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Atmospheric Moisture (chapter 4)

Atmospheric Moisture (chapter 4). Water vapor (p. 84-93) Cloud formation (p. 93-98) Cloud classification (p. 101-111) Stability and clouds dry adiabatic processes (p. 70-74) moist adiabatic processes (p. 98-101) chinook (p. 122-123) Precipitation (p. 112-121) warm cloud precip growth

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Atmospheric Moisture (chapter 4)

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  1. Atmospheric Moisture(chapter 4) Water vapor (p. 84-93) Cloud formation (p. 93-98) Cloud classification (p. 101-111) Stability and clouds dry adiabatic processes (p. 70-74) moist adiabatic processes (p. 98-101) chinook (p. 122-123) Precipitation (p. 112-121) warm cloud precip growth cold cloud precip growth precip types

  2. Water is the source of all life on earth. The distribution of water is quite varied

  3. Water vapor is not uniformly distributed in the atmosphere Water vapor channel, GOES 8 dry humid

  4. Water (and only water!) exists in all 3 phases on earth: solid (ice), liquid or gas (water vapor)

  5. topics • The three phases of water • The hydrologic cycle • How to measure atmospheric moisture • How water vapor varies on Earth • Humidity and human comfort

  6. 1. the three phases of water

  7. questions • Is water vapor present in the air at temperatures below the boiling point? • Does evaporation occur from a pot of water, before it boils? Yes Yes

  8. Water vapor is present in the atmosphere at any temperature. open container, unsaturated air: evaporation > condensation

  9. saturation closed container: evaporation = condensation dynamic equilibrium

  10. The maximum water vapor concentration increases exponentially with temperature

  11. Phase Changes:Gain/Loss of Heat by the Air Process Changes Heat gained/lost From To Condensation vapor liquid 2500 J/g Evaporation liquid vapor -2500 J/g Freezing liquid ice 333 J/g Melting ice liquid -333 J/g Deposition vapor ice 2833 J/g Sublimation ice vapor -2833 J/g note: the specificheatofwater is 4.186 J/(g °C) [that is 1 cal/(g °C) ]

  12. the hydrologic cycle and global climate Evaporation (& melting) = cooling Condensation (& freezing) = heating This condensation heating is a major source of energy for the global circulation. Recall the equation R = H + LE

  13. Evaporation uses energy (LE)

  14. Pop quiz : sublimation is the transition from … to … • vapor  liquid • liquid  vapor • ice  vapor • liquid  ice

  15. 2. The hydrological cycle …some team work • Sketch the hydrologic cycle • Do not look at your textbook • Draw land, oceans, etc, and show how water is moved around • Write down your guesses of what fraction of the global water is held … • In the oceans ( ..%) • In ice caps & glaciers ( ..%) • In lakes & rivers ( ..%) • In the atmosphere ( ..%) • Also guess what the global mean rainfall is (inches per year) • Over the ocean, evaporation … precipitation (fill in > or <) • Over land, evaporation … precipitation (fill in > or <)

  16. the Hydrologic Cycle

  17. answer • The oceans contain 97.5% of the earth's water, • Ice accounts for 2.1% • Lakes and rivers is 0.3% • the atmosphere less than 0.001%. • if all the water vapor in the global atmosphere were to condense and rain out at once, you ‘d have 1’’ of rain (precipitable water)

  18. The Hydrologic Cycle Rivers carry water from land to oceans. What does this imply? • there must be more precip than evaporation over land • net transport of water vapor from ocean to land

  19. The Hydrologic Cycle • Water vapor links the surface to the atmosphere • Water vapor concentrations are extremely variable • The hydro cycle is closely tied to atmospheric circulation patterns.

  20. Relative Storage • The global mean annual precipitation P is about 40’’, • i.e. about 40 times the precipitable water PW • What is the average residence time of water vapor in the atmosphere ? • RT = reservoir content / flux =  water is rapidly recycled though the atmosphere.

  21. Residence time of H2O molecules in the hydrologic cycle • Atmosphere 9 days • Rivers (speed ~1m/s) 2 weeks • Soil Moisture 2 weeks - 1 year • Largest lakes 10 y • Shallow groundwater (speed ~1-10 m/d) 10s - 100s y • Mixed layer (~150m) of oceans 120 y • Global oceans (avg depth 4 km) 3000 y • Deep groundwater up to 10,000 y • Antarctic icecap 10,000 y - more

  22. 3. Water vapor variables • The three phases of water • The hydrologic cycle • How to measure atmospheric moisture • How water vapor varies on Earth • Humidity and human comfort • Mixing ratio • Precipitable water • Vapor pressure • Saturation vapor pressure • Relative humidity • Dewpoint • Wet-bulb temperature

  23. The mean vertical distribution of mixing ratio in the atmosphere Mixing ratio: grams of water vapor per kg of air Nearly half the total water in the air is between sea level and about 1.5 km above sea level. Less than 5-6% of the water is above 5 km, and less than 1% is in the stratosphere.

