Three days ago I was sitting by the window in early morning enjoying my coffee and reading blogs, when I noticed several jet trails against the dawn sky. These 'contrails' don't often form at all; there are many more planes passing overhead than the contrails you will see. But when atmospheric conditions are just right, the passing planes leave behind their straight narrow trails, really just another form of clouds.
Even as the other clouds moved in though, you could still see the remnants of contrails cutting straight across the sky. Gave me a perfect series of pictures for: