To clarify, I’m asking why when it is winter IN the southern hemisphere, why is it milder than winters in the northern.
Not asking why are the seasons reversed.
In: 2765
Southern hemisphere winters as a whole are actually colder than northern hemisphere winters. The difference is where the land is (ie. where the people are). Almost all southern hemisphere land is relatively close to the equator: only the far southern tip of NZ and far southern parts of Argentina and Chile extend south of 45° latitude. This means that almost all land (and thus people) living in the southern hemisphere are closer to the equator than the South Pole.
Compare with the northern hemisphere where billions of people live north of 45°N: most of Europe, almost all of Canada, fair chunks of the US, China and of course Russia and other central Asian an ex-USSR countries.
The major population centres of the southern hemisphere are all tropical, subtropical or mild temperate/oceanic (think southern Australia, NZ). They just aren’t far enough south to get persistent polar air masses in winter.
Furthermore, the fact that it’s pretty much all ocean south of 45°S all the way to Antarctica means that polar airmasses that do work their way to more temperate areas warm up a lot more than they would in the northern hemisphere. Water has a MUCH higher heat capacity than land does, particularly if that land is already snow covered. Compare an equally cold polar airmass leaving the polar regions and heading to a mid latitude city (say Chicago for the north and Melbourne for the south). In the north it comes down across land the whole way (Canada), and has little opportunity to pick up any heat from the frozen land below. In the south it’s absorbing heat from the unfrozen ocean the whole way up before it gets to Australia, and thus is substantially warmer by the time it gets there (note: Australia does get substantial snow still at higher elevations, but not in any of the major cities).
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