Working outdoors

Most of my recent work over the past number of years has been landscape painting, done 'en plein air' with a french easel. I find it very liberating and challenging working this way. Liberating, in the sense that it frees me from the confines of the 'white cube' studio space, and challenging because of all the problems that come with it. First, the weather becomes a major factor; almost a partner in the creation of the work. Once a painting is started under a certain light condition, or time of year, I'm forced to proceed with the completion of it under the same conditions, as close as that's possible. So, most of the time the weather is dictating to me what times I can work on a certain painting. It also puts me on a timeline that syncs my painting time with the rotation of the earth. That is, if there are certain shadows that are cast in the painting, I only have a small window of time to paint them under the same conditions, during the same time of the day. Recently, my paintings have moved from a looser impressionistic interpretation of color and light, to a descriptive, exact factual space. Whereas before I could complete a small painting in a few hours, I find myself now returning to work on larger, more detailed paintings, over many weeks.
One painting I started in November, had to be put on hold. I was painting under the elevated subway line (F/G) near my studio, in the morning. I was initially fascinated with how the light was raking across the front of a series of old factory buildings, causing the bricks to glow with a fire-like intensity. The light in November has such a magical, crisp quality, causing very dark, violet shadows. I immediately thought of Edward Hopper, and specifically "Early Sunday morning" in the Whitney's collection.
So after a couple of prep drawings, I started a painting about 18" x 24". I continued working on it a few days a week during that month, from about 9am until 11, until the sun just peeked out from behind the elevated track above and behind me, causing my canvas to be flooded with direct light. This obviously was a severe limitation I was putting on myself, given the fact the I only had about a 2 hour window to paint this scene, and the increasingly shorter days and dropping temperatures. Needless to say, by the beginning of December, I had to stop work on it. The light had become totally different, and it was too cold to paint. Usually my threshold to paint outdoors is about 50 degrees, not only for my own comfort factor, but the paint and the media I use (25% poppy oil, 75% turpentine) doesn't flow or stick to the canvas right. I tried taking some photos, but they left me disoriented and confused in relation to the painting I started. At some point, I'll write about the use of photographic material in relation to my paintings, but for now I'll just say that it wasn't going to help me. I had gone too far with the painting, and what was recorded in the photographs and what was happening on the canvas were miles apart. I threw them out and decided I'll continue working on it in the spring when the weather gets warmer, even if it means redoing the entire painting under different light conditions. Hence, the frustrations of working 'en plein air'.