Register to reply

Night coldest right before sunrise?

by fourier jr
Tags: coldest, night, sunrise
Share this thread:
fourier jr
#1
Jun2-14, 01:50 PM
P: 948
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?
Phys.Org News Partner Earth sciences news on Phys.org
russ_watters
#2
Jun2-14, 08:51 PM
Mentor
P: 22,303
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.
davenn
#3
Jun2-14, 09:57 PM
Sci Advisor
PF Gold
davenn's Avatar
P: 2,666
here's an interesting answer to your question, that I found on the www

http://www.weathernotebook.org/transcrip... old but erroneous proverb states that: itís always darkest just before the dawn. A related belief contends it is always coldest at the same time of day. Hi, Iím Bryan Yeaton for The Weather Notebook.

It seems logical to think that air temperature bottoms out just before sunrise and then begins to warm with dawnís early light. Observations and physical theory, however, show the coldest time of day generally occurs some time after sunrise.

To simplify why, letís only consider a clear night, with no passing fronts.

All objects gain heat from outside sources and radiate it away at the same time. When more radiant heat is lost than gained, the object cools. When more heat is gained than lost, it warms. If they are balanced, the temperature remains constant. OkayÖ thatís pretty basic.

Between sunset and sunrise, the Earthís surface gathers no solar energy but continues to radiate away its stored heat. During the night, the surface also loses radiant heat faster than it steals heat from other sources, and thus its temperature, and that of the air in contact with it, drops steadily.

At dawn, when the first light beams across the landscape, the incoming solar radiation is very weak. It does not yet have enough strength to counter all the heat escaping from the surface. As a result, the surface continues to lose heat for some time following sunrise, and the air temperature continues to fall.

At some point, the solar rays shine strongly enough to counter the heat loss. The gain-loss balance is shifted, and the air finally begins to warm up. As a rule of thumb: the coldest temperature is about an hour after sunrise.
so there ya go

Dave

fourier jr
#4
Jun3-14, 12:07 AM
P: 948
Night coldest right before sunrise?

I guess the suddenness is just my perception. Thx for the answers
DrJohnSmith
#5
Jun7-14, 10:53 AM
P: 3
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.
johnbbahm
#6
Jun12-14, 07:15 AM
P: 143
I think this was addressed well in this very forum about 10 years ago.
http://www.physicsforums.com/showthread.php?t=7408


Register to reply

Related Discussions
Absolute Zero, about -273.15ļC, is the coldest known temperature General Physics 12
Before Sunrise General Discussion 1
Is Ottawa really the world's coldest capital? General Discussion 17
Sunrise from west Astronomy & Astrophysics 33
Why is sunrise less red than sunset? Earth 19