  24. Terminology: mixing ratio and PW Mixing ratio Definition: the ratio of the mass of water vapor in a sample to the total mass of the air Units: g/kg The mixing ratio is conserved. Precipitable Water The PW is the vertically integrated amount of mixing ratio. (global mean = 1’’) Units : water depth (mm or inches)

  25. The higher the temperature, the greater the number of water molecules the air can hold. Saturation and temperature saturation vapor pressure, mb Clausius-Clapeyron equation temperature, ºF

  26. Terminology: Vapor Pressure The partial pressure of a given sample of moist air that is attributable to the water vapor is called the vapor pressure. Units: mb Distinguish the actual from the saturation vapor pressure.

  27. Pop quiz: vapor pressure is … • the weight of water vapor • the partial pressure of water vapor molecules in the air • the concentration of water vapor, expressed as a mass of water vapor per mass of air • the density of water vapor, expressed as a mass of water vapor per volume of air

  28. Terminology:Saturation Vapor Pressure The vapor pressure necessary to saturate the air is the saturation vapor pressure. Saturation vapor pressure increases rapidly with temperature: the value at 90°F is about double the value at 70°F. 1013 mb What is the boiling point in Laramie? Why is it different? mb 212ºF (100 ºC)

  29. saturation vapor pressure and boiling point 1013 mb Laramie pressure: 780 mb mb Lower boiling point! 197 °F 212ºF (100 ºC)

  30. Boiling Point Boiling occurs when the SVP of escaping bubbles is greater than the total atmospheric pressure.

  31. psychrometric chart

  32. Relative humidity Saturation vapor pressure (mb) Wet bulb temperature (°F) vapor pressure (mb) dewpoint (°F)

  33. psychrometric chart

  34. Terminology: Relative Humidity Definition: RH = actual vapor pressure (%) saturation vapor pressure RH is a relative variable: it relates the actual amount to the amount that would saturate the air. Example: 50% RH means the air holds half the water vapor that it is capable of holding; 100% RH means the air holds all the water vapor it can. Because of the temperature dependence of the SVP, warm air has more water vapor than cooler air for a given RH.

  35. question Exercise: T = 15ºC, e = 10.2 mb determine the RH (a) graphically (b) calculated Exercise: T = 15ºC, e = 10.2 mb determine the RH (a) graphically (b) calculated RH = 100 e/esat RH = 100 e/esat

  36. answer Exercise: T = 15ºC, e = 10.2 mb determine the RH (a) graphically (b) calculated Exercise: T = 15ºC, e = 10.2 mb determine the RH (a) graphically (b) calculated 17 RH = 100 e/esat RH = 100 e/esat RH = 100 e/esat = 100 (10.2/17) = 60 %

  37. Relative humidity paradox:an air parcel with less water vapor can have a higher RH Solve this paradox! water vapor content inside 8 outside 4 18 F 70 F temperature

  38. Relative humidity paradox:the air outside is colder and has less water vapor than the air inside. But because the svp is so much lower at lower temperature, the RH is higher outside. water vapor content inside: 35% RH 8 outside: 80% RH 4 18 F 70 F temperature

  39. Explain the variation of surface RH on a quiescent day dewpoint

  40. Testing your understanding of relative humidity • Go to: http://profhorn.meteor.wisc.edu/wxwise/relhum/rhac.html

  41. Global RH near ground level http://ingrid.ldgo.columbia.edu/

  42. RH and dewpoint temperature RH=100%

  43. Dew point temperatures • The dew point temperature is the temperature the air would have if it were cooled, at constant pressure and water vapor content, until saturation. • The difference between the actual temperature and the dew point is called the dew point depression, a measure of relative humidity.

  44. psychrometric chart 11 Find the dewpoint

  45. psychrometric chart the dewpoint is: Td = 8ºC What is the dewpoint depression? 8

  46. the dewpoint depression is : T - Td = 15-8 = 7ºC Question: if the vapor pressure increases, how does the dewpoint change? 8

  47. The dewpoint increases when the vapor pressure increases new vapor pressure e Td

  48. determine the dewpoint depression (°F)in Laramie

  49. determine the dewpoint depressionin Laramie now

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