The anvil is the cirrus blow-off from thunderstorms. The size and scope of the anvil will depend on the updraft strength, the upper levels winds, moisture and the age of the storm. The anvil is composed of ice due to the very cold temperatures high in the troposphere. The anvil will have a smooth appearance of moisture fanning out from the top of the storm.

The updraft strength determines how high into the troposphere the moisture will be carried. In very intense updrafts the moisture can rise into the lower stratosphere. Storms with an abundance of CAPE will have a more explosive updraft. These strong and severe storms will have pronounced overshooting tops and anvils due to the moisture violently rising high into the atmosphere. Once this moisture rises high into the troposphere it will be up to the upper levels winds and time to fan out the moisture.

The upper levels winds will have an important influence on the anvil. If the upper level winds are weak the anvil will not be as pronounced. The moisture will stay fairly close to the top of the storm and the storm will tend to not last as long. This is often the case with "air mass thunderstorms". When the upper level winds are strong the storm top moisture will be quickly advected away from the storm. This produces the classic anvil you will see with big storms. The edges of the storm top are most exposed to the upper level winds and these upper level winds will peal moisture away and entrain it into the upper level winds. The growth of an anvil can occur quite rapidly when the upper level winds are strong.

An updraft with more moisture is going to be able to supply more moisture to the anvil. The warm season storms that are tapping into rich low level moisture tend to have more well developed anvils since the moisture helps contribute to instability and thus a stronger storm updraft. Not only is the updraft stronger but more moisture is being carried into the upper levels for the winds aloft to fan out.

The age of a storm will determine how expansive the anvil is. A young storm has not been exposed to the upper levels winds as long as thus the anvil will not have developed yet. As a storm ages the updraft will continue to supply more moisture to the top of the storm that can be fanned out by the upper level winds. On satellite imagery the older storms have more expansive anvils while the younger storms are less expansive. Often in a line of storms, new storms will develop along the southern edge of the line of storms. This will produce a V-shape appearance of the clouds aloft. The bottom of the V will be the newer storms and the expansive part of the V will be the older storms. Also, moisture tends to fan out from a storm thus the anvil will be less expansive closer to the storm and more expansive further from the storm. Multiple storms fanning an anvil will add a continuing supply of moisture that will be fanned out downwind from the storms. This will also give the V-shaped cloud signature on satellite.