100° in Arizona vs. 100° in NYC-- NYC will be worse.
100° in Seattle vs. 100° in NYC-- Seattle will be worse.
90° in NYC vs. 110° in Arizona-- Arizona is worse.
90° in Seattle vs. 110° in Arizona-- I'll take Seattle, but this one divides people who have experienced both.
It's irritating for people who have actually experienced 120 degree weather listen to softies accustomed to cool ocean breezes and mediate temperatures complain about 90 like it's sweltering. It ain't shit. People in Seattle don't know hot. I've been there and elsewhere in Washington several times during the summer.
I don't even break a sweat walking in dry 90° weather. Hell, just a few weeks ago, on the 13th, I walked nearly 21 miles through the middle of the day in Wichita; the peak temperature that day was 93°, with a mean of 84°; an average humidity of 64%, and a peak humidity of 84% . Burnt like a crab since there was no shade, and I dumbly forgot to apply sunscreen. I wouldn't have made it past 10 miles where I live at 113° with no shade (and the avg humidity tends to fall in the 40's on hot summer days like this). It's just not the same. If you don't hide from a ~115° sun...it will simply destroy you.
The heat index is bullshit. I've never found anything more useless and out of step with my personal perceptions:
https://en.wikipedia.org/wiki/Heat_index#Table_of_values
Seriously. Have they ever had to literally shut down flights because it "isn't safe to take off" due to the Seattle heat? I strongly doubt it. I'd be interested to know.