Why is it colder in the morning than when you went to bed at night? I know this must have been asked before but I couldn’t find it archives.
Bob
In the evening, just after the Sun has gone down, it’s still pretty warm from having been exposed to the Sun having been up for the past twelve hours or so. In the morning, just before the Sun comes up, it’s been dark for the past twelve hours or so, and has had plenty of time to cool off. in general, while the Sun is up, the temperature is increasing, and while it’s down, it’s decreasing.
I’ve heard that explanation. But to me there seems to be a marked decrease in the temperature in the morning. That is if you stay up all night then the temperature just at dawn is markedly less than just an hour or so before. If it were only a matter of how long the Sun has been down, then the Sun just coming just coming up shouldn’t have an effect.
Bob
Are you looking at the thermometer to gauge the temperature, or is just that you feel cooler? As Chronos pointed out, dissipation of heat from earth and structures takes awhile, and there may be another factor at work as well. I’ve read in a few different places that the human body goes through a cyclic temperature range with the coldest point usually found very early in the morning. I notice this very much when I stay up all night. One citation for this can be found here.
I had a nice long reply on this subject, and then it was lost in the Great Parse Crash of Ought-One a few hours ago before I could submit it. So here’s the abbreviated version.
The earth’s surface is constantly losing/radiating heat into space. But the earth’s surface also gains heat (via the sun). In general, if the rate at which the earth is being heated via the sun exceeds the rate at which heat is being lost to space, then the temperature will rise. If there’s more cooling going on than heating from the sun, the temperature decreases. Simple enough, no?
But in actuality it’s not quite so cut-and-dried. The amount of heat/energy that the earth’s surface receives from the sun varies throughout the day. When the sun’s at a high angle in the sky (i.e. around noon), the surface receives more, and when it’s at a low angle (i.e. early or late in the day), we receive less heat/energy.
That’s why on a normal day, the warmest hours are in mid-afternoon, instead of right before sunset; the earth has had a full morning of warming up, but by late afternoon, the amount of heat being lost is already starting to exceed the amount being received from the sun, and so the temperature starts cooling well before the sun goes down.
Now, how this relates to the OP:
Throughout the night, the earth cools, since obviously there’s no energy from the sun at all to offset the cooling. But as rgclark has noticed, this cooling doesn’t magically stop once it starts to get light in the sky. Just like in the late afternoon, even though the sun’s coming up in the early morning, the surface is still losing heat faster than the sun can replace it. And so even though the sun is up or just about to come up, it keeps getting colder until the sun finally starts giving us more heat than we’re losing to space.
This phenomenon is easily observable - just get ahold of an hour-by-hour temperature graph for a given location. I’ll bet you a dollar that the coldest temperature of the day occurs right around, or just after, the local sunrise.
Schief, that was a very good explanation, and you are right about it getting colder yet immediately after the sun arises. I know because I start my run before it rises and it’s still getting colder even after it rises. Sometimes, a few degrees, not just one or two.
However, that’s if there are no other factors. As we well know the temperatures can actually rise during the night if a strong warm front passes. And, conversely, it can fall during the day if a strong cold front passes.
Exactly right, barbitu8. On a typical day the temperature will rise and fall as I have described, but I’ve been through enough Midwestern days where the temperature drops 20 degrees from morning to afternoon as an Arctic air mass passes to know there’s more than a few exceptions to my scenario.
Just to completely cover the scenario, a few other meteorological phenomena should be noted. If it is a cloudy night, there will not be as much radiation (since it is blocked by the clouds), and so not as much cooling. Similarly, if it is a cloudy day, the sun’s rays will not be as heating, as some will be blocked out. Also, if it is a windy night, there will not be as much cooling either, as the winds apparently prevent effective radiation. Schief, who obviously knows more than I do in this field, can confirm or otherwise state the reason.
This is correct. As the earth cools at night, it naturally brings down the temperature of the air surrounding it at the surface as well. Remember that warm air tends to rise, and cold air tends to sink. As a result, on a calm night this cooler air tends to stay right near the surface, and get increasingly colder as the night goes on, while air at higher elevations may actually stay warmer and form something of a temperature inversion.
You can sometimes observe this effect driving in hilly country on a cool, humid night - the cold air will tend to ‘drain’ into the low-lying spots. As a result, you’ll encounter fog in the dips and valleys (where the air has been cooled to saturation), but not at the higher elevations.
On a windy night, however, the air gets kicked up and mixed throughout the lower levels of the atmosphere, cooling the whole atmosphere more slowly and evenly and preventing any strong radiational cooling right at the surface.