Kevin Myatt is The Roanoke Times' weather columnist.

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We're not really done with frozen precipitation once it gets warm, as some residents of Floyd County were reminded Wednesday.

In fact, a potentially destructive and deadly form of frozen precipitation can occur only when there is enough warmth to cause forceful updrafts into colder layers high in the atmosphere.

Hail is formed when raindrops are carried upward by strong updrafts within thunderstorms. As the rain is carried into subfreezing air in a higher atmospheric layer, it freezes into pellets.

The updrafts keep lifting the hailstones until they become heavy enough to fall, adding new layers of ice with each trip upward. This is why hailstones have an onionlike appearance when cut open.

Hailstones, mostly small in size, piled up like snow up to 5 inches deep near Willis and Indian Valley in Floyd County on Wednesday. We seem to have a similar situation in narrow corridors within Southwest Virginia once or twice a year, usually in April or May.

Three factors played into the copious hail accumulation in Floyd County.

The first was the relatively low freezing level in the atmosphere. With an influx of cooler air from Canada, the freezing level sank below 11,000 feet Wednesday and Thursday. Raindrops didn't have to be carried very high in order to freeze.

Secondly, the thunderstorm producing the hail was extremely slow moving, going only about 10 mph. So rather than scatter a little hail over a larger area, as is the case with faster moving storms, it dumped much of its load of hail on a narrow corridor.

A third factor was the higher elevations along the Blue Ridge. As hail falls below the freezing level, it starts melting. Less of it can melt before hitting elevations above 2,000 feet, like those in much of Floyd County, than if it fell all the way to the Roanoke Valley's elevation, which is between 850 and 1,200 feet.

The elevation factor is why relatively mild storms can pummel backpackers with hail in the Rockies, or even on Mount Rogers or in the Great Smokies. It's also part of the reason why the High Plains from the Texas Panhandle to the Dakotas -- much of which is 3,000 to 5,000 feet in elevation -- experiences more frequent and larger hail than most other parts of the country.

But under the right circumstances, large and/or copious amounts of hail can fall all the way to sea level. I remember once seeing a photo from Florida of hail 12 to 18 inches deep, snarling traffic.

Violent thunderstorms have the ability to carry raindrops to extremely high levels of the atmosphere -- five to 12 miles -- repeatedly. The most vicious thunderstorms, called supercells, feature rotating updrafts that continue for hours. These are the kind that typically produce hailstones ranging from golf ball to softball size, which can do lots of damage to crops, cars and houses, and in some cases have even killed people.

The storm over Floyd County did not have violent or rotating updrafts, so it was not long-lived nor did it produce large hail. The largest stones reported to the National Weather Service were 0.88 inch in diameter, but most were 0.5 inch or smaller.

Hailstones of 0.75 inch or larger currently classify a storm as severe in our region, as would 58 mph wind gusts. But a proposal under consideration would raise the bar for severe-sized hail to 1 inch by next year, a policy that has already been implemented by many weather service offices in the central and western United States.

One more tidbit about hail: Not only does an accumulation of it look like snow at first glace, but it counts as snow for statistical purposes. Snow, sleet and hail accumulation are one category under National Weather Service reporting standards.

So the folks in Floyd County who saw the hail pile up can simply add that to their snowfall accumulation for the season.

Weather Journal appears on Monday, Wednesday and Friday.