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Name: Nolen
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Question:
At what time of day (summertime) it is usually the hottest? (I have heard it was about 3:00 P. M.) Why (assuming the above is substantially correct) is it in mid-afternoon that it is the hottest? Seems that it should be at noon, when the sun is highest in the sky.


Replies:
Good question! First of all, remember that most places use Daylight Saving Time during the summer, which means that true noon is actually some time around 1 PM. Then, even though the sun might be most intense during true noon, there still is much sunlight arriving at later hours, still pouring out energy upon the ground, still heating things up.

Richard Barrans


Nolen,

The highest temperature, particularly on a cloudless day in Summer occurs somewhere between 3 pm and 7 pm, with the most common time being around 5 pm, Daylight time. This is somewhat later than in Winter when the time of maximum temperature is somewhere between 1 and 3 pm.

You can think of it almost like baking something in an oven or warming something in a microwave. Even though the oven or microwave is putting a constant amount of energy into whatever you are cooking, it takes time for the food to warm up from the lower temperature that it started out at.

In the same way, it takes time for the Sun to heat the ground and therefore for the ground to heat the air above it. Since the maximum intensity of Sunlight occurs at local solar noon (which might be as much as an hour different from the Daylight time, depending on where you live in a time zone), the ground is warming throughout the morning, but its temperature can not catch up to the increasingly greater amount of energy that the Sun is providing (unlike the constant heat in the oven or microwave). After noon, the Sun continues to raise the ground and air temperature until more energy is lost to space than the ground can take in from the Sun. Therefore, there is a long lag between noon and the time of maximum air temperature. Having more than 12 hours of daylight in the Summer also contributes to pushing the maximum temperature time to the late afternoon.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory


Nolen,

The lag is due to the fact that heating of the air occurs, not from the sun's rays passing though, but from heating of the ground and infrared radiation leaving the ground in the form of heat. This process takes some time, and it is also the reason it does not get chilly immediately after sunset.....there is residual warmth in the ground being radiated into the air producing some residual heating even after the sun has set.

If the warmth of the air resulted from the sun's rays passing through, then you would observe highest air temp. when the sun's rays were at their daily peak.

Thanks for using NEWTON!

Ric Rupnik


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