Everyone shivers when they’re chilly, but how can we tell if temperatures are verging on dangerous? According to Rupe Hansra, OD, senior director of eye care at LensCrafters, the eyes are the windows to signs of extreme cold. "When hypothermia becomes severe, blood vessels in the eyes constrict to conserve energy," says Dr. Hansra. And that can result in temporary blindness, which model Kate Upton experienced firsthand during her Sports Illustrated Swimsuit Issue cover shoot in Antarctica. Temperatures reportedly dipped as low as -20°F while she was there.