I live in a latitude about the same as Spain, Southern Italy, Greece and Turkey, but in my part of the U.S., it's been above 30 degrees Centigrade for a few weeks, with high humidity. Our winters also get around 0 Centigrade (+- 8 or 9 degrees) pretty regularly, so we need to supply both heating and cooling.

Usually when it gets above 24/25 C we start to really put on the air conditioning. And below 20 C we put on the heat. A/C is generally important for not only cooling, but reducing humidity so your body's sweat actually does something useful.

I've lived in houses around here that didn't have A/C and it's pretty dreadful in the summer. You basically don't do anything during the day, and it's too hot to really sleep comfortably. Even fairly poor people usually end up with at least one room with A/C.

This isn't true for all of the U.S., the Pacific Northwest (e.g. Seattle) and Northern California tends to not have air conditioners because it doesn't usually get hot enough.

Quite a bit of the U.S. is at the same latitude as North Africa or the Middle East, the northern-most bits get maybe to Belgium? So even with the temperature gradients that ocean currents and global air movement affords, we get total sun that's more like areas of the world people associate with being "hot".

Having been to Europe numerous times, I notice most public areas, cars, buses, trains and offices seem to have A/C. So I'm sure you spend more time in it than you may realize.

"And below 20 C we put on the heat."

I'm pretty sure we don't put our heating on here in Edinburgh until October when nights are probably getting to be well below 10C.

Mind you for me 10C is "comfortable", 15C is "warm", 20C is "hot" and 30C means I'm on holiday somewhere hot where I can go capsizing to stay cool.

There's definitely an element of being locally acclimatized. I remember spending some time in the desert, finally got used to the hot temperature when December rolled around. I was freezing all the time and finally checked the thermostat, 19 C.

I had a situation like that when I first went to university. My parents house in the north of Scotland was rather old and rather ramshackle with no central heating - I can remember going to have a bath once and finding 2" of ice in the bath. However, I don't remember feeling cold as a kid.

However, I went to university and stayed in centrally heated halls of residence, which I didn't find warm. However, when I returned home for Xmas it felt like I had been shipped to Siberia - I was dying of cold! I remember being pinned to the bed by a pile of blankets as I desperately tried to stay warm at night.

I'm in Newcastle and I'll probably have heating on next month - basically the lowest I let my home temperature drop to naturally is ~17C, then I put the heating on, set to 19C.

Isn't that because Geordies are famous for never wearing warm clothes? :-)

http://news.bbc.co.uk/1/hi/england/tyne/7892733.stm

In greece, summers are atrocious without AC. But it gets even worse in the middle of polluted cities like athens.

Some corners of Athens are disgustingly unbearable. Tall buildings, narrow streets filled with parked cars. The entire thing is a dust, smoke and heat trap. Temperatures feel 10 degrees warmer in those corners. Not to mention it smells horrible.

I'm from Poland, last week we actually had 38C outside and I don't know anyone who has AC. Yes it's hot during the day and nights can be uncomfortable, but every conversation I had with friends about this always ends with "yeah it would be nice to have but it's not worth the expense for the few weeks of summer".

It's not all year round. The rule is pretty easy: when you can't think clearly any more you start your AC.

That's the reason some houses have not AC in the south of Europe but all the offices have.

I live in Louisiana, southern US. I generally leave the thermostat set to ~23°C, so the AC comes on periodically to try and maintain that temperature indoors. During the warmer months, that's a significant portion of every day. During the winter, the central heating comes on instead to do the same job. There are some times of the year where neither come on, but they are brief and it will likely be on at night. It would get far too humid indoors without something running, I wager.

Here's the thing: You'd save significantly by setting it to 25deg. when cooling and 19deg. when heating. Both are very comfortable once you've adjusted for a day or two - after that, going to 23deg. actually feels wrong (either too cold im summer or way too hot in winter).

Don't underestimate the thermal power needed for just one additional degree in a typical American house with poorly insulated windows, walls and doors.

Northern Italy (just below Austria). The hottest is around 37° Celsius, but I don't use AC until I go over 30 inside the house (which has happened just once or twice this summer). Workplaces usually keep AC on as long as temp is above 23°, but that's just my experience.

I live near Barcelona, those days the temperature reaches 27-28C.

We don't put the AC, we open the balcony door and the air current is enough.

In winter, we reach at most 5C, we don't put also the heating. Some neighbors have it very high and in our apartment the temperature is 20-21C.

I live in Australia.

We tend to use AC if the outdoor temp is above 30C, so most days November to April.

It gets to above 45C here, which is pretty horrible without AC.