Here in the northeast the summertime in dojos is much slower, student wise, than October through May. Has been for as long as I've been in the arts. The only exceptions that I know are college kids home for the summer who can't wait to get back on the dojo floor.
Besides vacation times, it's not just the weather, it's the light. When we turn the clocks back and it's suddenly dark at four thirty it's kind of depressing. Getting to the dojo seems to brighten everyone up.
Some dojo owners I know close for a week every summer, usually around the fourth of July. That's something I never saw, or even thought about, back in the day.
As for effecting my training, it depends. I used to fly to your fair city, L.A, once a month (90's) and train at Billy Blanks' place in Sherman Oaks. I did that for several years. It was an enormous place with more people than I've ever seen in a fitness place/dojo. Couldn't tell if the numbers were less in the summer, there were just too many folks. But it was always easier to train when you were away, less of the normal, everyday hassles of home.
At the time, I was living in Hawaii and training in a BJJ dojo. It was on the second floor of a two story building with a flat tar roof. Oh, man, the summer was so fricken hot in the place. If you were on bottom with a heavier guy in top position it felt like you were being cooked alive. But it wasn't just the heat, it was getting your butt to the dojo. If you had a long day or were especially tired from work, you would think about how damn hot it was going to be rolling....and sometimes you just wouldn't go. Especially since Brazilians aren't know for puctuality and you knew class would start late, again. (excuses - I got a million of them) So, yeah, summer definitely affected me there. I'm a pussy.