Nic Posted October 17, 2012 Share Posted October 17, 2012 I am a freshman in college, on my exam in my basic/intro level meteorology class the question was asked, Frost forms when: a) air cools below the dew point temperature. the dew point is 0c or below c) deposition occurs d) all of the above. The professor said the answer was D, but how can that be possible? It is not possible to have a air temperature below the dew point right? I asked the professor about this, she said something about condensation that really did not make sense to me. I think she misunderstood what I was asking. - Ignore the sun glass face it is supposed to be a b it keeps changing when I post it. Link to comment Share on other sites More sharing options...
Quincy Posted October 17, 2012 Share Posted October 17, 2012 Supersaturation occurs when the air temperature becomes lower than the dew-point. I don't think it's a common occurrence, but it can happen. Link to comment Share on other sites More sharing options...
LocoAko Posted October 18, 2012 Share Posted October 18, 2012 I think it is a poorly worded question (especially for an entry-level course), but perhaps your teacher was just being pedantic. Technically, water condenses/deposits from vapor to liquid/solid at supersaturation (because at saturation, the condensation/deposition and evaporation/sublimation rate is equal and there would be no net phase change). Air is not a "sponge" in that it can only "hold" a certain amount of water - the dewpoint/frostpoint is just the temperature at which the condensation/deposition rate equals the evaporation/sublimation rate, respectively. Once the temperature drops below the dewpoint temperature, the condensation/deposition rate exceeds the evaporation/sublimation rate and you get condensation/frost. The reason it seems like the temperature never goes lower than the dewpoint is just because it is corrected almost instantaneously - as soon as it happens, moisture condenses/deposits, releasing latent heat and warming the air back up to the dewpoint temperature. Link to comment Share on other sites More sharing options...
Isopycnic Posted October 19, 2012 Share Posted October 19, 2012 I think it is a poorly worded question (especially for an entry-level course), but perhaps your teacher was just being pedantic. Technically, water condenses/deposits from vapor to liquid/solid at supersaturation (because at saturation, the condensation/deposition and evaporation/sublimation rate is equal and there would be no net phase change). Air is not a "sponge" in that it can only "hold" a certain amount of water - the dewpoint/frostpoint is just the temperature at which the condensation/deposition rate equals the evaporation/sublimation rate, respectively. Once the temperature drops below the dewpoint temperature, the condensation/deposition rate exceeds the evaporation/sublimation rate and you get condensation/frost. The reason it seems like the temperature never goes lower than the dewpoint is just because it is corrected almost instantaneously - as soon as it happens, moisture condenses/deposits, releasing latent heat and warming the air back up to the dewpoint temperature. Thanks Jake, good stuff. I was trying to explain this to a few non scientific freinds a few weeks ago. i think i lost them at deposition and sublimation. Link to comment Share on other sites More sharing options...
Mallow Posted October 21, 2012 Share Posted October 21, 2012 I think it is a poorly worded question (especially for an entry-level course), but perhaps your teacher was just being pedantic. Technically, water condenses/deposits from vapor to liquid/solid at supersaturation (because at saturation, the condensation/deposition and evaporation/sublimation rate is equal and there would be no net phase change). Air is not a "sponge" in that it can only "hold" a certain amount of water - the dewpoint/frostpoint is just the temperature at which the condensation/deposition rate equals the evaporation/sublimation rate, respectively. Once the temperature drops below the dewpoint temperature, the condensation/deposition rate exceeds the evaporation/sublimation rate and you get condensation/frost. The reason it seems like the temperature never goes lower than the dewpoint is just because it is corrected almost instantaneously - as soon as it happens, moisture condenses/deposits, releasing latent heat and warming the air back up to the dewpoint temperature. One other slightly off-topic note--dew and frost can form when a station's temperature is several degrees above the dew point. This happens because on clear nights, radiation drives temperature drops, and radiation is most efficient at the surface. Thus, the temperature of the surface (and the air in contact with it) falls rapidly, and can reach the dew point even when the temperature two meters above the surface (the standard thermometer height) is several degrees warmer. Link to comment Share on other sites More sharing options...
famartin Posted October 24, 2012 Share Posted October 24, 2012 One other slightly off-topic note--dew and frost can form when a station's temperature is several degrees above the dew point. This happens because on clear nights, radiation drives temperature drops, and radiation is most efficient at the surface. Thus, the temperature of the surface (and the air in contact with it) falls rapidly, and can reach the dew point even when the temperature two meters above the surface (the standard thermometer height) is several degrees warmer. Just last night I witnessed a car top (near 2 meters height) which had ice on it, even though the 2 meter temperature measured nearby was 36F. Car bodies are very efficient radiators... Link to comment Share on other sites More sharing options...
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