What the Hell is the Pineapple Express?

Information you need about: the weather.

Looks like a blast from a plasma gun? It is plasma, actually. The northern lights, seen here over Canada's Wapusk National Park, occur when the solar wind enters the ionosphere above the North Pole, creating waves of red, green, and blue light as charged particles collide with gas molecules 50 to 200 miles above Earth. Ideal viewing is at the winter solstice, between 60 and 75 degrees north (e.g., Alaska). Solar cycles peak every 11 years, and the next big show is slated for 2011. Find solar-wind forecasts at gedds.alaska.edu/auroraforecast.     Photo: Daniel J. Cox/Corbis

Like St. Elmo's Fire, it's a Hollywood movie as well as an actual weather event. The real Express is a subtropical jet stream of warm, moist air that originates over the tropical Pacific and gets pulled north by pressure differences over the Gulf of Alaska, sending fat-bottomed clouds streaming to the West Coast several times each winter. "It's like a fire hose of moist air pumping two to five times the amount of precipitation you'd get in a normal winter storm," says Tanner. Since warm air holds more moisture than cool air, this means big snows in the Sierra. It also caused the November 2006 deluge that dumped 18 inches of rain on Mount Rainier National Park in 48 hours.

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