Wintry precipitation such as snow, sleet, and freezing rain can create hazardous conditions for outdoor exercise.
One of the main issues is traction. Any type of frozen precipitation can create slippery surfaces which can lead to falls causing sprains or fractures.
Icy surfaces caused by freezing rain or black ice are the most dangerous conditions in which to exercise outdoors. Freezing rain occurs when snow falls into a warm layer of air above the ground and melts into liquid rain. This liquid rain then falls to the surface where temperatures are below the freezing point (32*F) and the rain freezes on contact with this cold surface. This ice (also known as glaze) can build up as more precipitation falls to the ground and continues to freeze. The buildup of glaze results in treacherous conditions for most types of outdoor exercise. The other main type of icy surface called black ice forms during the night when either rain or melted snow on roads or sidewalks freezes due to cold temperatures. You may not be able to see these types of frozen surfaces which makes them extremely dangerous to those exercising outside. The early morning often is the most hazardous time for black ice; by later in the morning and afternoon (if temperatures move above freezing) much of this ice will tend to melt.
One other concern in wintry weather is visibility. This is especially important during a snow event. Snow can lower visibilities to less than 1/4 of a mile which is dangerous in two ways. If you are outdoors, it is easy to become disoriented and lose your sense of direction. There have been cases of people going out in very low visibilities and getting lost which in some instances lead to death from hypothermia. The other danger is not being visible to any vehicles that may be out on the roads increasing the chances of being struck and injured or killed. Low visibilities, combined with blowing snow and winds over 35 miles per hour constitute blizzard conditions making it extremely dangerous to exercise outside.