Weather Watch

What are those ‘jellyfish’ floating in Central Coast skies?

Jellyfish clouds photographed Oct. 9 by Cambria resident Elizabeth Bettenhausen.
Jellyfish clouds photographed Oct. 9 by Cambria resident Elizabeth Bettenhausen.

Clouds can look like hair, heaps or layers and exist in the atmosphere in thousands of different forms and sizes. Just by looking at them, they can often tell you what the atmosphere is up to, even without the use of sophisticated meteorological instrumentation like weather balloons or satellites.

Last Sunday, I received numerous emails from sky-watchers about clouds that resemble jellyfish, complete with tentacles that whimsically stretch downward. For me, they resemble the album cover art of Roger Dean on a few Yes records.

These unusual, but not entirely uncommon clouds along the Central Coast are known as jellyfish clouds, or altocumulus castellanus. These are typically middle-level clouds that are classified as altostratus or altocumulus, which form at altitudes from 6,500 to 23,000 feet. They are primarily composed of water droplets. However, they can also be made of ice crystals. Castellanus means towerlike turrets, like a castle, that billow upward from the base of the cloud.

So, what happened last Sunday to create these rare clouds? Well, the weather charts indicated a weak upper-level low pressure system off the Southern California coast that pulled in a small amount of subtropical air from the Gulf of California. Weather balloon data from Vandenberg Air Force Base on Sunday revealed a dry mass at the Earth’s surface and another dry layer at about 17,000 feet, with the subtropical air from the Gulf of California sandwiched between.

As the moist subtropical air rose and cooled, its invisible water vapor condensed on small nuclei from dust, forest fires, salt from ocean spray or even sulfite particles from phytoplankton, into visible heaps, or white puffy clouds.

However, as the subtropical air rose above 17,000 feet it ran into the layer of much drier air and stopped creating visible clouds due to evaporation. In other words, these clouds could not rise higher than 17,000 feet, where the top of these altocumulus clouds vaporized and created the castellanus.

As the air rose inside these jellyfish clouds, it formed increasingly larger water droplets that became too heavy to be suspended in the cloud by its updrafts and fell as rain. However, as the rain was pulled toward the Earth’s surface by gravity, it ran into the dry air near the Earth’s surface and evaporated before reaching the ground, forming the jellyfish-like tentacles. Over the past six years of drought we have all seen plenty of virga, or pillars of rain that dissipate before hitting the ground.

Due to the dry air above, there were no high-level clouds like cirrus, cirrostratus and cirrocumulus that form at altitudes from 16,000 to 43,000 feet. At the same time, Santa Lucia (northeasterly) winds drove the marine layer out to sea, leaving behind a clear canvas for these beautiful clouds to shine.

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With the gusty southerly winds and rain expected Sunday, it’s a good idea to prepare for power outages: Keep a battery-operated flashlight and radio within easy reach and make sure the batteries are fresh. Listen for updates on storm conditions and power outages. Use safer LED candles. Wax candles are not recommended. Don’t depend on a phone that requires electricity to communicate. Keep a standard handset or mobile phone ready as a backup. Store water-filled plastic containers in your freezer that you can use as blocks of ice to prevent food from spoiling.

John Lindsey is PG&E’s Diablo Canyon Power Plant marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.

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