Yes, the traffic does seem to go down for most people during the summer. People are more likely to search for articles online when they are bored with not much to do. During the summer, people tend to go on more vacations, relax at the beach, hang out with friends at cook outs, etc. During the winter, people are stuck inside a lot and do more searches.
Also, I hate to say this, but some people tend to use the internet a lot while working if they work at an office job. During the summer, people tend to take off more days from work, so they are not sitting at their computers as much.
What you can do to see your traffic remain steady is to write more "summer oriented" hubs. Topics like loosing weight for bikini season, vacation tips, summer clothing styles, and other warm weather related topics tend to bring in traffic throughout the summer.