Well, I like to model outside, nothing beats airbrushing on the patio.
The problem here in Alberta, as it is in much of Canada's Prairie region, is the total unpredictability of the weather, it rarely goes with the forcast.
More than once I've planned on airbrushing outside because the sky was clear, the temperature was comfortable and no wind. Well, there I am setting up my airbrush and clouds move in and it starts raining, or I've gotten far enough into it to start painting and the wind comes up. I remember a couple of years ago in late May, all the snow was gone, the temperature was decent, no wind.... No sooner did I start spraying than an unforcast flurry of snow hit }:-(
I don't like heat and humidity much, but I could sure appreciate some consistant weather.
I agree with maddafinga, spring and fall are the best times to be outdoors, not too hot or humid and not so many bugs to get stuck in your wet paint!