Cirrus fibratus looks striated and is the most common cirrus species; Cirrus floccus species looks like a series of tufts; Cirrus spissatus is a particularly dense form of cirrus that often forms from thunderstorms. Cirrus uncinus clouds are hooked and are the form that is usually called mare's tails. Each species is divided into up to four varieties: intortus, vertebratus, radiatus, and duplicatus: Intortus variety has an extremely contorted shape, with Kelvin–Helmholtz waves being a form of cirrus intortus that has been twisted into loops by layers of wind blowing at different speeds, called wind shear; Radiatus variety has large, radial bands of cirrus clouds that stretch across the sky; Vertebratus variety occurs when cirrus clouds are arranged side-by-side like ribs; Duplicatus variety occurs when cirrus clouds are arranged above one another in layers. Cirrus clouds often produce hair-like filaments called fall streaks, made of heavier ice crystals that fall from the cloud. These are similar to the virga produced in liquid–water clouds. The sizes and shapes of fall streaks are determined by the wind shear. Cirrus cloud cover varies diurnally. During the day, cirrus cloud cover drops, and during the night, it increases. Based on CALIPSO satellite data, cirrus covers an average of 31% to 32% of the Earth's surface. Cirrus cloud cover varies wildly by location, with some parts of the tropics reaching up to 70% cirrus cloud cover. Polar regions, on the other hand, have significantly less cirrus cloud cover, with some areas having a yearly average of only around 10% coverage. These percentages treat clear days and nights, as well as days and nights with other cloud types, as lack of cirrus cloud cover. Formation Cirrus clouds are usually formed as warm, dry air rises, causing water vapor to undergo deposition onto rocky or metallic dust particles at high altitudes. The average cirrus cloud altitude increases as latitude decreases, but the altitude is always capped by the tropopause. These conditions commonly occur at the leading edge of a warm front. Because absolute humidity is low at such high altitudes, this genus tends to be fairly transparent. Cirrus clouds can also form inside fallstreak holes (also called "cavum"). At latitudes of 65° N or S, close to polar regions, cirrus clouds form, on average, only above sea level. In temperate regions, at roughly 45° N or S, their average altitude increases to above sea level. In tropical