1. GY205 Weather and Climate
Lecture 3

2. Moisture in the Atmosphere
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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 Vapor in 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 be colder than the surrounding environment
An air parcel can not rise because it will always
be colder than the surrounding environment

39. Absolute Instability is always warmer than the surrounding environment
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