Chapter 7 Water & Atmospheric Moisture

1. Chapter 7 Water & Atmospheric Moisture

2. Hydrological Cycle

3. Hydrogen Bonding Water is a polar or charged molecule + charge on the hydrogen side & a - charge on oxygen polar covalent bonds hydrogen bonding is the attraction between oxygen and hydrogen atoms on adjacent molecules Credit: http://bio.winona.msus.edu/berg/ILLUST/H-bond5.gif

4. Phenomena due to hydrogen bonding surface tension – cohesive force that pulls a surface closer together – water has highest surface tension capillary action - ascent of water up a tube thanks to the ‘pull’ of water molecules on one another

5. Phenomena due to hydrogen bonding Solvent – Water is an excellent solvent as it can electrically “surround a salt” http://www.brooklyn.cuny.edu/bc/ahp/SDPS/SD.PS.water.html

6. 720 calories required --> <-- 720 calories released

7. Latent Heat Exchanges in the Environment

8. Show water phases shockwave

9. Measures of Humidity humidity - refers to water vapor in air relative humidity - ratio of the water vapor content of air to the maximum possible water vapor content at that temperature dew point temperature - temperature at which air is saturated with water vapor specific humidity - mass of water vapor (g) divided by the mass of air (kg) – independent of changes in pressure, volume, and temperature

10. water vapor capacity is set by air temperature

11. rule of thumb - slope is ~ 1.5 mbar/°C in typical ambient range How much water can air hold? think of this like the amount of sugar you can dissolve in coffee or tea before it precipitates out saturation vapor pressure is the maximum amount of water air can hold

12. air in the tropics can hold about 10x as much water vapor as air at the poles the saturation vapor density (or pressure) roughly doubles for every 10 C increase Figure Credit: “Earth’s Climate” by W. Ruddiman

13. Specific & Relative Humidity RH = (SH/max SH)*100 RH = (20/28)*100 = ~71%

14. Diurnal Variations in Relative Humidity temperature & RH can be anti-correlated -if the specific humidity is constant & daily heating occurs

15. Dew Point if dew point is close to the ambient air temperature, relative humidity is very high

16. Dew Point Temperature air is saturated when its water vapor content equals its water holding capacity (RH = 100%) this temperature is the dew point RH < 100% Tdew

17. Measuring Relative Humidity hair absorbs water vapor and this increases its length think about how your hair gets in very humid climates this system works so well that it was used for over 200 years!

18. Measuring Relative Humidity II the principles of sling psychrometry sling psychrometer 2 thermometers side by side one is a ‘dry-bulb’ thermometer which measures ambient temp. would the wet bulb temperature be more depressed in a dry or a wet environment? other is a ‘wet bulb’ thermometer that is wetted by a wick - twirling drives evaporation and depression of temp.

19. Atmospheric Stability think of a packet or parcel of air to describe a mass with certain humidity and temperature characteristics stability refers to the tendency of an air parcel to rise or sink

20. air rises because pressure decreases with height - rising air expands and does work, releasing heat This is called adiabatic heating

22. DAR – Dry Adiabatic Rate

23. DAR – Dry Adiabatic Rate

24. Different kinds of lapse rates condensation releases heat (~2500 J/g at 0°C) this is the environmental lapse rate we discussed MAR ~ -6°C/km Avg lapse rate ~ -6.5°C/km Altitude (km) DAR ~ -10°C/km Air temperature (Tair) DAR - dry adiabatic rate MAR - moist adiabatic rate

25. dry adiabatic rate (DAR) – the rate at which dry air temperature decreases with height moist adiabatic rate (MAR) – the rate at which saturated air temperature decreases with height environmental lapse rate (ELR) - the local, background change in air temperature with height unstable air - describes an air parcel that is more buoyant than its surrounding air (will rise) stable air - describes an air parcel that is the same temperature or colder than surrounding air (will fall or stay at the same elevation)

26. we think of three ‘regimes’ of stability rising parcel cools faster than surrounding air (it stays more dense) rising parcel cools more slowly than surrounding air (it stays more buoyant)

27. Unstable air buoyancy>gravity surrounding air cools at 12 °C/km

28. Stable air buoyancy<gravity surrounding air cools at 5 °C/km

29. buoyancy>gravity if MAR buoyancy<gravity if DAR Conditionally unstable air surrounding air cools at 7 °C/km

30. Shockwave of stability

31. Precipitation across the Sierras Credit: S. Booth, Sierra College http://geography.sierra.cc.ca.us/booth/Physical/chp6_precip/oro_precip.jpg

32. MAR Elevation (km) DAR LCL DAR Temperature what about temperature gradients? cross section of the Sierras 9 C 2 km Elevation (km) 19 C 1 km air is compressed and is heated during descent 15 C 25 C 29 C air temperature on the east side is greater DAR MAR DAR DAR lapse rate type LCL = lifting condensation level - the height at which an air parcel reaches 100% RH and condensation occurs

33. Cloud formation what are clouds? aggregations of suspended microscopic water droplets and ice crystals how do clouds form? air must be saturated with water vapor (RH = 100%) and there must be microscopic nuclei for the vapor to condense onto

34. a typical raindrop is 2 mm in diameter, or ~a tenth of an inch

35. How much does a cloud weigh? this cloud weighs ~1000 metric tons (106 kg) how much energy was released when water vapor condensed into all these cloud droplets? condensation releases 585 calories/g or ~2500 J/g This is 2.5 trillion Joules!! (for comparison, a 100 W light bulb consumes only 360,000 Joules in an hour)

36. raindrop formation

37. snow formation

38. Cloud Types and Identification clouds are flat (stratus), puffy (cumulus), or wispy (cirrus) horizontal, layered clouds are stratiform vertically developed clouds are cumuliform high, wispy clouds are cirroform

40. altocumulus clouds

41. stratus clouds

42. Cirrus clouds

44. Cumulus clouds

45. Cumulonimbus clouds

46. photo of a cumulonimbus cloud taken from the space shuttle

47. high level winds shear the tops of these clouds, giving them an anvil appearance

48. Fog types and fog formation fog is clouds in contact with the surface if you are right in the middle of a cloud/fog bank, what is the relative humidity? ok, what does this tell you about the ambient temperature and the dew point temperature? fog is confined to a restricted layer - this is because it is ‘capped’ by a thermal inversion there are, of course, different kinds of fog, but the basic mechanism of fog formation is the same

49. AdvectionFog this is an especially common type of fog along the CA coast Santa Barbara locals refer to the early summer fog banks as the “June gloom” the name gives a big hint about how this kind of fog is formed - remember that advection is simply horizontal mixing of air in response to pressure gradients think about what would set up a horizontal pressure gradient between the coast and offshore air

50. warm, moist air offshore is pulled to the coast and fog condenses over the cold waters of the CA current along the CA coast, most of the fogbanks are advection fog - as the central valley warms up, air rises and a pressure gradient is established