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Actually, statistically it is July 31 in the northern hemisphere and January 31 in the Southern Hemisphere, the day on which the sun no longer heats the earth and because of axial tilt the ground in that hemisphere starts cooling as the cooling action of longer nights and decreased angle of the sun is a stronger factor than the heat added during daylight hours.

There are no dramatic changes at that point, as the differential is very small, but accumulates each day until January 31 in the northern hemisphere and July 31 in the southern.

A: July 31st?? B.S. There had to be ZERO effort in that. The hottest day of the year USED TO BE July 21st, about 20 years ago, and a few hundred years before that, BUT, now it is July 19th, for sure! I spent quite a bit of time working on it. This would make July 19th the coldest day of the year in the Southern Hemisphere, and it would make January 19th the coldest day of the year (exactly six months) in the northern hemis. (and the hottest day of the year in the southern hemis).

These last dates might vary a day or two (but not more!) due to differences in the hemispheres, but July 19th as hottest is certain. The reason it changed from July 21st to July 19th over the past 20 years? My educated guess is that it is because of the destruction of the Amazon Rainforest, and the paving over of more of the world because of the population increase.

IF the Earth heated up instantly, like the Moon comes close to doing, then the hottest day of the year would be June 21st, since THAT is the longest day of the year (more sunlight means more heat), BUT, because of the Earth's atmosphere, and especially the oceans (and forests, etc, etc), the Earth slowly heats up and slowly cools off (which is definitely a good thing). R. Hovis 5/20/16

Older answer:

The fact that the Earth is heating up two days quicker (to the hottest day) means the destruction of "buffers", like forests, etc, has had a detrimental effect. The climate places that keep this data say that, on a grand average, the hottest day of the year is July 21st. (for hundreds of years of recorded temperatures), Some scientists say that, for the past few years, is July 19th, but I would stay with "July 21st" until the "July 19th" crowd provides more data. (and, that would mean that July 21st is also the coldest day in the Southern Hemisphere

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9y ago

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