De-icing chemicals work by lowering the freezing point of water. Before a dry de-icing chemical can act, it must dissolve into a brine solution. The necessary moisture can come from snow on the road surface or from water vapor in the air (humidity). Changing ice or snow into water requires heat from the air, sun, pavement or traffic friction. Even when the pavement is below freezing it holds some heat and can help melt snow and ice.
As air and pavement temperatures drop, the amount of salt needed to melt a given quantity of ice increases. Normally, a temperature of around 20 degrees F is considered the lower limit for salt effectiveness. Once reaching this temperature, snow plow trucks simply can't apply enough salt to the roadway to achieve the necessary melting action.