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GEO 203 Notes – 10/7/10 (Lecture #8: Condensation)Ideal conditions for dew to form:Still, clear nightHigh humidity in air next to groundLow humidity in air aboveDew is typically seen at night and in early morning when the temperature is low and relative humidity is high.If Td is at/below freezing, saturation or moistening of cooled air will lead to deposition (gas  solid) rather than condensation.Frost point: temperature to which air must be cooled at constant pressure for frost to formAverage first day of frost in East Lansing: 10/4Jackson: 10/7Frozen dew = condensation + freezingFrost = depositionWhy does frost form in an open field but not under a tree?Longwave radiation during evening hours is countered by radiation of tree—no such radiation inopen fieldWhy is the dew point temperature a good indication of the minimum temperature at night?Lower dew point temperature = less water vapor = higher temperatureFOGGround based radiation fog: occurs at ground when dew point temp is reached by radiational coolingTypically between 0-1000 feetFavorable conditions: Strong radiational cooling, high surface humidity, calm/light wind, clear night, late fall/winter, valleys/basinsAdvection fog: warm moist air that moves, or advects, above a cold surface may become cooled to its dew point temp, creating an advection fog.Often occurs due to mixing currents/when warm ocean air rolls into cooler waters at Pacific CoastlineADVECTION FOG:Caused by moist/cold air drifting into a cold/moist environment.Associated with advection of airCan form both night and dayRADIATION FOG:Caused by radiational cooling of groundCalm and clear nights and early morningsNo motion necessaryEvaporation/mixing fog: can form by mixing warm unsaturated air with cool unsaturated airi.e. steam fog(mixing ratio unit: r or w)Parcel A, at T=20 oC, Ws =15.0g/kgTd=15 oC, W=10.8 g/kgRH = 72% UnsaturatedParcel B, at T=-10 oC, Ws=1.8g /kgTd=-15 oC, W=1.2 g/kgRH = 66% Unsaturated After mixing, T=[20 +(-10)] /2 =5 oC Ws=5.5 g/kg, W=(10.8+1.2)/2= 6 g/kgRH = W/Ws=6/5.5=1.09=109%>100% saturatedUpslope fog: moist air flows up mountain slopes/hillside to level where condensation occursMost days of dense fog in west coast, mountains, Great Lakes20-40 days of heavy fog in East LansingAttempted fog dispersalFog seeding: putting condensation nuclei into the atmosphere (large particles) to rain the particles down (largely unsuccessful and


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MSU GEO 203 - Lecture #8: Condensation

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