Anvil cirrus is ice-cloud material that spreads away from a thunderstorm anvil under the upper-level flow. It is the fibrous, downstream extension of the parent storm's top and can extend for hundreds of kilometers into otherwise clear sky.
How it forms
A mature thunderstorm updraft reaches its equilibrium level and can no longer accelerate upward. The rising air spreads horizontally, and the ice condensate at the top of the storm is caught by the winds near and above the tropopause. That ice is transported downstream as a broad, thin sheet of cirrus and cirrostratus.
Anvil cirrus is composed almost entirely of ice crystals and is dry compared with the parent cloud. Because upper-level winds are usually much stronger than winds near the surface, the anvil often streams far ahead of or beside the storm below.
How forecasters use it
The direction and extent of anvil cirrus is a live indicator of upper-level flow. Storm chasers use it to orient themselves relative to a storm's tilt, and forecasters use satellite imagery of anvil blowoff to estimate how deep the convection reached and how strong the outflow at the storm top is.
Broad anvil canopies also shade the surface downstream. That shading can suppress heating and inhibit new convection under the shield, which sometimes locks a storm complex into a single dominant mode until the anvil clears.
Where it misleads
Anvil cirrus persists in the upper atmosphere long after the storm that produced it has died. A large canopy overhead can be exhaust from a storm that collapsed an hour ago and hundreds of kilometers away, not evidence of anything active nearby.
The size of an anvil is also not a clean measure of severity. Weaker but slow-moving storms can build large anvils by exhausting steadily into a light upper wind, while very severe storms in strong flow may have compact, sharp anvils. Storm intensity has to be judged from the updraft and low-level structure, not from the anvil alone.