The previous weather before a winter storm is an important consideration for accumulation potential. Soil and ground temperatures take much longer to adjust to air temperatures than elevated structures. Soil temperatures will be fairly warm if the winter storm occurs early in the winter season or the weather has been warm for several days previous to the colder weather. The duration of below freezing air temperatures before a winter storm occurs has an influence on accumulation. Heat energy moves from warmer toward colder objects. When air temperatures are below freezing and soil temperatures are above freezing, heat will be conducted from the ground toward the air. This heat has the capability of melting a significant amount of winter precipitation. If a winter storm occurs just after a warm period, there will be a significant buildup of ice or snow on vegetated surfaces but concrete and urban structures (such as ground-connected roads and parking lots) can melt a significant proportion of the ice or snow that falls on them. If several days of below freezing temperatures precede the winter storm, very little of the precipitation will melt once reaching the surface. This is especially important in a freezing rain situation. Frozen soils along with below freezing surface temperatures will allow rain to freeze on both elevated and ground surfaces. Freezing rain is much more dangerous to transportation if the soil and ground temperatures allow for an accumulation of ice on all roads. |