I mentioned contrails earlier because they are also caused by frozen water droplets hanging in the air, gleaming in the sunlight. But contrails happen when warm air is released with jet exhaust and collides with supercooled water in the stratosphere. This causes the water to freeze instantly, and thus you can see a long trail of frozen vapor where a plane just flew overhead.
http://www.sott.net/article/288430-This-bizarre-glittering-cloud-just-appeared-in-the-sky-over-Australia