2 Moisture in the Atmosphere To view this animation, click “View” and then “Slide Show” on the top navigation bar.
3 Evaporation, Condensation, and Saturation Air is saturated when evaporation andcondensation are in equilibrium
4 Saturation Vapor Pressure Pressure exerted by water vapor when theair is saturated
5 Temperature and Water Vapor in Saturated Air As temperature increases, the amount of watervapor in saturated air also increases
6 Specific HumiditySH is the mass of water vapor present in a given mass of airSH is unaffected by temperature
7 Relative HumidityRelative humidity – amount of water vapor present compared to the amount that COULD be present at a given temperatureRH = SH / saturation SH x 100RH is expressed as a %RH tells you how close the air is to being saturated, 100% RH = saturationTo change RH, change temperature and/or quantity of water vapor present
8 Temperature Affects Relative Humidity With SH held constant:Cooling the air increases RHWarming the air decreases RH
9 Specific Humidity and Relative Humidity With temp. held constant:Adding water vapor will increase RHRemoving water vapor will decrease RH
10 Humidity and ComfortIn the winter we warm the air, lowering RH indoors and have to use a humidifier or we get dry skinIn the summer, we cool the air, raising RH and the excess moisture is removed by the air conditionerBasements 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 TemperatureThe dew point is the temp. at which the air will become saturatedAlways < 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 RHDew point >70°F is muggy; ~50°F is comfortable; <30°F is dry
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 airDew will form (cold-can, fogged bathroom mirror)If dew point is <32°F, frost will form by deposition
15 MythbusterDew does not “settle” onto a surface, it forms directly on the surfaceFrost is not frozen dew, it forms by deposition directly from water vapor
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 systemAdiabatic processes – temperature changes without adding or removing heatAdiabatic processes occur as a result of the compression or expansion of a gasCompression 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 RatesA parcel of air rises at the DALR until it is cooledto 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 tempDrier air allows more evaporation thus a greater wet bulb depression indicates lower RHPsychometric 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 FogClouds and fog are composed of tiny droplets of water suspended in the airWhen air is cooled to the dew point, condensation occurs, and liquid water forms on condensation nucleiCondensation nuclei are hygroscopic aerosols (natural and anthropogenic)Condensation nuclei are always abundant
24 MythbusterClouds 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 coolingThe 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 LiftedOrographic 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 risingThe air can be:Absolutely stableAbsolutely unstableConditionally unstable
37 Lapse Rates ReviewedEnvironmental lapse rate – change in air temperature with heightDry adiabatic lapse rate – change in the temp of a rising, unsaturated air parcelSaturated (wet) adiabatic lapse rate – change in the temp a rising, saturated air parcelSALR<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 alwaysbe 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 itis 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 heightForms on clear nights when the heat emitted by the surface easily escapes to spaceInversions place a cap on rising air parcels
42 Cloud Types Clouds are classified by height and form Heights: low, middle, high, or vertically developedForms: cumulus = puffy; stratus = flat, layered; cirrus = thin, wispy
44 Medium Clouds 6000 - 19000 feet “Alto-” prefix indicates medium Altostratus - medium level stratusAltocumulus - medium level cumulus; “sheep’s back”AltostratusAltocumulus
45 High Clouds > 19000 feet, made of ice crystals Cirrus - thin, wispy “mares’ tails”Cirrostratus - filmy overcast; halos around sun & moonCirrocumulus - high puffy clouds; “mackerel sky”CirrostratusCirrus
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