What causes an avil shape at the top of cumulonimbus clouds?

Question

What causes an avil shape at the top of cumulonimbus clouds?

Answer 1

The updraft within the thunderstorm goes up until it reaches a level where the air becomes stable (the tropopause).  At that level the air can no longer rise much, if at all.  The clouds that compose the top of the thunderstorm get blown downwind by the winds at that level, forming the anvil shape.  The upper winds are typically much stronger than lower levels, thus those high clouds can get blown quite far downwind of the storm.  In other cases, the upper level winds can be quite light, in which case the clouds at the top of the storm spread out in a circular pattern.

Answer 2

Thunderstorms occur only under unstable atmospheric conditions. Instability (or stability) is characterized by the temperature lapse rate, how fast the temperature changes as you move up in the atmosphere.

Let's just start at the beginning.

As air rises, it expands as a result of decreasing pressure, and cools down as a result of expansion. This cooling occurs at two different, but predictable rates.These are the dry adiabatic lapse rate and the saturated adiabatic lapse rate. The dry lapse rate (approximately 9.8°C per kilometer) occurs whenever the air is below 100% relative humidity, and the saturated lapse rate varies with respect to atmospheric pressure, but is always less than the dry lapse rate because of the energy released by the condensation of water.

An unstable atmosphere, such as where thunderstorms occur, is characterized by how far a parcel will rise if it is pushed from it's current location. Consider this, any height level in the atmosphere will have a specific temperature, pressure, and density. If a parcel of air from one level gets pushed up to a different level it will do one of two things:

1- After it is pushed up, the parcel of air will expand and cool. If this new temperature in our parcel is warmer than the rest of the air at this higher level, the parcel will be less dense and continue to rise.

2- If however the parcel is cooler than the air at the new level, it will sink back down to its original level.

This first situation is call unstable, and the second is of course called stable.

Thunderstorm Formation:

Thunderstorms begin with a disturbance of air and an unstable atmosphere. When air rises, it does so at the dry adiabatic lapse rate up until it reaches saturation. At this point, you will have the base of your clouds. The air will continue to rise and form clouds along the way so long as it is less dense than the background air. In the case of a thunderstorm, the air rises all the way to the tropopause. Above the tropopause is the stratosphere (where the ozone layer is) and temperatures actually increase with height, so it is an incredibly stable layer. Winds are high in the upper troposphere, so the top of thunderstorms usually spread out and form an anvil shape if they are not blown away completely.

Just an aside:

I went on a lot about instability, and you are probably wondering if there is some sort of measurement for it. There is! (Actually there are a few different measurements for it.) The most common is called the CAPE index (pronounced just like the word "cape".) Most sounding diagrams will list the measured CAPE somewhere on them. For example, this one shows a CAPE of 515 at the top.

http://www.rap.ucar.edu/weather/upper/fw...

The higher the CAPE, the more likely a storm will form. And, if a storm does form, higher CAPE values will indicate faster formation and more severe weather.

Answer 3

Stability condition of the atmosphere(which prevents the growth of the cloud) and winds(which causes spreading of the top portions of the cloud)) at that particular level.