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If Summer Solstice around June 21 every year is the longest day of the year and closest to the sun for the Northern Hemisphere (maybe I should say receive the most direct facing sunlight), then why isn't it plus and minus 90 days around this date the hottest? Why does it seem like it is hottest between Summer Solstice and Nov 1st?

That's because we know if the summit of a evenly distributed mountain is at Spot A, then plus and minus 300 feet away from Spot A should be equally near the summit.

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If Summer Solstice around June 21 every year is the longest day of the year and closest to the sun for the Northern Hemisphere, then why isn't it plus and minus 90 days around this date the hottest? Why does it seem like it is hottest between Summer Solstice and Nov 1st?

First things first: Summer solstice in the Northern Hemisphere is very close to the furthest rather than closest the Earth comes to the Sun. Earth perihileon (closest distance to the Sun) is in early January and Earth aphelion (furthest distance to the Sun) is in early July. The smallish eccentricity of the Earth's orbit about the Sun plays a very minor role in the Earth's seasons. Despite the apparently contradictory nature of Earth perihelion and Earth aphelion, it is the Northern Hemisphere rather that Southern Hemisphere that experiences stronger seasons. This is primarily the result of the disparate mix of land versus ocean in the two hemispheres. The Southern Hemisphere is predominately covered by oceans while the Northern Hemisphere is closer to even (but even then, it's not quite 50-50).

The key reason that June 21 is only mildly hot in the Northern Hemisphere is because the Earth's climate has "momentum". It takes some amount of time for the increased heating in Northern Hemisphere springtime to take effect. Similarly, it takes some amount of time for the decreased heating in Northern Hemisphere autumn to take effect. August is the hottest month in many parts of the Northern Hemisphere. North Atlantic hurricanes peak in September, which is well after the June 21 solstice.

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    $\begingroup$ I'd emphasize a bit more on that "momentum". It's clearly more than just warming the air, there are cool sources that take longer to warm like bodies of water. coastwatch.glerl.noaa.gov/statistic/avg-sst.php $\endgroup$
    – rtaft
    Commented Jul 3, 2023 at 12:37
  • $\begingroup$ that's a bit funny in San Francisco Bay Area, where the day can be 85°F (29°C) during the day and 55°F (12.7°C) during the night, so during the night, it already feels like winter and you may think the momentum has "gone away". But I guess you can say there is a bigger momentum (seasons) and a smaller momentum (within a day), such as during winter, the night can be 39°F (3.8°C), so I suppose it is like Mandelbrot or Elliott Wave $\endgroup$ Commented Jul 4, 2023 at 16:45
  • $\begingroup$ However, if the energy re-emitted by the planet does not almost exactly equal the solar energy received, the planet would warm up. I am suspicious that in the northern hemisphere between June 21 and the end of July energy would accumulate (because that would warm the planet globally, including the southern hemisphere). Is then the balance of absorbed and re-emitted energy balanced each day or not exactly? $\endgroup$
    – Davius
    Commented Jul 4, 2023 at 19:21
  • $\begingroup$ @user910130: Lakes and coastal ocean water (especially in straits and stuff) warm up over the course of the spring / summer, and cool down over the winter. (Ever go to the beach in mid June and find the water still way too cold?) So over a whole year the average energy incoming balances the energy outgoing, but the peaks and troughs of incoming vs. outgoing for any given local area are not in phase with each other. Areas near large bodies of water are farther behind in phase. (e.g. spring comes earlier farther inland, without an ocean's thermal mass holding back change.) $\endgroup$ Commented Jul 5, 2023 at 3:10
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It's all about thermal storage/momentum.

Consider spring and fall equinox. They should be the same temperature, right?

  • On March 20, why are the lakes still frozen? Because the hemisphere spent the previous 6 months with the least possible solarization, and the top few feet of topsoil, groundwater, and water bodies are pretty cold.
  • On September 23, why is it so warm? Because the hemisphere spent the previous 6 months with the most possible solarization. The planet's surface has been soaking in that heat for 6 months.

So it's really about energy storage in the planet's surface. You probably know if you go to a certain depth, tempertures are consistent all year - but above those depths, the heat (or lack) of prior months affects the surface and atmosphere.

You get a micro-example of this with home heating and A/C.

Half the day's solarization happens before noon. (1 PM daylight savings). So why doesn't it start getting miserably hot in the summer until 2-3 PM (and/or that's when the A/C system really starts working in earnest)? It's our buddy thermal momentum/storage again. This time, in the structure of your house. It takes hours for solarization to warm the exterior of the house against the prior night's cooldown, and then hours for that heat to soak through insulation.

Now why doesn't that A/C just run when the sun is strongest from 7 AM to 5 PM? Because we're using ancient designs that presume fossil fuel and hydro power, and focus on comfort - aiming for 23°C/73°F at all times. Solarization accounts for the lion's share of need to run A/C, so this time delay in "solarization vs. A/C need" is a huge grid management problem, called the "Duck Curve". Smarter people would realize the value of the thermal mass inside the house's insulation envelope... and have the A/C run when advantageous, e.g. exactly when the sun shines. If everyone did that, the Duck Curve would go away.

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