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De-icing of roads is carried out proactively in order to prevent the occurrence of slippery driving conditions. Proactive de-icing usually consumes less salt and also takes effect more quickly. If the weather forecast promises snowfall while the temperature is near zero, roads are salted in advance to prevent the snow from sticking to the surface of the road and freezing. This also speeds up snow ploughing. When the temperature is above zero, roads can also be salted in anticipation of heavy winds. Heavy wind increases evaporation from the road surface, which can cause wet roads to freeze even when the air temperature is above zero.
The development of weather conditions is constantly monitored by contractors based on information received from road weather stations and weather forecasts. Road weather stations provide accurate data on the road surface and air temperature as well as on the formation of slipperiness on the road surface. In addition to this, road weather stations also provide information on the effects of salt on the road surface and the rate at which the salt loses its effectiveness. Contractors also monitor changes in the weather by driving around their respective areas. However, weather forecasts can also change, as a result of which proactive maintenance procedures cannot always be optimally timed throughout the entire contract area.
The temperature difference between the road surface and the air above it can sometimes be quite significant. Furthermore, both of these temperatures can also fluctuate by several degrees even over short periods of time. The potential freezing of a damp or wet road surface is entirely dependent on road surface temperature; ambient air temperature has no effect on it. On a clear winter morning, the road surface temperature can be as much as 8°C lower than ambient air temperature. For example, even if the ambient air temperature is +5°C at an altitude of 2–4 metres, the road surface temperature can still be as low as -3°C. The temperature sensors mounted on cars are of course closer to the road surface, but even then the temperature difference between the sensor and the road surface can be as much as 3–6°C depending on weather conditions. Because of this, some car sensors will start warning the driver about potentially slippery conditions even at temperatures as high as +3–6°C.
Often, the reason for salting roads in above-zero temperatures is that the temperature and weather conditions are expected to change to the point that the humidity in the air will start to condense on the colder-than-air road surface, making it very likely that the road surface will begin to freeze (forming so-called black ice). This usually happens when clouds clear away, allowing the heat on the road surface to escape through the clear layers of air and out into space in the form of thermal radiation, which causes the surface temperature to drop below the air temperature. This is the same phenomenon that sometimes causes the windows of parked cars to freeze over. Even if the air temperature is above zero, the road surface temperature can be or eventually drop below freezing under these kinds of conditions.
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