It’s slowly starting to look like the next great influx as seen from Fedecan stats.

People reply with “people lived in tropical climates before air conditioning”, but forget to account for not comparing apples to apples. The tropics do not have the same conditions post-AC invention than they did pre-AC. If you lived in Texas 500 years ago it is not the same as living in Texas today.

Yeah Perth, Melbourne, and Adelaide can get quite hot, but usually in those instances it’s hot desert air that typically causes the dew point to drop drastically. That means the heat index in some instances is lower than the air temperature. See here for an example where the air temp climbed to 41°C / 106°F, but the heat index is only 38°C / 101°F.

Here in Boston the heat index typically hits 38°C / 100°F every summer, and in recent years it’s happened multiple times. but that’s usually due to the humidity that typically prevents the air temperature from climbing above 36°C / 96°F. There was a day last summer in Boston where the heat index was 38°C / 101°F, but the air temp was only 32°C / 89°F, because the dew point that day sat at 24°C / 75°F for the entire day. We’ve had 40°C / 104°F heat indices every other summer and have topped at 42°C / 107°F last summer.

There is also the case of overnight lows that are also a lot higher than in the major Australian cities save Brisbane. Since the dew point is lower in most Australian cities even if it gets hot, Australian cities tend to have cooler nights than those in the US. Here in Boston when we get days with heat indices approaching 38°C / 100°F, the overnight heat index tends to hover no lower than 23°C / 74°F. Boston had an overnight heat index of 31°C / 88°F following the day it hit 42°C / 107°F. I’m not aware of any of the 5 major Australian cities with that high of a nighttime low, but I would imagine the situation in the rest of the U.S. further south to be much worse than what Boston had to endure that day.

One thing Australians got going for them is that all 5 major Australian cities all have colder summer averages than any American sunbelt city. Brisbane, Perth, and Adelaide all have average summer heat index maximums below 31°C (87°F).

NYC has the same average summer heat index 31°C (87°F), and dew point as Brisbane, so Brisbane is not any more hot or humid than NYC in the summer. But that means a large swath of the eastern US south of the 40th parallel are all hotter and more humid than much of QLD and NSW. Perth has the same average summer heat index as Chicago and Minneapolis at 29°C (85°F), and those 2 American cities are the furthest north and closest to the Canadian border. Sydney and Adelaide have average summer heat indices colder than Boston (27°C / 81°F vs 28°C / 83°F), which the latter is the northeasternmost major city in the US closest to New Brunswick. Melbourne (26°C / 78°F) has colder summer averages than Ottawa, Toronto, or Winnipeg (27°C / 80°F).

This.

Lets say a particular random hypothectical location has a average summer daytime maximum of 32°C (89.5°F) and a dew point of 24°C (75°F) for the most recent climatological period (1991-2020).

That is an average heat index of 38°C (100°F) using the most recent averages.

Now let’s say in the time since 1850, this random hypothectical location has seen average summer maximums climb by 1.25°C (2.25°F), and average summer dewpoints climb by 1°C (1.8°F) as an example.

That means that back in 1850 in the era before air conditioning, people living in the same exact location would have only experienced an average summer heat index of 35°C (95°F), since the average summer maximum back in 1850 would have only been 30.7°C (87.2°F), and the dew point only 23°C (73°F).