Cirrus
Delicate white streaks and hooks of ice, the highest ordinary cloud and the best for sunsets.
What it looks like
Fine white filaments, tufts and hooks made entirely of ice crystals. The classic shape is a comma: a small tuft where crystals form, and a long tail where they fall into slower wind below. Old names include mares' tails.
What it means
On their own they mean nothing. A sky that thickens over a day from scattered cirrus to a milky veil and then to a grey sheet often means a warm front and rain within a day. Mares' tails that are followed by a clearing sky are simply a jet stream passing over.
Not to be confused with
Aircraft contrails, which are straight and appear where nothing else is, and cirrostratus, a continuous veil rather than separate streaks.
Seen one? Add it to your log or check with the identifier.