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Temperature begins to fall at dawn

  1. Oct 18, 2003 #1
    I heard somewhere that temperature actually begins to fall at dawn and then goes up later on. Does anyone know why this is? Thankyou.
  2. jcsd
  3. Oct 18, 2003 #2
    Well, the temperature should normally be falling at a steady pace during the night, assuming clear skies, no wind. This is heat loss due to IR radiation that is not compensated by inwards radiation from clouds or the sun. So as soon as the rising sun gains some IR radiation strenght, the temperature will stop dropping. So sunrise is indeed the coldest moment of the day, but it is a gradual process.
  4. Oct 19, 2003 #3
    That "sunrise is indeed the coldest moment of the day" is what one would naively expect. But the fact is the temperature actually continues to drop for a number of hours immediately following sunrise. So my question was why does this nonintuitive thing happen?
  5. Oct 19, 2003 #4


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    For a clear sky and nearly zero wind at the surface, I can see it. Under these conditions the ground will radiate heat during the night, and a thin layer of very cold air will form next to the cold ground, But air is not a very good conducter of heat, so in the absence of convection and mixing, the temperture a few feet off the ground won't be any where near as cold.

    Now when the Sun comes up and begins to heat the ground, suddenly there's mixing. And the very cold air mixes with the warmer air and lowers the temperature. Meteorological thermometers are usually placed six feet above the surface, so they would show the effect.
  6. Oct 19, 2003 #5


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    Do you have any references for that statement? Looking at weather data (the NWS has the last 2 days of temperatures for everywhere in the country) that appears to not be the case.
  7. Oct 21, 2003 #6
    Unfortunately I can't remember where I first heard about this, so maybe its wrong but I don't think so.

    Your idea of a thin layer of cold air next to the ground set off some of my neurons. So here is my resulting idea. Over night the ground cools so any sufficiently thin layer of air next to the ground will be at the same temperature as the ground.

    So my idea corresponding to this is that maybe the sun doesnt start heating the ground much until it is high enough in the sky. But it does heat columns of air far above the ground. These columns would then rise because this heating lowers their density. To avoid a vacuum air rushes in close to the ground. Then this new thin layer is cooled by the ground and then begins to rise and so on. The result is that columns of cool air are maintained near the ground until the sun raises the ground temperature ending the process.
    Last edited: Oct 21, 2003
  8. Oct 21, 2003 #7


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    I like this question eigenguy and would like to try answering tho without any assurance

    I think one can explain the effect without air

    One can just assume a rotating black ball of imperfectly-conductive material

    the equilibrium temp will depend on the angle of the sun, for simplicity assume point on the equator and axis normal to sun (as at the equinox) so 12 hour day and nite.

    during nite the ground will try to radiate away heat till it is
    down to 2.7 kelvin (the temp of space) but it wont make it because nite is only 12 hours, say.

    at dawn the angle of the sun is so little that the equilibrium temp is still quite low, so it will still be radiating away more heat than it receives

    for a black ball of perfect conductor the equilibrium temp is 1/sqrt 2 times the equilibrium temp for flat surface facing sun

    because of imperfect conduction the temp is, like, oscillating above and below that "average"

    one can calculate the angle, or the time of morning, when the flat equilibrium crosses that "average", it is around 9AM or when a point on equator sees sun at 45 degrees. this is very rough and does not even assume air (which is all-important) but makes it plausible that even without air there would be this cooling for a little while still after dawn
    Last edited: Oct 21, 2003
  9. Dec 20, 2003 #8
    Temperature at dawn

    temperature begins to fall at dawn
    I heard somewhere that temperature actually begins to fall at dawn and then goes up later on. Does anyone know why this is? Thankyou.

    Due to the increasing light, the molecules of water begin to liven up.
    This requies energy which is drawn directly from the surrounding air.
    When there is frost on the ground in the early morning the temperature is fairly constant.
    When the light starts to increase the temperature drops.
    The energy required to change the state of water from ice to liquid is termed the "latent heat of liquification" and is drawn directly from the surrounding air.
    Dew starts to evaporate and this also requires energy.
    This is termed the "latent heat of vapourisation".
    I hope this helps.
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