How does fog work?

  1. daniel_i_l

    daniel_i_l 866
    Gold Member

    Where I live, sometimes in the morning there's heavy fog just as the sun is coming up but after the sun somes up it all goes away. Why does it get foggy right before the sun comes up and why does the fog disappear shortly after?
    Thanks.
     
  2. jcsd
  3. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    Water vapur is held in the air. Water vapour is invisible but when it condenses into water droplets like clouds or 'steam' from a kettle you can easily see it.
    The amount of water vapour that air can hold depends on the temperature.
    At night as the air is cool, all the water vapour cannot be held n the air and it condenses out into fog/clouds. As the air heats up these droplets evaporate back into vapur and are absorbed by the air - as vapour they are invisible.

    The same amount off water (roughly) is present it's just that as widely spread individual water vapour molecules as a gas you can't see it, condensed into liquid water droplets in a fog it reflects and scatters light so you can.
     
  4. daniel_i_l

    daniel_i_l 866
    Gold Member

    Thanks for that explanation.
    But how does that explain the fact that the fog starts just before (+-1 hour) the sun rises and not in the middle of the night?
    Thanks.
     
  5. mgb_phys

    mgb_phys 8,952
    Science Advisor
    Homework Helper

    Depending where you are, it can be the ground cooling down to draw wet air in from the ocean or mountains, or locallly it can be water from ground plants/dew.

    You do get fog in the middle of the night - you just don't tend to notice it (unless driving!)
     
  6. Danger

    Danger 9,878
    Gold Member

    It's generally cooler at night than in the morning, so the liquid water settles to the ground as dew. With the sun up, evaporation starts. A lot of the water changes back and forth between the liquid and vapour states as it ascends, which is why you see mist rising from the ground in the morning.
     
  7. russ_watters

    Staff: Mentor

    It is coldest outside just before sunrise.
     
  8. How does fog differ from a cloud?

    Can a cloud exist above a substantial fog bank?
     
  9. Danger

    Danger 9,878
    Gold Member

    The only difference is in height above the ground. Both are composed of water droplets, water vapour, and sometimes ice crystals suspended in air. A lot of clouds are far more dynamic than fog simply because of the air currents at higher altitudes. Clouds will generally be colder as well, also due to altitude.

    edit: I don't know where you guys live. What's really cool here is to see a couple of mountains in the Rockies engulfed in clouds, then visit one of the towns up there and find it foggy from their perspective.
     
    Last edited: Aug 30, 2007
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