Nearly every summer rainstorm comes with thunder and lightning. Yet during even the blusteriest blizzard, there's nary a spark in the air. It can occur (although snow lightning strikes just six times a year on average in the U.S.), but winter air doesn't make for prime lightning-forming conditions, says meteorologist Robin Tanamachi of the University of Oklahoma.During the summer, the lower atmosphere is full of warm, humid air. Above that, it's cold and full of ice crystals. As the warm air rises, it carries water vapor with it, these molecules brush against the ice crystals, and this friction creates an electric field in the cloud -- like scuffing your feet across a carpet. The ice crystals gain a slight positive charge, and the updraft carries them to the top of the cloud, giving the cloud's bottom a net negative charge. Once the difference between the negatively charged cloud bottom and the positively charged ground becomes great enough, a bolt arcs between them.
But in snowy months, the atmosphere is cold and dry throughout, so there's no updraft to create friction within the clouds. Wind stirs the molecules and crystals some, but that action rarely generates a strong enough electric field to spark lightning.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.