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Fog, clouds in winter

  1. Nov 26, 2010 #1
    I'm not a science student. Please keep your answer simple and straightforward so your help can be appreciated to the fullest. Thanks.

    In Winter when there are clouds in the morning and have been there for some time during the night, it will result in more cold. Because earth has lost its much of its warmth to the atmosphere and which has escaped into upper layer of atmosphere. Is it correct? Would the same thing happen when it's summer?

    When hot air mixes with cold air, it results in fog. Suppose hot/warm air flows into a cold area, it will result in fog. Wouldn't it result in rise in temperature because cold has extracted some of the warmth of hot/warm air and on average the temperature of the mixture would be high now. Do I make sense?
  2. jcsd
  3. Nov 26, 2010 #2


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    Quite the opposite. In general cloudy nights are much warmer than starry nights, as clouds slow down loss of energy through radiation.
  4. Nov 26, 2010 #3
    Hi Borek

    What would happen in the morning? Won't it be little colder?
  5. Nov 26, 2010 #4
    During the day, clouds will MOST OF THE TIME make earth's surface cooler. However, during the night, clouds serve as a "blanket" to help prevent radiational cooling from earth's surface keeping the nocturnal temperatures warmer than they would be without this "blanket". In addition, water vapor and tiny cloud droplets are an outstanding greenhouse gas and substance as they also enable earth to retain its daytime heated surface by absorbing and re-radiating long wave radiation back towards earth's surface.

    In the winter months, low clouds and fog are often much more difficult to "burn off" as the low sun angle has a much less efficient insolation effect, thus limiting evaporation and maintaining the low stratus cloud deck near the surface of the earth. This positive feedback often allows low clouds and fog to remain longer in the winter than they would otherwise do.

    I hope this helps....
  6. Nov 26, 2010 #5
    You missed the fog part:
  7. Nov 27, 2010 #6
    As for the fog part...when hot/warm air enters a colder a region of the atmosphere, a fog does NOT always develop. Adequate water vapor is required for saturation to occur to create the fog. It is true that warmer air has a greater capacity to evaporate water and thus will most likely posses enough water vapor to create the fog....but that is not necessarily true ALL of the time! This type of fog creation is often referred to as ADVECTION FOG.

    As for warming the atmosphere, fog is created by the physical process of condensation. When water vapor undergoes this physical change, latent heat is released as the vapor condenses into tiny cloud droplets, thus warming the atmosphere through the latent heat of condensation. The latent heat that is is released during condensation is ~540 calories per gram of water which quite frankly is a huge amount of heat energy over a fog laden patch of the earth!

    Thus when fog does develop, the air will warm from 2 processes....1. Latent heat of condensation and 2. long wave radiation being re-radiated back to the planet.

    I hope this is helpful.........
  8. Nov 27, 2010 #7
    Caveman, thanks a lot. You explained it very effectively and that too in simple language.

    One last question, though. I hope you won't mind it.

    I have always thought fog only builds up up to a certain height less than 1km above the surface. So, would the re-radiating part really play a mentionable role? Please let me know.

    And by the way, when warm/hot air enters a colder region, don't you think the change in temperature would also result from the mere mixing of the two besides condensation heat? Suppose, hot/warm air was 40C and the cold was 10C, then obviously after mixing up averge temperature will lie between 10 and 40 C.

    Reminding you, I'm not a science student! Thanks.

    Best wishes
  9. Nov 28, 2010 #8
    We are talking semantics here with the depth of the fog. Fog is simply a stratus cloud at or slightly above the surface of the earth. Admitedly, a shallow layer of fog would re-radiate less than a thicker, deeper fog.

    As for mixing of 2 different mass fields of air, this is actually a very complicated topic as the 2 different mass fields would not want to mix due to their inherent physical properties being so different. The density of the colder air would hold nearer the earth while the warmer and thus less dense air would overrun the colder air mass field at the surface of the earth. Mixing could effectively occur when the winds would mix the boundary layer air and most certainly, the original cold, dry dense air would most certainly gain kinetic energy (temp), but as to how warm and moist it would become is a function of all of the initial qualities of the air mass fields.

