Ski and snowboard helmet: wear one.
Do ski helmets affect reaction time to peripheral stimuli? No. At least not according to a study just published in Wilderness and Environmental Medicine. Several reasons not to wear a lid have been floating around: too much weight on the neck for kids, increase level of risky behavior, and impaired hearing and vision. The study showed that ski goggles imparied vision more. In addition to protecting your noggin, an inch of polystyrine and plastic on your head keeps you very, very warm.
Above: on location, Portillo, Chile; this pic, Tony Knowles Coastal Trail, AK