Wispy cirrus clouds are seeded mostly by airborne mineral dust
Those thin white streaks high overhead are made of ice, and each crystal needs something to grow on. Research aircraft sampling cirrus over North and Central America found mostly mineral dust, along with metal particles and possibly biological ones. Cirrus covers roughly a third of Earth at any time.
Cirrus forms when warm, dry air rises and water vapour freezes directly onto tiny particles, typically between 6,000 and 20,000 metres up, highest in the tropics and lowest near the poles. The dust sampled in these clouds contained aluminium, calcium, iron and silicon; the metallic particles included nickel, copper, zinc and lead. The researchers concluded that mineral dust supplied most of the ice nuclei. Luke Howard first defined the cloud scientifically in 1803, and its name is Latin for a curl or fringe.
The clouds are thin enough to see the sun through and look white, although sunrise and sunset can tint them yellow or red. Five species are recognised. Fibratus, streaky and striated, is the most common; uncinus, with hooked ends, is the familiar mare's tail; spissatus is dense and often spills from thunderstorms. Varieties add more detail: intortus is tangled, sometimes twisted into looping Kelvin-Helmholtz waves by winds blowing at different speeds, while vertebratus lines up like ribs.
Cirrus often trails fall streaks, strands of heavier ice crystals whose shapes are set by wind shear. Those crystals melt and evaporate in warmer, drier air below and never land. Still, cirrus can spread from the tops of thunderstorms and tropical cyclones and often signals approaching rain. Light bending through its ice creates halos and sun dogs. Its relatives, cirrostratus and cirrocumulus, differ in form, and cirrocumulus holds supercooled liquid droplets as well.
Satellite data from CALIPSO put average cirrus coverage at 31 to 32 percent of Earth's surface, ranging from as much as 70 percent over parts of the tropics to about 10 percent in some polar areas, with more at night than by day. Because the clouds warm the planet, they may contribute to climate change. Cirrus has also been seen on Mars, Jupiter, Saturn, Uranus, Neptune and Saturn's moon Titan, some of it made of ammonia or methane ice.
Source: Cirrus cloud
