Cirrus clouds are atmospheric clouds

Cloud

A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

 generally characterized by thin, wispy strands, giving them their name from the Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 word cirrus meaning a ringlet or curling lock of hair. The strands of cloud sometimes appear in tufts of a distinctive form referred to by the common name of mares' tails.

Cirrus clouds generally appear white or light grey in color. They form when water vapor undergoes deposition at altitudes above 16500 ft (5,029.2 m) in temperate regions and above 20000 ft (6,096 m) in tropical regions.