Cirrus clouds are thin, wispy clouds that form high in the sky, often looking like delicate white streaks or curls painted across blue. They sit so high up (around 20,000 feet or more) that they're made of ice crystals instead of water droplets, which gives them their feathery appearance.

Sometimes people call them “mare's tails” because they resemble the flowing tail of a horse.

Cirrus clouds often appear when fair weather is present, but they can also signal that a change is coming. When you see them thickening and spreading across the sky, a storm system might arrive within a day or two. Pilots and meteorologists pay attention to cirrus clouds because they can form at the boundary between different air masses, like scouts appearing before the main weather front arrives.

If you're lying on your back watching clouds, cirrus are the high, wispy ones that look almost transparent, very different from the thick, puffy cumulus clouds that look like cotton balls floating much lower in the sky.