Fibratus – A Cloud Combed by Steady Winds
Clouds up near the top of the troposphere – that part of the atmosphere where most of our weather occurs – can sometimes appear in long, straight, parallel filaments. This pattern, or cloud species, is known as fibratus and occurs in two of the ten main cloud types: Cirrus and Cirrostratus. The formation’s name comes from the Latin for ‘fibrous’, and it results from the cloud being brushed across the sky by a particularly even distribution of winds.
High clouds like Cirrus and Cirrostratus are composed of ice crystals cascading through the air up near the cruising altitude of aircraft. As they fall, the ice crystals pass through differing winds which determine how they spread across the sky. The neatly combed appearance of fibratus happens when the crystals fall through winds whose speeds or directions change gradually with altitude. Up where the crystals start to fall, the wind is blowing faster than, and often in a different direction to, how it blows further below. As they descend into the gradually slowing winds they trail behind, giving the cloud the appearance of streaks that often extend many miles across the sky. But you have to look carefully at the ends of the streaks to be sure you’ve spotted a fibratus.
This species of high cloud never has hooks or tufts at the ends of its streaks. The strands of fibratus just peter out at each end. Other forms of Cirrus have hooks at their ends, a species known as uncinus (from the Latin for ‘hooked’). These result from a more abrupt change in wind speed or direction at a certain point in the ice crystals’ descent. When the streaks appear to trail from little tufts of cloud, each looking like a strand of hair with a fluffy knot at one end, they’re an example of the species called floccus. Fibratus clouds have no such tangles. Their celestial strands are combed straight and knot-free by the steady hand of the wind.
Cirrus fibratus spotted by ‘kmgoslin‘ over Cambridge, Vermont, US.