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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?

Patches
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    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
  • 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
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    In researching this question, I came across [this](http://wiki.answers.com/Q/What_time_of_day_does_the_low_temperature_occur), 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
  • @Jason Plank: Three months late to the game, but: HAH! Love it! – erekalper Jun 20 '11 at 12:36
  • I wish this were asked at Earth Science SE, because I pieced together a great answer when revisiting quite a few micrometeorology topics, but don't have great enough sources to hammer them home (perhaps I can find some course notes from my undergraduate days, I wonder if they would be considered valid) – JeopardyTempest Aug 06 '18 at 09:05
  • But the wording of every answer is subtly wrong. All get the central idea right, that there's still a net energy loss going on. But none give the right precise explanation from there. Basically the energy in that first little bit after sunrise is so pitiful that it's not able to prevent the Earth's surface from continuing it's own natural cooling (as all bodies emit [thermal radiation](https://en.wikipedia.org/wiki/Thermal_radiation). This is why the surface cools through the night) – JeopardyTempest Aug 06 '18 at 09:15
  • Basically the Earth only receives a few W/m² just after (and even before) sunrise... compared to typically into the hundreds of W/m² within an hour if the sun angle is decent. [The Oklahoma mesonet site](http://www.mesonet.org/index.php/weather/meteogram/) is great for comparing radiation to temperature (and then google the sunrise) (note: appears the Mesonet site uses your local clock, so you'll have to convert to CDT/CST (or convert their sunrise to your time, or change your clock to Central timezone!) – JeopardyTempest Aug 06 '18 at 09:21
  • So the pitiful incoming energy isn't enough to prevent the Earth's natural cooling. It'd be sort of the same idea as taking your pan out of the oven, and then putting it out in the sun to try to keep it from cooling off. It's just losing too much energy by radiation. But because the ground is relatively cool, soon the is providing way more than enough radiation to stop the loss, and start the gaining. – JeopardyTempest Aug 06 '18 at 09:28
  • (Note I actually speak of the ground, even though the question is about the air temperature, because the ground is the key to the near-ground atmosphere via conduction/absorption of the ground's radiated energy/convection. The air can be warmer than the ground at sunrise, but if the ground is warmer, the air is still not receiving enough energy from the ground to counteract its own losses by radiation). – JeopardyTempest Aug 06 '18 at 09:44
  • (Best source I have off hand is a [blog by meteorologists at University Wisconsin](http://wxguys.ssec.wisc.edu/2017/01/10/morning-low-temperature/). Otherwise bits and pieces picks up from a wide selection of journal articles and related sites) – JeopardyTempest Aug 06 '18 at 09:57

2 Answers2

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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

enter image description here1

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.

Russell Steen
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    It's the same reason why late january-early February is the coldest part of the year. – MeBigFatGuy Mar 27 '11 at 18:38
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    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 02:57
  • right source, wrong conclusion. – jwenting Mar 29 '11 at 07:31
  • what about days that start at, say 50F and drop to 24F by evening? – warren Mar 29 '11 at 15:35
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    Edited to correct assertion to bring it in line with the references provided. – Russell Steen Mar 29 '11 at 19:28
  • @MeBigFatGuy Note that while the broad idea (still losing more energy) is the same, the details really aren't. Winter lags primarily because water has a lot of energy stored up, and it's slowly transfered to the atmosphere (in other words, we'd be coldest nearer New Year if not for water keeping the NHem warmer than it otherwise would be). On the other hand sunrise lag is because dawn sun just isn't giving enough energy to overcome natural cooling. In other words, winter is an externalish factor, dawn is more an internal factor (ground isn't quite internal, but it controls 2m temps mostly) – JeopardyTempest Aug 06 '18 at 10:06
  • @warren: I say MOSTLY in my last comment because airmass [advection](https://en.wikipedia.org/wiki/Weather_front) (wind bringing in cold or warm air from other places) can occasionally drive large local changes. Kind of like how opening the front door and leaving it open will overwhelm whatever heating processes you've got going on inside. When the air from much nearer the Poles comes calling, it dominates all the other stuff. – JeopardyTempest Aug 06 '18 at 10:16
  • (Clouds and even precipitation temperature/evaporation/outflow can also have pretty big influences sometimes, but rarely as extreme and long-lasting as a front) – JeopardyTempest Aug 06 '18 at 10:17
  • @RussellSteen: Note that by sunrise, I believe convective and (upward) conductive transfer around 2 meters are very tiny (or even positive, on occasion?)... and it's the radiation flux that's the big negative factor around sunrise in the absence of great transfer from the ground. – JeopardyTempest Aug 06 '18 at 10:32
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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.

N4TKD
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