The summit of Mauna Kea, at nearly 14,000-feet altitude, presents unique challenges to visitors. Visits to the summit require a 30 minute acclimatization stop at the 9,200-foot Onizuka Visitor's Center, warm clothing for the summit, sunscreen for protection from excessive UV radiation, and water. A four-wheel-drive vehicle is required beyond the 9,200 foot level as the air is too thin to adequately cool a vehicle's brakes upon descent. Temperatures on the summit can fall to 20 (F) ( -4 (C)) within a matter of hours; it is highly recommended that all visitors to the summit take warm clothing and sturdy footgear sufficient to withstand the elements should the vehicle break down. High winds on the summit can often result in blowing snow, sleet, fog and sometimes, flying rocks. Winds can reach 150 mph; beyond 75 mph vehicles are at risk for being blown off the road. Medical and auto help is a minimum of two hours away.