Storm packed some unusual punch

Rime ice is the opaque coating of tiny white granular ice particles.

Rime ice is the opaque coating of tiny white granular ice particles. Photo by News Tribune.

As lightning bolts lit up the sky over the Capital City while blizzard conditions prevailed, Mid-Missouri experienced an uncommon phenomenon called “thundersnow” Thursday.

photo

Hoar frost is a heavy frost that forms on objects exposed to open air on cold days.

“It’s not particularly common, but it’s not rare, either,” said Scott Truett, senior forecaster for the National Weather Service.

Generally, cold air masses are more stable than warm air masses, which is why thundersnow events are less common than thunderstorms. Thermodynamically, both events are similar, since thunder is created by rapidly expanding gases in a lighting discharge. But when temperatures are freezing, those same clouds “can pour snow.”

“And today is snowing very, very heavily,” Truett said.

“We’re seeing lots of thundersnow today. It happens when you get strong instability in the atmosphere,” said Tony Lupo, chair of the University of Missouri’s atmospheric science department on Thursday. “Today is a very unstable situation. It’s exciting.’”

Lupo said thundersnow does make forecasting snow accumulations slightly more challenging. He predicted the snow was falling at a rate of about two or three inches per hour at its heaviest. “But that’s occurring in very narrow bands,” Lupo added. “That makes it difficult to predict exact amounts.”

He said some locales might experience accumulations of six to eight inches; 10 miles away, only half those amounts might fall.

Scientists have developed a variety of terms to describe icy precipitation and snowfall.

Some of those terms, while easy to confuse, have different specific meanings.

For example, “sleet” occurs when rain passes through a deep layer of colder air; the rain then freezes and falls as small pellets.

“Freezing rain” is composed of water droplets that fall through a shallower layer of cold air and freeze upon contact with objects on the ground. “It can be dangerous,” Truett said.

One effect of cold temperatures and humidity are the enchanting, lacy ice formations that often settle on trees, glass windows and other objects at this time of year.

“Rime ice,” for example, is the opaque coating of tiny white granular ice particles caused by the rapid freezing of supercooled water droplets on impact with an object. It’s generally not caused by falling precipitation, but by fog or mist. It’s more common in wetland or lowland situations.

“Hard rime ice” is somewhat less milky in appearance; “soft rime ice” is more snow-like.

Both are “very brittle and whitish,” Lupo explained.

Clear ice is a thin coating of ice on terrestrial objects, caused by rain that freezes on impact. Clear ice — as opposed to rime ice — has less air and is relatively transparent because of the large drop size, rapid accretions of liquid water or the slow dissipation of latent heat of fusion.

“We haven’t seen a lot of it with this storm, but we should be careful. It could change,” Lupo cautioned. “Jefferson City is close to that line where we’ll get some clear ice.”

Another icy condition is “hoar frost,” the heavy frost that forms on objects exposed to the open air on cold days. The deposition of hoar frost is similar to the process by which dew is formed, except that the temperature of the frosted object must be below freezing.

One snow phenomenon Mid-Missourians probably won’t experience this week is “pogonip,” or a dense winter fog containing frozen particles. That kind of fog is created mostly in the deep mountain valleys of the western United States.

Additional:

Weather-related school closings, announcements for Friday, Feb. 22, 2013

Area forecast, radar and advisories

Jefferson City snow route conditions

MoDOT Traveler Information map

Winter photo gallery submitted by our readers

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