FARGO -- There is a layer of air from the ground up to a few hundred feet called the planetary boundary layer. The boundary layer is heated by the convection of heat from the ground after the ground is heated by radiation from the sun. It is not unusual for the air temperature within the boundary layer to be quite different from the air above. During winter, the presence of snow cover greatly reduces the amount of radiation received on the ground, which then greatly reduces the convection process.

This is why the introduction of snow cover suddenly makes our weather colder. Even when our weather warms up from a warm front, the warming is usually tempered. This is also why winter precipitation sometimes falls as freezing rain. Weather systems can blow mild air up and over the top of the freezing air within the boundary layer. Liquid drops then freeze upon contact with the frozen ground.