    I hope this is helpful..........
  10. Nov 28, 2010 #9
    Thanks a lot, CM. Of course, it's very much helpful.

    Yes, you are right about the mixing of the gases. To come up with some exact model would a lot of difficult mathematical functions and other physics of gases related topics. But unless warm/hot air has mixed up well with cold air there won't be any fog. Actually the appearance of fog is an indication that the mixing has taken place. I think wind speed should be enough that the it can force the mixing. Do I make any sense?

    but as to how warm and moist it would become is a function of all of the initial qualities of the air mass fields.

    But you would agree that the final temperature would lie between the temperature of hot/warm air and colder air. And if we assume that the colder air was quite dry, then overall moisture level would decrease? I'm not sure. Your opinion.

    Best wishes
  11. Nov 29, 2010 #10
    I don't think the mixing of two air masses is necessary at all for fog. You just need air to cool to lower than the dew point. I think the two most common types of fog are radiation fog, where air near the ground is cooled at night when there are no clouds, and little wind, and advection fog, when wamer air is cooled by snow or water
  12. Nov 29, 2010 #11


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    There are a number of ways that fog can be produced. You can have air streams meet, you can have temperature drop, you can have a single air stream rise over a hill or other obstacle, and so on.

    What's interesting is that yes, fog formation results in temperature increase, because water vapor releases heat when it condenses. It won't go past the dew point, of course.
  13. Nov 29, 2010 #12


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    It is interesting, I think, regarding night time "overnight' temperatures, to discuss the phenomenon, mentioned above by Borek and Caveman717, known as 'radiational cooling'. Maximum radiational cooling occurs when overnight skies are clear, winds are calm, and there is snow cover on the ground. It is not unusual, for example, to see nightime low temperature differences of up to 15 degrees F (8 degrees C) between 2 towns, at the same elevation, a mere 10 miles apart. A city near the coast for example, might have an air temperature of 15 degrees F, with no snow cover, a 5-10mph wind, and cloudy skies, whereas a town a few miles away with clear skies, no wind , and a snow cover, might have a temperature of 0 degrees F. Rule of thumb is to subtract 5 deg F for clear vs. cloudy, 5 degrees F for no wind versus light wind, and 5 degrees F for snow cover vs. no snow cover. Effects are cumulative.

    Regarding Fog , it typically occurs when air temperatures drop down to, or close to, the dew point 'saturated air' temperatures. On a hot summer day with high dewpoints, say 90degrees F daytime high with dew point about 75 degrees F (rather uncomfortable!), if the temp at night drops into the 70's, and the moisture content of the air is still there, fog is pretty much a guarantee. On a snowy or moistury winter day, with temps in the high 20's and dewpoints about the same, again...fog.
  14. Nov 30, 2010 #13
    Am I correct in stating that fog can form in an inversion layer where the warmer air is below the colder air? I think this is what happens with an occluded front.
    Last edited: Nov 30, 2010
  15. Dec 1, 2010 #14


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    I think you've got that reversed. Typically, warm air is below the colder air...it gets colder the higher up you go. During inversion layer, you've got the cold air on the bottom, and the warmer air above. There are different things that happen in this scenario....many different ones. For example, in winter, warm moist air override above the colder layer results in the warm water vapor condensing to snow in the colder air below , and ultimately perhaps to sleet and freezing rain as the colder layer reduces in thickness. This is not necesssarily a fog condition, because the cold entrenched surface air is relatively dry with low dew points. But during an occluded front, with cold air behind a cold front forcing itself under the warm moist air behind a warm front, typically at the storm's end, then yes, that is an ideal fog situation, with cloud formation at the surface.
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