ACCUWEATHER - One to two inches of snow fell Tuesday night over Shippingport, Pa.,as a result of an unusual cause: steam from a nearby nuclear power plant.
The snow was not fluorescent and not radioactive, as some people joked onsocial media, but it did take a special set of "ingredients" to form.
"The snow that fell yesterday is not common, but when the weatheringredients are favorable, it can form," AccuWeather.com Expert SeniorMeteorologist Bernie Rayno said.
"That's what happened yesterday. It's a unique situation, but notrare."
Low temperatures and concentrated moisture in the air were two keyingredients in the event, Rayno explained.
First, the temperature needed to drop to at least 5 degrees F -- thetemperature necessary for a cloud to form.
The power plant was releasing steam from its stacks into the frigid air andthe difference between those two air masses were key.
For snow to develop, the moisture -- in this case, steam -- needed to stayconcentrated in low levels of the atmosphere.
Over the small city last night, located about 35 miles from Pittsburgh, a phenomenon called"inversion" occurred at around 5,000 feet. This kept the steam fromrising upward and dispersing in the atmosphere.
Had the warm air been dispersed, as it frequently is, the snow would nothave developed.
Additionally, the calm night, accompanied by almost no wind, allowed thewarm air to remain nearly stationary horizontally.
"These factors prevented the moisture plume from rising anddispersing," Rayno said.
When the two air masses collided in the lower level of the atmosphere, onewarm and one cold, a narrow band of snow formed and stretched as far as 30miles from the power plant.
"It was very low in the atmosphere and, if it were farther from theradar, we wouldn't have even see it," AccuWeather.com Expert SeniorMeteorologist JesseFerrell wrote in his blog.