Plein air season for me is when the weather is warm enough to be outdoors most of the day. In the Pacific or Inland Northwest, that's pretty much summer, at least for me.
Mornings and evenings can be quite cool even in the summer. They are my favorite time to capture the colors and mood of a scene, however, whether in a little oil sketch like this one, or something a little larger.
(upper left)"View of Adams" Oil, 3"x4", framed ©2012 Diana Moses Botkin