I actually see increased traffic in many niches during the summer. I'd say it goes both way; yes people are outside more, but when they're not outside, they're spending more time inside on their computers.
I see about a 10%-15% increase in the peak months of the summer and of winter. Whether or not it has anything to do with the weather, I don't know, but it has been happening for two years, so I'd hope to say that "The Summer Effect" is not true.