You don't tour Yellowstone during the winter months in a four wheel drive SUV. So much snow falls each winter they don't even try to plow the roads. During the winter months only over snow vehicles with tracks have access to the interior of the park. One option available to visitors is a snowmobile. Visitors to Yellowstone on snowmobiles are required to have a licensed guide and remain on roads at all times. Unlike snowmobiles of the past, the snowmobiles I saw in Yellowstone in 2010 were all new models using engines which are surprisingly quiet and produced little exhaust or odor. Currently only 318 snowmobiles are allowed in the park each day. In a park with over 2.2 million acres the number of snowmobiles permitted is actually rather modest. And there are still some who are trying to prohibit the use of any snowmobiles within the park.