Early Wednesday about 1 a.m., the Doppler weather radar at Vandenberg Air Force Base indicated a thick red line of severe weather over Cayucos moving southeast toward San Luis Obispo.
Radar — an acronym for radio detection and ranging — sends out short pulses of radio waves, while receiving the reflected energy coming back from water droplets suspended in clouds or falling to the ground.
Like a dentist using X-rays to look for cavities, radar allows a meteorologist to examine areas of the atmosphere previously inaccessible. If the radio waves strike rain, hail, snow or even insects, a small portion of reflective beam is directed back toward the radar antenna.
Computers analyze the strength and frequency shift of the returned radar waves and produce a display of the location and intensity of precipitation that we see. Rainfall is shown as blue and green; heavier rainfall is indicated by yellow, with orange and red indicating the heaviest precipitation.
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That morning, the area over Cayucos turned to red with shades of magenta on the radar display, indicating rainfall rates of eight-plus inches per hour or hailstones.
You see, hailstones are usually coated with a thin layer of water as they travel through the thunderstorm cells. This condition can cause a storm's reflectivity to be higher, leading to over-estimating the amount of rain received.
These reddish shades of color are just about the level where three-quarter-inch or larger hail can occur.
Later that morning, I received photographs and emails from numerous residents in Cayucos documenting 1-inch in diameter hailstones. An extensive review of weather records for San Luis Obispo County indicates that these were probably the largest hailstones ever documented along the Central Coast.
However, for other parts of the country, this isn’t a particularly big deal. Back in July of 2010, a monster hailstone plummeted to Earth from a slate-gray South Dakota sky. It was found by ranch hand Leslie Scott near a small town of about 100 people named Vivian, on the high plains of South Dakota.
The hailstone was 8 inches in diameter and weighed almost 2 pounds! That broke the previous record for North America, when a 7-inch hailstone smashed down in Aurora, Neb., in June 2003.
I spoke with Leslie Scott about the 2010 hailstone discovery at his home near Schoolhouse Hill in Vivian. Leslie and his wife, Debra, heard what sounded like bricks smashing against their home from an angle.
What they heard that evening were enormously large hailstones, which smashed through their roof, leaving baseball-sized holes in their ceiling, and broke out many of their windows. After the deafening noise stopped, Leslie and Debra ventured outside and saw countless large hailstones lying on the ground.
One hailstone caught his eye because it had numerous fingers of ice sticking out of it, but there were other stones that size and even bigger that fell, Leslie said.
So, how are these hailstones formed? Hail is produced in cumulonimbus clouds, otherwise known as thunderstorm clouds. Strong winds within the cloud will keep supercooled (less than 32 degrees Fahrenheit) raindrops suspended thousands of feet in the sky.
Supercooled 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 1-inch diameter hail in Cayucos required updraft winds of between 37 and 56 mph. The hailstone that Leslie found in South Dakota probably took violent updrafts that exceeded 160 miles per hour to remain suspended in the cloud over the course of five to 10 minutes!
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