I live in a climate where, even in summer's dead heat, you need to carry a sweater cause of the air conditioning in restaurants. In Madrid, are the restaurants and/or museums super-air-conditioned, moderately A/C, or none at all? Trying to figure out if I need to carry something at night either due to A/C or evening air in Late August or early Sept.

Now that you say it, I recall when, talking to some friends, we all agreed how freezing cold A/C was in the US.
In the summer, you can expect A/C in Spain to be regulated a few degrees above the American standard, however it's true that in some places (i.e. VIPS and other) I've noticed that you can stand only the first 20 minutes. It seems a marketing idea about how to make people eat fast and leave quickly.

Its always a good idea (at least for museums)... but if you're there in the late summer, early fall, eat outside everytime you get the chance! Its a wonderful atmosphere, and you can people watch while taking all in the taste and smell of the food and wine all the while enjoying the sights and sounds of everything going on around you!
Happy travels!

In metro and buses could be a bit cold if they're almost empty. Restaurants usually have a nice temperature.

Hi Prez!
I think there's no need to carry an extra sweater to go to restaurant, except in very few "unlucky" occasions. As Espe3 said, in summer it's very nice to have dinner in the outside tables of the restaurants ("terrazas"). There are some good places south of Plaza Mayor,in La Latina area.

August two first weeks are the hottest. It´s hot even late at night. As for the AC the only time I have really been bothered by it was at a cinema and this happended 7 years ago.

Hope it helps,
Andrés