<10: Bloody cold, avoid going outside if possible. Rug up if you do.
15: Winter. Probably rain and wind.
20: Cool Autumn / Spring, you might need a jumper outside.
25: Vaguely warmish, t-shirt weather.
30: Getting warm, wear shorts and t-shirt.
35: Beach weather, avoid going outside for too long if you're in office attire and don't want sweat patches.
40+: Hot beach weather, stay out of the sun (or in the water) in the middle of the day.
Here in Melbourne we'll have several 40-degree days each summer, and the only large city with more that I'm aware of is Phoenix. There are heaps of large cities that are warmer in general, but they don't peak quite as high. Record is 46.2, set a few years ago. Not a pleasant day, especially since it was after a few similar days...
I’ve lived in both the low and highlands of Papua New Guinea and they have two types of weather: dry and wet. In the lowlands you do not go outside without wearing a hat as the sun will actively try and kill you. When a PNG friend visited us in Melbourne, it was a 30°C day, we were in shorts and t-shirts, he was in jeans and wearing a parka to keep warm.