Jeff Masters, guru at Ann Arbor’s Weather Underground, thinks we’re in for another cold winter. But don’t count Masters among the global-warming skeptics–on the contrary, the U-M meteorology Ph.D. has blogged against what he’s called a corporate-driven campaign to deny the evidence of human-driven environmental change. In fact, a key piece of that evidence underlies his prediction: the Arctic ice pack shrank so much last summer, he says, that it may bring more cold air down to the United States. That’s because the warmer Arctic weakens the jet stream–the pattern of upper-air winds that steers weather systems. He calls 2009-10 an “upside-down winter” because it was one of Canada’s warmest on record, yet the coldest in the Lower 48 in a generation. He’s betting on a repeat this year–along with plenty of snow. This is a strong La Nina year, and the last one, in 2007-08, brought us twice as much white stuff as usual.