curvature effect Small droplets (0.2  m) require supersaturations on the order of 1% (i.e. RH=101%) to start growing - but this level of supersaturation is rarely seen – suggests that homogenous nucleation won’t work. Cloud droplets grow Cloud droplets shrink Cloud droplets on the line are in unstable equilibrium

15. Condensation nuclei To form the smallest droplets of pure water by direct condensation (homogeneous nucleation) requires super- saturations of well over 100%. This never happens. There may be 10 3 - 10 9 such nuclei per cm 3 with radii <= 0.2  m. These particles (aerosol particles) may be salt from sea spray, or particles from pollutant emissions Instead these droplets are formed by condensation around existing small particles (heterogeneous nucleation) which are called condensation nuclei (Aitken nuclei).

16. Asian Brown Cloud - Biomass burning from forest fires, vegetation clearing and fossil fuel burning and ‘dirty’ industries produce particulate material for the shrouding haze, perhaps 3 km thick. Kuala Lumpur Brown cloud reaches up to top of Himalayas London smog

17. Volcanic ash – volcanoes also release SO 2, which oxidises to H 2 SO 4

18. Natural sources: Fires (not always natural) Volcanoes – SO 2 Layer of sulphuric acid aerosol in the stratosphere from Mt. Pinatubo, 1991

19. Hemel Hempstead, Dec. 2005 Black carbon aerosol from incomplete combustion

20. Buncefield oil depot fire, 2005

23. Satellite image at 1045 GMT, ~5 hours after start of fire

24. Top of BL

25. Nottingham Skew T-log P on day of Buncefield Fire (12Z, 11 Dec 2005)

26. Aerosol properties Hydrophobic Neutral Hydrophilic (or hygroscopic – same thing) –to act as a cloud condensation nuclei, the aerosol must be hydrophilic – i.e. have an affinity for water –often also soluble (e.g., NaCl, H 2 SO 4 )– presence of a dissolved solute lowers the equilibrium vapour pressure Greek for ‘water’Greek for ‘wet’

27. Some nuclei are hygroscopic and attract water molecules very strongly, especially while the water droplets are still small and there is still a high concentration of chemicals from the original nucleii (eg salt in the forming water droplet). The salt ions bind closely to water vapour molecules, making it more difficult for them to evaporate – hence lower equilibrium vapour pressures over impure water droplet. This is the Solute Effect and permits condensation at relative humidity < 100 %.

28. Summary There are 2 theoretical paths for water to condense to form sub-micron droplets Homogeneous nucleation can be ruled out because it requires super-saturation levels much higher than observed Heterogeneous nucleation, where existing particles form the ‘core’ of droplets, is always the path followed. The pre-existing particles are aerosol particles, formed from natural and man-made emissions, and are everywhere Hygroscopic aerosol particles are particularly effective As drops grow, the physics of homogeneous nucleation becomes more relevant in determining droplet size

29. http://www.metoffice.gov.uk/weather/uk/surface_pressure.html

30. Analysis 0000 Friday 30 Oct

32. Forecast 0000 31 Oct

33. 1200 Saturday 31 Oct

34. 0000 Sunday 01 Nov

35. 1200 Sunday 01 Nov