Do you find your audience is more 'seasonal' than consistent?

On my older projects it was reasonably consistent, but i find on my new projects only time will tell but at this moment in time its on and off, i think depending on what you are marketing depends on when the audience / people will come to a website. if they enjoy they will be their more often if not so much going on they may leave it for a bit before coming back.

Summer is usually a down period as most people go away on holidays so the internet apart from those on mobile devices.

I think most communities can perceive this in different degrees, depending on the ages and situations of the majority of its active members. Once you get used enough you can anticipate the dry periods and when it can be an opening to try to grab back your members' attention while they are free so they still feel compelled to check back on you even after they go back to their school, work, projects, etc. 

I do tend to get more traffic in the summer time since schools are out and people have more time to burn, unlike later parts of the year where there is more work, school and holidays.

Yes. I would say it is definitely Seasonal. Summer is usually a downtime as most people go on holiday, so apart from those on mobile devices, the internet goes away.

Unlike the later part of the year where there are more jobs, schools and vacations.

usually summer is high for my forum, I guess it depends on the members.