1. Wednesday, 9/16/091 ATMO 1300-006 Class #8 Wednesday, September 16, 2009 Chapter 4, Water in the atmosphere

2. Saturation vapor pressure depends only on Temperature Wednesday, 9/16/092

3. Table 4-1, p. 92 3Wednesday, 9/16/09

4. Condensation In the lab with perfectly clean air (no aerosol) takes a relative humidity of more than 200%. RH > 100% is supersaturation Condensation is inhibited by the curvature effect Small, very curved droplets have molecules with few neighbors, and are quick to evaporate Wednesday, 9/16/094

5. Fig. 4-7, p. 93 5Wednesday, 9/16/09

6. Condensation in the atmosphere Is inhibited by the curvature effect Is enhanced by the solute effect Some aerosol, salt particles for example, dissolve and have the ability to hold on to water molecules and suppress evaporation Other aerosol particles form nuclei, or small surfaces for condensation Wednesday, 9/16/096

7. Condensation in the atmosphere (continued) A cloud nucleus gives water molecules more neighbors, by acting like a small flat surface There are always abundant cloud condensation nuclei in the atmosphere— dust, salt, pollen, pollutants The solute effect permits condensation at RH < 100%. This is called haze Wednesday, 9/16/097

8. Condensation in the atmosphere (continued) Supersaturation (RH > 100%) does not occur in the atmosphere When the relative humidity reaches 100%, cloud particles form. Cloud at the surface is called fog. Fog reduces visibility to less than 1 km or 0.6 miles Heavy fog is a travel hazard Wednesday, 9/16/098

9. Frozen dew When the dew point is above the freezing point of water and cooling occurs, dew forms when the temperature reaches the dew point Sometimes further cooling occurs after the dew forms If the temperature falls below 0°C, the dew can freeze. When liquid dew freezes, it is called frozen dew Wednesday, 9/16/099

10. Frozen dew (continued) Frozen dew is also called “black ice” It is a major traffic hazard It also causes slips and falls for people on foot Frozen dew is hard to see Frozen dew frequently forms on roads where there is a significant slope, as well as bridges and overpasses Wednesday, 9/16/0910

11. Fog in a city Fig. 4-8, p. 95 11Wednesday, 9/16/09

12. Fig. 4-9, p. 96 12Wednesday, 9/16/09

13. Radiation fog forms at night in valleys due to cooling Fig. 4-10, p. 97 13Wednesday, 9/16/09

14. Advection fog: warm moist air and a cool surface current Fig. 4-11, p. 97 14Wednesday, 9/16/09

15. Steam fog/evaporation fog: cold air and warm water Fig. 4-12, p. 98 15Wednesday, 9/16/09

16. Ice in clouds Deposits (deposition) on small particles called ice nuclei There is a scarcity of ice nuclei at high temperatures (near but below 32ºF) Many water droplets do not freeze at subfreezing temperatures. This is called supercooling. Below -40ºC (or F), all water drops freeze Wednesday, 9/16/0916

17. Ice in clouds (continued) Ice takes on different crystal shapes in clouds, depending on temperature and supersaturation Clouds are saturated with respect to water, supersaturated with respect to ice The saturation vapor pressure over ice is less than that over water There can be ice fog (inland Alaska) Wednesday, 9/16/0917

18. Fig. 4-30, p. 112 18Wednesday, 9/16/09

19. Fig. 4-35, p. 117 19Wednesday, 9/16/09

20. How clouds form above the surface: lifting and cooling Fig. 4-13, p. 99 20Wednesday, 9/16/09

21. Fig. 4-14, p. 100 21Wednesday, 9/16/09

22. Fig. 4-15, p. 101 22Wednesday, 9/16/09