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GY205 Weather and Climate

GY205 Weather and Climate. Lecture 3. Moisture in the Atmosphere. Evaporation, Condensation, and Saturation. Air is saturated when evaporation and condensation are in equilibrium. Saturation Vapor Pressure. Pressure exerted by water vapor when the air is saturated.

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GY205 Weather and Climate

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  1. GY205 Weather and Climate Lecture 3

  2. Moisture in the Atmosphere

  3. Evaporation, Condensation,and Saturation • Air is saturated when evaporation and • condensation are in equilibrium

  4. Saturation Vapor Pressure • Pressure exerted by water vapor when the • air is saturated

  5. Temperature and Water Vaporin Saturated Air • As temperature increases, the amount of water • vapor in saturated air also increases

  6. Specific Humidity • SH is the mass of water vapor present in a given mass of air • SH is unaffected by temperature

  7. Relative Humidity • Relative humidity – amount of water vapor present compared to the amount that COULD be present at a given temperature • RH = SH / saturation SH x 100 • RH is expressed as a % • RH tells you how close the air is to being saturated, 100% RH = saturation • To change RH, change temperature and/or quantity of water vapor present

  8. Temperature Affects Relative Humidity • With SH held constant: • Cooling the air increases RH • Warming the air decreases RH

  9. Specific Humidity and Relative Humidity • With temp. held constant: • Adding water vapor will increase RH • Removing water vapor will decrease RH

  10. Humidity and Comfort • In the winter we warm the air, lowering RH indoors and have to use a humidifier or we get dry skin • In the summer, we cool the air, raising RH and the excess moisture is removed by the air conditioner • Basements are often damp because they are cooler and thus have a higher RH than the rest of the house, so we use a dehumidifier to lower RH

  11. Dew Point Temperature • The dew point is the temp. at which the air will become saturated • Always < or = to air temp. • The closer the air temperature is to the dew point, the closer the air is to saturation, and the higher the RH • Dew point >70°F is muggy; ~50°F is comfortable; <30°F is dry

  12. Average July Dew Point Distribution

  13. Average January Dew Point Distribution

  14. If the air cools to the dew point temperature: • RH becomes 100% (the air is saturated) • Condensation will occur on any surface cooled to the dew point of the surrounding air • Dew will form (cold-can, fogged bathroom mirror) • If dew point is <32°F, frost will form by deposition

  15. Mythbuster • Dew does not “settle” onto a surface, it forms directly on the surface • Frost is not frozen dew, it forms by deposition directly from water vapor

  16. Remember Phase Changes? • Evaporation absorbs heat • Condensation releases latent heat

  17. How Refrigerators/Dehumidifiers Work • Compressor (B) raises pressure of refrigerant gas, causing it to heat up. • Gas flows through coils, dissipating heat. As at cools, it condenses to liquid. • Liquid passes through expansion valve (C) into low-pressure environment, causing evaporation. Gas passes through coils inside the frig, absorbing heat, lowering temp. • Gas moves back to compressor to repeat the cycle.

  18. Processes that Change Air Temp. • Diabatic processes – heat is added to or removed from a system • Adiabatic processes – temperature changes without adding or removing heat • Adiabatic processes occur as a result of the compression or expansion of a gas • Compression increases temp., expansion lowers temp.

  19. Adiabatic Processes in the Atmosphere • Adiabatic processes are responsible for forming clouds, a type of fog, and some wind systems

  20. Adiabatic Lapse Rates • A parcel of air rises at the DALR until it is cooled • to the dew point, above that it rises at the WALR

  21. Measuring Relative Humidity and Dew Point • Sling Psychrometer – evaporative cooling on the wet bulb lowers its temp • Drier air allows more evaporation thus a greater wet bulb depression indicates lower RH • Psychometric tables used to convert readings to RH and dew point • (They got the diagram wrong!)

  22. Hair hygrometer – hair expands and contracts in response to changes in RH (bad hair days)

  23. Clouds and Fog • Clouds and fog are composed of tiny droplets of water suspended in the air • When air is cooled to the dew point, condensation occurs, and liquid water forms on condensation nuclei • Condensation nuclei are hygroscopic aerosols (natural and anthropogenic) • Condensation nuclei are always abundant

  24. Mythbuster • Clouds are made of liquid water droplets, NOT water vapor. Water vapor is an invisible gas.

  25. Condensation Nuclei, Cloud Droplets, and a Raindrop

  26. Types of Fog • Fog is basically a ground level cloud • Radiation fog – forms due to overnight radiational cooling • The ground cools, and lowers the temp of the air directly above it to the dew point

  27. Advection fog – warm, humid air crosses a cold surface, loses heat, and is cooled to its dew point

  28. Steam fog – forms when cold air moves over warmer surface, usually water • Evaporating water rises and is cooled by the cold air to its dew point

  29. Upslope fog – air is forced uphill, and cools adiabatically to its dew point

  30. Precipitation fog – rainfall raises relative humidity to saturation due to evaporation and cooling

  31. Cloud Formation • Air is lifted and cools adiabatically • When it is cooled to its dew point condensation occurs, forming cloud droplets

  32. How the Air is Lifted • Orographic lifting – wind rises up a topographic barrier

  33. Frontal lifting – air rises along, or is pushed upward by, a front

  34. Convergence – air flows into areas of lower pressure and rises

  35. Localized convection – air is warmed at the surface and rises

  36. Convection Depends on Atmospheric Stability • Stability – refers to the tendency of a parcel of air to resist rising • The air can be: • Absolutely stable • Absolutely unstable • Conditionally unstable

  37. Lapse Rates Reviewed • Environmental lapse rate – change in air temperature with height • Dry adiabatic lapse rate – change in the temp of a rising, unsaturated air parcel • Saturated (wet) adiabatic lapse rate – change in the temp a rising, saturated air parcel • SALR<DALR due to the release of latent heat during condensation

  38. Absolute Stability • An air parcel can not rise because it will always • be colder than the surrounding environment

  39. Absolute Instability • A rising parcel of air will continue to rise, because it • is always warmer than the surrounding environment

  40. Conditional Instability • An air parcel is stable if unsaturated; • it is unstable if saturated

  41. Temperature Inversions • Layer of air with increasing temp with height • Forms on clear nights when the heat emitted by the surface easily escapes to space • Inversions place a cap on rising air parcels

  42. Cloud Types • Clouds are classified by height and form • Heights: low, middle, high, or vertically developed • Forms: cumulus = puffy; stratus = flat, layered; cirrus = thin, wispy

  43. Low Clouds • < 6000 feet • Stratus - flat, layered clouds, “gray skies” • Nimbostratus - stratus clouds producing precipitation • Stratocumulus - stratus with some vertical development Stratus Stratocumulus

  44. Medium Clouds • 6000 - 19000 feet • “Alto-” prefix indicates medium • Altostratus - medium level stratus • Altocumulus - medium level cumulus; “sheep’s back” Altostratus Altocumulus

  45. High Clouds • > 19000 feet, made of ice crystals • Cirrus - thin, wispy “mares’ tails” • Cirrostratus - filmy overcast; halos around sun & moon • Cirrocumulus - high puffy clouds; “mackerel sky” Cirrostratus Cirrus

  46. Clouds w/Vertical Development • Cumulus - puffy clouds • Cumulonimbus - thunderstorm clouds, “thunderheads” Cumulus Cumulonimbus

  47. GY205 Weather and Climate End of Lecture 3

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