From The Denver Post (John Ingold):
Topography creates updrafts, keeping stones aloft longer
The largest hailstone ever recorded in America spent close to an hour aloft in a cloud growing to the size of a small volleyball, then plunged to earth at more than 100 mph, struck the ground in South Dakota weighing nearly 2 pounds, left a divot, was scooped up by a local rancher and placed in a freezer, melted a bit during a power outage, was packed in dry ice and driven cross country, and finally arrived at a lab in Boulder where Charles Knight, one of the nation’s premier authorities on hail, added it to a research collection that also included the two previous record-setting hailstones.
Even by Knight’s high standards, though, the golf ball-sized hail that hammered the western metro area earlier this month was something to behold.
“Large hail is pretty rare this close to the Front Range,” said Knight, a senior scientist at the National Center for Atmospheric Research. “It’s really pretty rare anywhere.”
But when the clouds twirl just right, Colorado’s unique geography and climate are capable of producing spectacular amounts of hail.
The state resides in what meteorologists call “Hail Alley,” a swath of land that also includes parts of Nebraska and Wyoming that is frequently bedeviled by hail. Areas of the Front Range and Eastern Plains can see 10 or more days of severe hail per year, on average. A study published last year by the National Insurance Crime Bureau ranked Colorado second nationally, behind Texas, for hail loss claims between 2013 and 2015.
And if it sometimes seems like hail is nature’s way of betraying you, there’s a reason.
The science of hailstone formation reveals that you have been told a lie your whole life: Water – really, really pure water – doesn’t necessarily freeze at 32 degrees Fahrenheit. It can, in fact, stay in liquid form at temperatures down to almost minus 40 degrees, a phenomenon that scientists call “supercooled water.”
It’s these tiny droplets of supercooled water, suspended in clouds towering as much as 9 miles above the ground, that start joining together to form hail. But first they need an instigator because supercooled water, Knight said, “doesn’t know how to start forming molecules into ice crystals.”
The instigator could be a fleck of dust that changes a droplet’s structure or a speck of water that freezes spontaneously at a colder elevation, said Andrew Heymsfield, a hail expert and colleague of Knight’s at NCAR. Either way, once there is an ice particle in the cloud – what scientists call a “hail embryo” – it exerts a kind of chemical peer pressure on other droplets, pulling them in, turning them into ice and gradually building up a hailstone…
The Front Range’s topography turns out to be a perfect petri dish in which to create hail.
Hailstones don’t form or grow very big without massive amounts of air billowing up from below. These updrafts keep the embryonic stone aloft long enough to gather up water into an ice ball, and the stronger the updraft, the bigger the stones can grow. Heymsfield said hailstones can spend a half hour forming in clouds, with the absolute largest in the strongest updrafts taking close to an hour.
Mountainous areas – such as Colorado’s Rockies – promote these updrafts by acting as elevated heat sources that pump warm air from the ground high up into the atmosphere. These storms might at first loom over Colorado but they also carry out onto the plains to the east, sucking in more moisture as they go.
Updrafts, though, only go so far in forming hail. The air needs to stay cold enough closer to the ground for the hail to actually fall as ice instead of melting on the way down.
This is why, Heymsfield said, some of the biggest hail comes from storms not in warm southern states like Florida that can produce huge thunderstorms but in comparatively cooler South Dakota and Nebraska. On May 8, the day the hailstorm struck the metro area, the temperature in Denver topped out in the 70s. The day the record hailstone fell – on July 23, 2010, near Vivian, South Dakota – the high temperature barely crested into the 80s, well below average for the date.