gy205 weather and climate lecture 3. moisture in the atmosphere
TRANSCRIPT
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GY205 Weather and GY205 Weather and ClimateClimate
Lecture 3Lecture 3
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Moisture in the AtmosphereMoisture in the Atmosphere
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Evaporation, Condensation,Evaporation, Condensation,and Saturationand Saturation
• Air is saturated when evaporation and condensation are in equilibrium
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Saturation Vapor PressureSaturation Vapor Pressure•Pressure exerted by water vapor when the air is saturated
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Temperature and Water VaporTemperature and Water Vaporin Saturated Airin Saturated Air
•As temperature increases, the amount of water vapor in saturated air also increases
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Specific HumiditySpecific Humidity
• SH is the mass of water vapor present in a given mass of air
• SH is unaffected by temperature
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Relative HumidityRelative 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
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Temperature Affects Relative HumidityTemperature Affects Relative Humidity
With SH held constant:•Cooling the air increases RH•Warming the air decreases RH
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Specific Humidity and Relative HumiditySpecific Humidity and Relative Humidity
With temp. held constant:•Adding water vapor will increase RH•Removing water vapor will decrease RH
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Humidity and ComfortHumidity 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
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Dew Point TemperatureDew 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
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Average July Dew Point DistributionAverage July Dew Point Distribution
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Average January Dew Point DistributionAverage January Dew Point Distribution
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If the air cools to the dew point temperature: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
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MythbusterMythbuster
• 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
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Remember Phase Changes?Remember Phase Changes?
•Evaporation absorbs heat
•Condensation releases latent heat
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How Refrigerators/Dehumidifiers WorkHow 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.
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Processes that Change Air Temp.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.
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Adiabatic Processes in the AtmosphereAdiabatic Processes in the Atmosphere
• Adiabatic processes are responsible for forming clouds, a type of fog, and some wind systems
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Adiabatic Lapse RatesAdiabatic 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
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Measuring Relative Humidity and Dew PointMeasuring 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!)
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• Hair hygrometer – hair expands and contracts in response to changes in RH (bad hair days)
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Clouds and FogClouds 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
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MythbusterMythbuster• Clouds are made of liquid water
droplets, NOT water vapor. Water vapor is an invisible gas.
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Condensation Nuclei, Cloud Condensation Nuclei, Cloud Droplets, and a RaindropDroplets, and a Raindrop
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Types of FogTypes 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
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• Advection fog – warm, humid air crosses a cold surface, loses heat, and is cooled to its dew point
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• 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
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• Upslope fog – air is forced uphill, and cools adiabatically to its dew point
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• Precipitation fog – rainfall raises relative humidity to saturation due to evaporation and cooling
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Cloud FormationCloud Formation
• Air is lifted and cools adiabatically
• When it is cooled to its dew point condensation occurs, forming cloud droplets
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How the Air is LiftedHow the Air is Lifted
• Orographic lifting – wind rises up a topographic barrier
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• Frontal lifting – air rises along, or is pushed upward by, a front
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• Convergence – air flows into areas of lower pressure and rises
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• Localized convection – air is warmed at the surface and rises
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Convection Depends on Convection Depends on Atmospheric StabilityAtmospheric Stability
• Stability – refers to the tendency of a parcel of air to resist rising
• The air can be:
• Absolutely stable
• Absolutely unstable
• Conditionally unstable
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Lapse Rates ReviewedLapse 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
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Absolute StabilityAbsolute Stability
• An air parcel can not rise because it will always be colder than the surrounding environment
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Absolute InstabilityAbsolute Instability
• A rising parcel of air will continue to rise, because it is always warmer than the surrounding environment
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Conditional InstabilityConditional Instability
• An air parcel is stable if unsaturated; it is unstable if saturated
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Temperature InversionsTemperature 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
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Cloud TypesCloud Types• Clouds are classified by height and form• Heights: low, middle, high, or vertically
developed• Forms: cumulus = puffy; stratus = flat, layered;
cirrus = thin, wispy
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Low CloudsLow Clouds• < 6000 feet• Stratus - flat, layered clouds, “gray skies”• Nimbostratus - stratus clouds producing precipitation• Stratocumulus - stratus with some vertical development
Stratus Stratocumulus
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Medium CloudsMedium Clouds• 6000 - 19000 feet• “Alto-” prefix indicates medium• Altostratus - medium level stratus• Altocumulus - medium level cumulus; “sheep’s back”
Altostratus Altocumulus
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High CloudsHigh 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”
Cirrus Cirrostratus
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Clouds w/Vertical DevelopmentClouds w/Vertical Development• Cumulus - puffy clouds• Cumulonimbus - thunderstorm clouds,
“thunderheads”
CumulusCumulonimbus
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GY205 Weather and GY205 Weather and ClimateClimate
End of Lecture 3