Cirrostratus Clouds Over San Francisco

Saw this streaking overhead outside Chase Center: cirrostratus clouds, the thin, fibrous lines that stretch across the upper atmosphere. They form from ice crystals at altitudes above 20,000 feet, which is why they stay so thin and pale even under direct sun.

This batch had a clean striped pattern, close enough together that you could still see blue sky between the bands. Cirrostratus clouds sometimes form a visible halo around the sun when the ice crystals bend light at the right angle, though that wasn’t happening here.

Meteorologists treat cirrostratus cover as an early signal that a warm front or rain system might be moving in over the next day or two. It doesn’t always pan out — sometimes it really is just a good-looking sky over Mission Bay.