Plein air is a term derived from the French phrase en plein air, which literally means 'in the open air'. It's a familiar concept today, but in the late 1800s when the Impressionists ventured out of their studios into nature to investigate and capture the effects of sunlight and different times of days on a subject, it was quite revolutionary.
An early Fall is a great time to paint outdoors. Bright colours and fresh air are very exciting opportunities to practice your painting skills. Focus on what you see, not what you can imagine or intellectualize about the subject (otherwise you may as well be back in your studio).
Consider scouting out locations in advance to decide what you're going too paint and where you would set up. I choose a park very close to my home-studio. This way when you head out to paint you can spend the whole day painting and take along the best selection of colors for that particular scene. Look right around, 360 degrees, so you don't miss the possibilities 'behind' you.
The ideal spot to set up will be in the shade, out of the wind, but this often isn't possible. There's something about seeing an artist at work that makes people extremely inquisitive, more likely to talk to a stranger, and prone to giving unwanted opinions. It can be disconcerting, especially if your painting isn't going well, and quite disruptive if it happens a lot. Considering positioning yourself where people can't come up behind you, such as against a wall or in a closed doorway. In the park it is impossible, so I tried to set myself far from walking paths.
If you don't wish to chat, be politely non-responsive along the lines of "I'm sorry I can't talk right now I've only a limited time to do this". Most people simply want a closer look at what you're doing, and so saying "Feel free to have a look" then getting on with what you're doing is all it takes. Some people will be keen to give you all sorts of uninvited advice; be thick-skinned and try to get rid of them extreme politeness, for instance with a "thank you, but I'm fine with what I'm doing".
In my opinion a plein-air painting needs to be started and finished outside the studio, because any of them is simply studies and can be used in the future as base for bigger and more serious paintings.
My painting outdoor experience this fall was great.
