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Temperature begins to fall at dawn

by eigenguy
Tags: begins, dawn, fall, temperature
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eigenguy
#1
Oct18-03, 11:06 AM
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I heard somewhere that temperature actually begins to fall at dawn and then goes up later on. Does anyone know why this is? Thankyou.
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Andre
#2
Oct18-03, 01:37 PM
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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.
eigenguy
#3
Oct19-03, 09:00 AM
P: 97
Originally posted by Andre
sunrise is indeed the coldest moment of the day
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?

selfAdjoint
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Oct19-03, 12:18 PM
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Temperature begins to fall at dawn

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.
russ_watters
#5
Oct19-03, 02:09 PM
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Originally posted by eigenguy
But the fact is the temperature actually continues to drop for a number of hours immediately following sunrise.
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.
eigenguy
#6
Oct21-03, 09:49 AM
P: 97
Originally posted by russ_watters
Do you have any references for that statement?
Unfortunately I can't remember where I first heard about this, so maybe its wrong but I don't think so.

Originally posted by selfAdjoint
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
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.

Originally posted by selfAdjoint
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.
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.
marcus
#7
Oct21-03, 10:48 AM
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Originally posted by eigenguy
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?
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
leijen
#8
Dec20-03, 02:28 AM
P: 7
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.
Sam


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