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September also seems to be warmer than May in much of southern Africa.
The biggest lead seems to be inland. Pretoria's September - May difference is 3.9C, Bloemfontein 3.9C, Windhoek 3.5C, Harare 2.5C. The biggest I can find is Gaborone (4.4C).
On the coast from Durban (0.1C) to Maputo (0.1C) and Xai-Xai (0.5C) there is a small lead.
The lead is absent from the south. September is cooler than May in East London (-0.8C), Port Elizabeth (-0.7C) and Cape Town (-1.1C)
All the seasonal lead climates have a winter dry season and the inland ones with the most seasonal lead have a very strong summer rainfall bias - perhaps this has something to do with it? Those without either have more or less evenly spread rainfall, peaks in spring and autumn or a winter rainfall bias.
My coastal climate has a slight seasonal lead, with September 0.2C warmer than May, and also has a strong winter rainfall
peak.
Other climates in the area vary between a 0.2C lag and others sharing the 0.2C lead of my climate.
Interesting, it was just an idea that it had something to do with a winter dry season.
Thinking about it though, perhaps these are not true seasonal leads as September is much further from the June solstice than May is. 15 Sep is 87 days after the winter solstice while 15 May is only 38 days before, so Sep is going to have a lot more insolation than May.
In Sydney, seasonal lead is more pronounced inland in the western suburbs. September is warmer than May by 2C in Penrith. It's probably an east coast phenomena in Australia and it has to do with climates that have drier winters. I think Bega is the southmost area in the east coast to feature this seasonal lead. But only in areas east of the Great Dividing Range. Going inland, or crossing the Range, the lead is non-existent.
As for why, I think it's because September is much more prone to high pressure systems, which bring warm winds from the interior and provide clear conditions. After all, on the coast of NSW and QLD, July-October have the highest sunshine percentages. And August is the sunniest month in Sydney. Westerlies tend to dominate at that time, and they can be (relatively) warm. May is prone to easterlies that would bring cool rainy days and even the dreaded east coast lows. So you can't expect a lot of bright, warm, 22C+ days in that month. Whereas September is the complete opposite of that (although nights can be cooler due to clear skies).
What I dislike about our seasonal lead is the fact that November (late spring) can at times be warmer than February (an actual summer month) - You'd more likely have a 40C+ day in November than in February. It's a bit "betraying" and annoying. Again, this is due to active (anticlockwise-rotating) high pressure systems found on the subtropical ridge in spring, which bring us desert heat from the interior. They're more common around that time.
In Sydney, seasonal lead is more pronounced inland in the western suburbs. September is warmer than May by 2C in Penrith. It's probably an east coast phenomena in Australia and it has to do with climates that have drier winters. I think Bega is the southmost area in the east coast to feature this seasonal lead. But only in areas east of the Great Dividing Range. Going inland, or crossing the Range, the lead is non-existent.
As for why, I think it's because September is much more prone to high pressure systems, which bring warm winds from the interior and provide clear conditions. After all, on the coast of NSW and QLD, July-October have the highest sunshine percentages. And August is the sunniest month in Sydney. Westerlies tend to dominate at that time, and they can be (relatively) warm. May is prone to easterlies that would bring cool rainy days and even the dreaded east coast lows. So you can't expect a lot of bright, warm, 22C+ days in that month. Whereas September is the complete opposite of that (although nights can be cooler due to clear skies).
What I dislike about our seasonal lead is the fact that November (late spring) can at times be warmer than February (an actual summer month) - You'd more likely have a 40C+ day in November than in February. It's a bit "betraying" and annoying. Again, this is due to active (anticlockwise-rotating) high pressure systems found on the subtropical ridge in spring, which bring us desert heat from the interior. They're more common around that time.
But Nov gets more insolation than Feb so you'd expect (in the absence of lag) for Nov to be warmer than Feb. According to https://www.timeanddate.com/sun/australia/sydney Sydney gets 13:53 hours of daylight on 15 Nov and 13:18 hours on 15 Feb.
The way the year is divided into quarters by months has an inbuilt lag with the spring month of Nov getting more insolation than the summer month of Feb etc, so a lot of the apparent seasonal leads may just be weak lags.
Obviously the amount of energy at ground level will be modified by clouds, which could vary seasonally. But it should show why Sep should, in the absence of seasonal lag, be considerably warmer than May in the SH.
At the Tropic of Capricorn (closest marked latitude to Brisbane) mid September gets about as much energy from the sun as the end of March/start of April does. September as a whole is roughly equivalent to 15 March to 15 April.
Brisbane seems to have very mild winters but its a rainy climate with a avg annual rainfall of almost 40 inches. Where I live in CA the winters are mild too (not as warm as brisbane) but the avg annual rainfall is only 13" (if lucky enough). I would love to visit Australia during the winter season (june or july).
Even for Gaborone which looked as though it had a large seasonal lead, the spring equinox ~22 Sep has an average high of 29C and low of 15C, while the autumn equinox ~21 March is 29 and 18C. So there is actually a small lag rather than a lead.
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