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
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  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 3,811
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    here's an interesting answer to your question, that I found on the www

    so there ya go :smile:

    Dave
     
    berkeman likes this.
  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!
     
  9. Dave's post is correct. It's a simple matter of the heat budget of the atmosphere. As long as heat (thermal energy) loss exceeds heat gain, temperatures will continue to drop. As soon as heat gain exceeds heat loss, temperatures start to rise.

    I do take exception with the "one hour after sunrise" statement. In tropical deserts, the nadir can be only minutes after sunrise; while at the Poles it can be a matter of days. A lot depends on cloud cover, humidity, vegetative cover, the movement of frontal systems, and the like. A nice, vague term like "shortly after sunrise" is probably more accurate.
     
  10. In some parts of the world (where I live being one), it is not the case that temperatures at ground level consistently fall during night, with a minimum before dawn.
    Changes of wind direction and the passage of frontal systems frequently can result in temperature rising during the night.
     
  11. jim mcnamara

    jim mcnamara 1,515
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    An issue meteorologists local to Albuquerque TV have noted is that the that coldest temperature of the "night" is often after dawn. AFAIK nobody has given a satisfactory explanation - mostly a comment about radiative cooling changing as the upper atmosphere changes when then terminator moves off. Anybody know specifically?

    ABQ is at 5000 feet elevation with very low humidity, so radiative cooling is a big factor here - with larger diurnal temperature changes than a lot of lower, wetter places. Our temperature range today was 65F -> 96F, more than 30 degrees F.
     
  12. Not sure what you are asking. If you are on a flat area, the surface should warm and cool during the day/night cycle as in post 3. Even during the day there is radiation leaving the surface to space, or the upper atmosphere, which is at a lower temperature.

    With uneven ground, there is some interplay between the higher and lower surfaces, with movement of air and different radiation factors, since the surfaces will have an exchange of radiation with each other.
     
  13. jim mcnamara

    jim mcnamara 1,515
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    Not really asking - "wondering" more fits the bill. Is there a definitive answer to: why is the lowest temp here frequently - that means more than 50% of the time - well after sunrise - when this appears not to be the case in other areas. 'well after' means several minutes, circa 10-20 minutes.
     
  14. russ_watters

    Staff: Mentor

    Post #2 covers why the coldest temperature can happen after sunrise. I suspect that's a matter of sky clarity (clearer skies reflect less heat back to earth), since it isn't something I see here in Philly.
    [edit]
    Humidity lower down also plays a role in a place like Philly because humidity limits how cold it can get. When you start getting dew, it takes more energy to cool the air and fog (like clouds) inhibits the heat transfer.
     
    Last edited: Jun 30, 2015 at 5:41 AM
  15. It certainly points up the fact that few temperature records are kept with short enough intervals to test the hypothesis. My experience was based on what the thermometer says, so there wouldn't be a "me factor".

    When I worked in Utrecht there was a sign on the wall that said, "Hier hebben we geen ruzie over dingen, gaan we berekenen het antwoord" (Here, we don't argue about things; we go compute the answer). It's disappointing that this could be debated for 10 years without anyone doing that.
     
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