Night coldest right before sunrise?

  1. Of course night is colder than daytime in general but it seems to me that maybe half an hour before sunrise it seems to suddenly get much colder, like freezing cold. Maybe it's just my perception. Is it just that all the heat that has been absorbed during the day has dissipated by that time or is there something else going on there?
     
  2. jcsd
    Earth sciences news on Phys.org
  3. russ_watters

    Staff: Mentor

    It isn't sudden, but the surface of the earth gets cooler for almost the entire night becuase the sun isn't out and it radiates.
     
  4. davenn

    davenn 3,689
    Science Advisor
    Gold Member
    2014 Award

    here's an interesting answer to your question, that I found on the www

    so there ya go :smile:

    Dave
     
  5. I guess the suddenness is just my perception. Thx for the answers :smile:
     
  6. Well, that's interesting because I've said the exact same thing, verbatim, for 40 years. I have to wonder how many people are regularly up at sunrise and just sitting around watching the thermometer. I do regularly as I work from home and it is an extremely reliable phenomenon, particularly if there is no wind.

    The answers are all pretty obvious but none address our central observation. Namely, the slope of the curve changes dramatically just before sunrise. We understand about the gradual cooling, but why the acceleration in the drop- right before it goes up. If it was slow radiation away, wouldn't it take a while to recover? Yet that quick drop comes right before it starts to rise again. Suffice to say something definitely appears that way, since we've both noticed the exact same thing.

    Here's a theory. I wonder if the air pressure drops too. The sun will start to heat the atmosphere over an area before it actually rises enough to strike the ground. Maybe the increased molecular motion at higher altitudes leads to a small decrease in pressure at the ground and that leads to a faster cooling. But just for a short while because then the heating starts at the ground. Kind of far fetched, but at least it would explain what it feels like we're seeing, that the sunrise and heating actually causes the drop right before the actual sunrise at ground level.
     
  7. Maggnum likes this.
  8. Thanks for the link from 10 years ago - I thought it essentially answered the question.
    Maybe I missed it but the 'me' factor was not mentioned. At and following sunrise, the sun shines directly on my skin. I am directly receiving the IR from the sun, therefore I feel warm and don't really care that the air around me is still cooling. The direct affect of receiving IR from the sun is easily perceived - on a hot day (at a train station) with the sun beating down on your face and body, step into and out of the shade of a telegraph pole (or whatever shade you choose) - wow - what a difference!
     
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