Lecture DS1 Condensation of water as droplets in the atmosphere: How do clouds form?

1. Lecture DS1Condensation of water as droplets in the atmosphere:How do clouds form? David Stevenson, Crew 314, dstevens@staffmail.ed.ac.uk

2. Ahrens ‘Meteorology Today’ 9th 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 What happensat the liftingcondensation level? How does saturated water vapour convert to liquidwater droplets? Liftingcondensationlevel T • 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: • Homogenous nucleation • Pure water drops form (overcome surface tension effects) and then grow by further condensation • 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. Embryonic waterdroplet – largeenough to remainintact Homogeneous Nucleation Chance collisions ofH2O molecules –more likely as vapourpressure increases 1. When a droplet forms, latent heat of condensation is released. 2. However, energy is required to overcome the surface tension neededto 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. about the same number of molecules are re-entering water surface as are escaping = equilibrium. vapour pressure equilibrium saturated For a curved surface of a droplet, get higher vapour pressure for same temperature unsaturated 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 mm, droplet 20 mm, raindrop 2000 mm. Typical raindrop 2000 mm Typical cloud droplet 20 mm The important point to note is that small cloud droplets have a greater curvature and hence have a greater rate of evaporation. condensation nuclei  to stop them evaporating away, smaller droplets require an even greater vapour pressure (i.e. higher supersaturation). 0.2 mm

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

11. Embryonic waterdroplet – largeenough to remainintact Homogeneous Nucleation Chance collisions ofH2O molecules –more likely as vapourpressure increases 1. When a droplet forms, latent heat of condensation is released. 2. However, energy is required to overcome the surface tension neededto hold a drop together. If energy from (1) > (2) then drop forms & grows; if (2) > (1) drop shrinks

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

13. States of equilibrium Stable Unstable Neutral Conditionalinstability

14. Cloud droplets on the lineare in unstable equilibrium The curvature effect Cloud droplets grow Cloud droplets shrink Small droplets (0.2 mm) 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.

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. Instead these droplets are formed by condensation around existing small particles (heterogeneous nucleation) which are called condensation nuclei (Aitken nuclei). There may be 103 - 109 such nuclei per cm3 with radii <= 0.2 mm. These particles (aerosol particles) may be salt from sea spray, or particles from pollutant emissions

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. London smog Brown cloud reaches up to top of Himalayas Kuala Lumpur

17. Volcanic ash – volcanoes also release SO2, which oxidises to H2SO4

18. Natural sources: Fires (not always natural) Volcanoes – SO2 Layer of sulphuricacid 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, H2SO4)– 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