Weather Watch

A ‘fire rainbow’ spread across the SLO County sky this week. Here’s what caused it

Caused by diffraction or the bending of the light waves through the ice crystals in cirrus clouds, a spectrum of colors produces circumhorizontal arcs much like the way that light passes through a prism.
Caused by diffraction or the bending of the light waves through the ice crystals in cirrus clouds, a spectrum of colors produces circumhorizontal arcs much like the way that light passes through a prism. Bob Bowles

Bob Bowles took this beautiful photograph Monday at Suey Creek Road about five miles east of Nipomo.

“It was a horizontal rainbow, never seen anything like it. It hung out in the sky to the east of us and changed colors for about an hour,” he said. “I don’t know if we were the only ones that got to enjoy this; didn’t hear anything.”

At around the same time, Teri Hunter tweeted a similar image of a weird rainbow from Nipomo High School.

Looking at Bowles’ photo, I was astonished by its mother-of-pearl-like iridescence; it was almost like looking at the inside of an abalone shell.

I would classify this phenomenon as a circumhorizontal arc or sometimes referred to as a fire rainbow, even though they have nothing to do with fires.

On the day of the photo, a weak cold front moved southward along the California coastline and produced gentle southerly winds and more importantly increasing mid- to high-level clouds.

One of the most common types of clouds that develop at high altitudes is cirrus. These clouds are frequently blown about into feathery strands called “mare’s tails.” On a side note, mare’s tails can sometimes be an indicator of future rains. Later that night into Tuesday morning, rain showers were reported throughout the Central Coast.

There are several types of clouds, these four are the core of them all.

Cirrus clouds usually develop at 17,000 feet and higher over most of the United States and at temperatures near minus-40 degrees Celsius or Fahrenheit (Fahrenheit and Celsius are equal at -40 degrees) or colder and consist almost entirely of hexagonal shaped ice crystals.

Caused by diffraction or the bending of the light waves through the ice crystals in cirrus clouds, a spectrum of colors produces circumhorizontal arcs much like the way that light passes through a prism.

The ice crystals often descend slowly, and these rainbows of colors can persist for hours. The colors can be brilliant, much like the feathers on a hummingbird.

Like observing the brilliant blues-greens and magenta flashes of Anna’s Hummingbird, you need to be at the correct angle to see the brightest colors.

For example, the circumhorizontal arc cannot be seen in locations north of 55 degrees of latitude, which covers a big chunk of Canada and nearly all of Alaska or south of 55 degrees toward the south pole.

Also, due to earth’s axial tilt of 23.5 degrees, which produces the seasons, the sun needs to be higher than 58 degrees above the horizon in the sky that occurs between late March and late September at our latitude. Other phenomena that are related are iridescent clouds that are also called rainbow clouds. Those iridescent clouds can form when updrafts in cumulus clouds lift and cool the air mass causing water droplets to condense, which can also act as prisms.

Tribune archive photos of rainbows over San Luis Obispo.

Spring river safety tips

With the exceptional California snowpack melting as temperatures rise, rivers and streams are full of dangerously cold and swift moving water this spring. PG&E encourages water enthusiasts to take extra precautions when in or near waterways, especially around hydroelectric facilities and dams, where water flows can change rapidly. Anglers are also encouraged to take precautions as trout season opens April 27 for most California rivers.

California’s snowpack measured 175 percent of normal in early April, ensuring cold runoff well into summer. “Public safety is our highest priority. We encourage everyone recreating in or near water to know at all times how they can quickly get out or away. Put safety first, especially while outdoors,” said Debbie Powell, PG&E’s vice president of power generation.

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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