Summer thunderstorms are ubiquitous throughout Virginia, but those with anvil clouds (cumulonimbus
from the Latin for “pile” and “rain cloud”) are particularly dangerous. They get their name as they resemble the anvils used by blacksmiths – or think of the heavy metal object that often falls on Wile E. Coyote in cartoons.
The Development of a Thunderstorm with Anvil Clouds
On a typical summer afternoon in Virginia, surface air in any given town or city is warmed by the sun-heated ground and rises. Water in the air will change from gas to a liquid state, forming droplets as it hits cooler temperatures at higher altitudes. With an average humidity of 91 in the mornings and 60 in the afternoons, Virginia is prone to this sort of storm cloud formation. If you watch, you can see the clouds grow taller and taller and darken at the bottoms as they condense and prevent sunlight from penetrating. These “towers” are the hallmarks of summer thunderstorms.
Many times you’ll see the very top of the cloud flattening out, creating an anvil shape that protrudes above the main cloud mass.
This anvil part is made of ice particles. The flat top forms when the cooler air in the cloud expands and spreads out when it reaches the warmer tropopause – the boundary between the troposphere* and the stratosphere.
What makes storms with Anvil Clouds more dangerous
Lightening
The greatest threat from an anvil cloud is the lightning it produces. Typically, lightning originates in the lower portion of a thunderstorm. In an anvil cloud, the lightning that comes from the energy produced as the air hits the tropopause wall can be ten times more powerful than a typical lightning strike.
A “bolt out of the blue” is lightning that strikes when the sky above you is blue. These strikes originate in anvil clouds that can be up to 30 miles away. Because they are more powerful and unexpected, this makes them particularly dangerous.
Hail
Anvil clouds are composed primarily of ice particles, a good indication that this storm will include hail. At times, you may see streaks of snow called “virga” falling from the edges of the anvils. This snow evaporates as it falls through the relatively dry air surrounding the upper part of the thunderstorm. Anvil clouds often contain heavy snow that turns to rain as it falls. It is estimated that 50% of the rain produced by the average thunderstorm begins as ice and snow.
Because anvil clouds are mostly ice, when these particles begin to fall, they can often thaw in the warmer air and refreeze as it goes through the denser thunderstorm clouds, forming hail.
A recent storm in Texas produced the largest hail on record at nearly 6” across. The typical hail size in Virginia is pea-sized, or less than ½”.
Wind, Rain & Tornadoes
Summer storms with anvil clouds will produce more violent storms characterized by heavier rains, hail, and tornadoes. The excess energy, created as the ice crystals and warm air collide in the tropopause, causes more violent air rotation and higher air ionization.
While anvil clouds are interesting phenomena to see, use caution in planning any outdoor activities if you see one.
* The troposhere is the lowest region of the atmosphere, extending from the earth's surface to a height of about 3.7–6.2 miles (6–10 km), which is the lower boundary of the stratosphere.