Why Is 4pm the Hottest Hour?

Alright, during Daylights Savings Time (in the U.S.) it is at a slightly different time. But one way or another, around 4pm is typically the hottest hour of the day. Why is this? I mean, think about it. During true solar noon (which is typically around 12 Noon, Standard Time, depending on where you live), the sun is directly over head. Its rays are most focused then. So why is 4pm then the hottest hour?

Thank you in advance to all who reply :slight_smile: :slight_smile: :slight_smile:

I recently had a conversation about this with a friend who has moved from a major coastal town to a large inland city. She said what she hated about the new place was that in summer it just keeps getting hotter all afternoon whereas on the coast the temperature would start dropping about 1 or 2 pm. Checking the weather readings on the bureau of meteorology site this seems to be true. Inland more than about 20 miles the temperature continues to rise after 2pm but not on the coast.

The standard explanation for coastal effects is that the earth cools off and heats up faster than the ocean. Water has a greater heat capacity than dirt and there is mixing in the ocean which keeps it’s temperature more uniform Dirt on the other hand doesn’t mix and so the top surface heats up and cools down relatively quickly. OK. So at night the ground cools down quite quickly and the air above cools down and tends to settle creating an offshore breeze. In the morning the sun heats the earth and by noon its quite hot so the air above it starts to rise brining in an onshore breeze of cooler air from the ocean. However, by 20 miles or so inland the cool ocean air has mixed with the warm land air and the breeze isn’t cool any more.

WAG. It’s hottest in mid afternoon because the sun is heating the ground. Sure, the sun continues to heat the ground after local noon, but the angle gets less and less as the sun drops lower and by, say, 4:00 PM the sun’s input is less than the amount of heat radiated by the ground and the temperature begins to drop.

The earth doesn’t lose heat instantly, which is a good thing because otherwise we’d get awfully cold at night. At noon, the earth is hot because the sun has been adding energy to it all morning long. It’s not just because of the energy that is being added at that instant. All through the afternoon, energy is continuing to be added to the earth. The energy being added by the afternoon sun is greater than the energy being lost by the earth, so the earth continues to get hotter. At some point, though, the energy being added by the sun drops below that being radiated off by the earth, and then it finally starts to cool down.

It’s a bit more complicated than this, because the air moves and the water moves and some of the energy goes into evaporating water and such, but that’s the basic idea.

As a geography major I was always given the same answer that these guys and girls have provided here. It’s the same reason that it’s often colder at four or five in the morning than at midnight.

A way to look at it is that the earth will heat when the sun’s angle is, lets say anywhere from 40 to 90 degrees overhead. now when it’s directly overhead it will heat up the fastest, but even when it starts sinking, it’s still heating up the earth till it falls to 40 degrees, where it is now in balance.

On the coast you have evaporative cooling, mixing of currents and wind patterns that form due to different temps over water and land, so the angel here may be from 70 to 90 degrees.

It’s analogous to the question why isn’t June 21st (summer solictice in the northern hemisphere) the hottest day of the year. The basic answer has been mentioned: the earth and its atmosphere have a lot of “thermal mass” so there will be a significant delay between the times of maximum insolation and maximum temperature.

Previous thread on the subject here