# Is the coldest part of the day an hour after the sun comes up?

Popular lore states that generally the lowest temperature of any given day occurs an hour after sunrise, because it takes some time for the heat from the Sun to reach Earth. But, it only takes about 8 minutes for the energy from the Sun to reach Earth.

Is there something more complex going on that causes the Earth to take longer to heat up, or is this claim false?

• This is just a guess, but even after the sun's rays start hitting you, isn't the earth still acting as a heat sink (absorbing heat, but not radiating that much)? – Andrew Grimm Mar 27 '11 at 11:58
• I'd guess that during the first hour the earth radiates more energy into space than it receives from the sun at that low angle. – starblue Mar 27 '11 at 12:27
• At sunrise the light has already made it to earth, so there isn't even an 8 minute delay. – starblue Mar 27 '11 at 12:29
• In researching this question, I came across this, which gave me yet another reason to believe that Stack Exchange is a superior Q&A engine. – Jason Plank Mar 27 '11 at 17:30
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The theory is surprisingly simple.

The air and the ground get colder when no sun shines on them and hotter when the sun does. What we measure when we say it's colder is actually the air temperature. The sun's radiation has no effect on thermometers.

As you can see in the graph, minimum temperature occurs about 30 minutes to 1 hour after sunrise (hard to be exact with the ticks used).

For all dates, minimum temperature occurs at sunrise. Temperature drops throughout the night because of two processes. First, the Earth's radiation balance at the surface becomes negative after sunset. Thus, the surface of the Earth stops heating up as solar radiation is not being absorbed. Secondly, conduction and convection transport heat energy up into the atmosphere and the warm air that was at the surface is replaced by cooler air from above because of atmospheric mixing. Temperature begins rising as soon as the net radiation budget of the surface becomes positive. Temperature continues to rise from sunrise until sometime after solar noon. After this time, mixing of the Earth's surface by convection causes the surface to cool despite the positive addition of radiation and heat energy.
--source

1

The above is a schematic of the typical diurnal cycle of surface temperature (red) and the net energy rate due to incoming solar (black) and outgoing longwave radiation (blue).

1: The source of this material is the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2010 University Corporation for Atmospheric Research. All Rights Reserved.

• It's the same reason why late january-early February is the coldest part of the year. – MeBigFatGuy Mar 27 '11 at 18:38
• Actually, if you just read the chart you posted, you can see that the minimum temperature occurs just after sunrise, not just before. This is because something else you quoted "Secondly, conduction and convection transport heat energy up into the atmosphere and the warm air that was at the surface is replaced by cooler air from above because of atmospheric mixing. Temperature begins rising as soon as the net radiation budget of the surface becomes positive." The surface net radiation does not become positive as soon as the first ray of light hits it. – Russell Steen Mar 28 '11 at 2:57
• right source, wrong conclusion. – jwenting Mar 29 '11 at 7:31
• what about days that start at, say 50F and drop to 24F by evening? – warren Mar 29 '11 at 15:35
• Edited to correct assertion to bring it in line with the references provided. – Russell Steen Mar 29 '11 at 19:28

Why Does Cold Air Fall and Warm Air Rise? My weather station says yes, remember cold air falls and without the ground being warmed there is nothing to push the cold air away.

• Welcome to Skeptics! We strive for high quality content. In order to achieve this goal, we ask users to back up any significant claim with a reliable source, so that the claim can be verified. Please edit your answer to include the references on which you based your answer. Thank you. – Borror0 Mar 27 '11 at 14:38

As I understand it, the suddenness you refer to is likely a combination of the steady decrease in air temperature until the ground has warmed enough to start heating the air instead of cooling it, as explained above, with wind chill associated with the air pushing eastward along the ground to replace air which has already been warmed, and is therefore lighter and rising.