Weather Watch

Thunderstorm clouds, ‘downpour of rocks’ create intense Central Coast weather

A cumulonimbus cloud with an “anvil” top rises above the hills in San Luis Obispo County.
A cumulonimbus cloud with an “anvil” top rises above the hills in San Luis Obispo County.

In Greek mythology, Daedalus constructs a set of wings for his son Icarus made from feathers and wax to escape from Crete.

Daedalus warns his son not to fly too low nor too high, so the wetness of the Mediterranean Sea would not dampen his wings nor the sun’s heat melt them away. Unfortunately, Icarus ignored his father’s advice and flew too close to the sun. As the tale goes, the sun’s heat melts the wax, which held the feathers in place, and gravity pulls the featherless Icarus into the sea.

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This fable reminds me of the layers of the atmosphere and the stories that clouds within them can tell. Experienced weather watchers who saw “anvil” shaped clouds Wednesday got a taste of that, and they were reminded that they were capable of creating severe weather. Let me explain.

The troposphere is the lowest layer of our atmosphere that starts at sea level and extends upward to about 33,000 feet, depending on your latitude and the atmospheric conditions; some days, this level can be higher, other days lower. Excluding inversion layers, as you climb higher in the troposphere, temperatures decrease with the lower air pressure. Pretty much all the weather we experience on earth occurs in the troposphere, which contains approximately 99 percent of the atmosphere’s water vapor.

On top of the troposphere is the stratosphere, which extends to about 164,000 feet above the ground. The ozone layer, which absorbs ultraviolet light from the sun and converts most of it to heat, is in the stratosphere. In this layer, the higher you go, the warmer it gets. Perhaps this is why Icarus’ wings melted. The air is thin at these altitudes; consequently, the atmosphere is mostly stable, which produces a smooth ride for jet airliners.

Last Wednesday, a vigorous upper-level low-pressure system swung through the Central Coast and created a highly unstable atmosphere. In Creston, quarter-sized hail reached the ground, and a well-defined funnel cloud was photographed in Santa Maria. Jessica Brezden of Creston said “it was like a downpour of rocks.”

So how are these hailstones formed?

Hail is produced in cumulonimbus clouds, otherwise known as thunderstorm clouds. Strong winds within the cloud will keep super-cooled (less than 32 degrees Fahrenheit) raindrops suspended thousands of feet in the sky. The super-cooled liquid will freeze on contact with particles such as dust or even insects and grow larger and larger in the strong updrafts as they collide with other subfreezing liquid droplets.

When the hailstones become too heavy to be supported by the updrafts or carried away by winds aloft, they fall to the earth. Often in San Luis Obispo County, hail will melt before reaching the ground. The thunderstorms that produced quarter-sized hail in Creston required updraft winds of between 37 and 56 mph. The height at which the air stops to rise is known as the “equilibrium level.” The equilibrium level can coincide with the boundary layer between the troposphere and stratosphere, which can create “anvil” tops.

The vast majority of thunderstorms aren’t robust enough to reach the top of the troposphere. However, on Wednesday, the updrafts were strong enough to do just that. Weather balloon data indicated that the top of these anvils was about 30,000 feet. On rare occurrences, the rising air can burst into the stratosphere and produce overshooting tops on the anvils. Intense updrafts indeed!

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PG&E note: Nearly 70 percent of the electricity PG&E delivered to its customers in 2016 came from greenhouse gas-free resources.

John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter: @PGE_John.

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