Daily Temperature Lag
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.
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.
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
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.
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Update: June 2012