I just learned something new - I'd not heard of thundersnow et al, or at least I don't remember hearing about it -
link =
http://www.theguardian.com/news/2014/feb/24/weatherwatch-electric-snow
The article is short, so for a change I'll copy all of it.
Weatherwatch: Electric snow
David Hambling
The Guardian, Monday 24 February 2014 16.30 EST
The Willis Tower in downtown Chicago provides a focus for the electrical field, with spectacular results.
Photograph: Scott Olson/Getty Images
Hot weather often brings dramatic thunderstorms. They are less common in winter, but there have been an unusual number over the past few months.
In this country, heavy rains have been accompanied by thunder and lightning, and the US has seen a spate of rare electrical snowstorms, known as thundersnow. Scientists have been studying these unusual winter phenomena with special radar and other sensors.
Christopher Schultz of Nasa's science research office has found that lightning is typically associated with heavy snowfall of around 5-7 centimetres an hour. The conditions for electrification are complex, depending on temperature, humidity, and the presence of supercooled water droplets and ice particles.
Individual particles pick up positive or negative charge and fall at different rates. When masses of positive and negative charge are separated far enough, electrical breakdown of the air occurs, and this is accompanied by thunder and lightning.
In cool winter storms there is less vertical air motion than in warmer summer ones. This means the area of charge separation tends to be a large flat sheet or band, resulting in horizontal lightning. This can be triggered when the band passes over a tall structure such as a television transmitter. High buildings such as the Willis Tower in Chicago may also extend into the cloud base and provide a focus for the electric field.
Lightning is initiated at this point and travels horizontally though the band; it may terminate at another tall structure, sometimes more than 70km away.
