By Dale Keep
In my September column, I discussed the importance of understanding how surface temperature affects melting capacities and usage rates of deicers. Another important part of the equation is the relationship between surface temperature and air temperature movement.
Pavement surface temperatures typically move similar to the air temperatures but with thermal lag. The temperature curve usually looks like the diagram shown below, with the air temperature often changing rapidly, while the ground temperature follows.
This is an idealized diagram, and it should be noted that several factors significantly affect the pavement-air temperature relationship, including subsurface or ground temperatures and the presence or lack of solar radiation.
The subsurface
If the subsurface is warmer or cooler than the air and surface temperature, it can affect the air-surface temperature relationship. For example:
Fall. In the fall, the subsurface is still above freezing when the air temperature is below freezing. Often during these conditions, the cooler air temperature cannot remove enough heat from the surface and the subsurface to drop the pavement below freezing. The pavement surface is drawing heat from the subsurface. Contrary to this, the lack of a subsurface, such as on bridges and parking facilities, normally means that the surface temperature moves very quickly and tracks the air temperature very closely, especially with even minimal air movement around the structure.
Winter/early spring. In the late winter or early spring, the subsurface temperature is often below freezing while the air temperature is above freezing. In this case, the air cannot transfer enough heat to the pavement to offset the colder subsurface. This is especially true with the reduction of solar radiation such as cloud cover and/or shading. Here again, bridges and parking structures with free air movement around them are often the exception.
Solar radiation
Sunshine can significantly affect the pavement temperature, depending on the time of year, cloud cover, shading from vegetation or buildings, and the color of the surface. Typically, the darker the surface color the warmer the surface temperature will become. Without the presence of shade, sunshine can raise the pavement temperature significantly above the ambient air temperature.
In the winter, bright sunshine on a pavement surface, even when the air temperature is below freezing, can raise the surface temperature to well above freezing. However, the angle of the sun and the presence of cloud cover and shadows can prevent sunshine from reaching the surface. These situations can allow the air temperature to rise while the asphalt surface stays cold and possibly ice- or frost-covered.
Learning to anticipate the movement of surface temperatures based on a few basic principles as presented here can make a great impact on the overall use of deicer chemicals. It is nearly impossible to predict what maximum or minimum surface temperatures will be reached without using some very expensive equipment and very complex algorithms, but understanding the basics of its movement is very useful.
May all of you have a safe and prosperous winter season.
Dale Keep owns Ice & Snow Technologies, a training and consulting company based in Walla Walla, WA.
