This is a guide to help you understand the air and pavement temperature measurements displayed on the Highway Driving Conditions Web Site. We have installed air and pavement temperature sensors at our four highway camera locations along the Trans Canada Highway (TCH). These sites include Foxtrap near St. John’s, Goobies, 
Grand Falls - Windsor, and Pynn's Brook. When network communications with the remote locations are available, the temperatures on our web site are updated every three minutes. Anytime current temperature readings are temporarily unavailable from a location, the display will show N/A °C .
 

Air Temperature

The temperature of the air is something with which we are all familiar. This is similar to the temperatures reported by meteorologists in daily weather reports, and what we feel when outside. Our air temperature sensors are positioned just outside the highway camera enclosures. The sensor is exposed to the air, yet housed suspended in an instrument casing away from any other material that may affect an accurate air temperature reading. The instrument casing shelters the sensor from direct solar radiation and other weather related influences. 
   
 

Pavement Temperature

We have also installed temperature sensors in the pavement (see image left) near the street poles holding the cameras. These sensors measure the temperature of the surface of the roadway pavement. Since water freezes at 0° Celsius, be careful driving when the pavement temperature is at or below 0° C. Anytime the pavement temperature falls below freezing, beware. During winter, de-icing applications are used on our roads lowering the freeze point of water. However, motorists should always use caution.

 
The Importance of knowing pavement temperatures verses air temperatures:

Air temperatures are not usually good indicators of what the roadway surface temperature actually is. During the fall the pavement is often kept warmer than the surrounding air because of the warm soil. During the spring the reverse may be true. The pavement temperatures can be colder than the air because the soil is still frozen from the low winter temperatures. The sun also has a strong influence on the pavement temperatures. It can help heat the pavement and speed the melting process. Air and pavement temperatures can often differ by many degrees. On a bitterly cold early winter day when the air temperature is well below freezing, the pavement or surface temperature may be somewhat warmer, primarily because the subsurface temperature has not yet cooled.


Black Ice
  
Another important consideration in knowing the difference between pavement and air temperature is the formation of Black Ice.

Black Ice forms when the air temperature is below 0° C but warmer than the pavement temperature. For example 0°C air temperature and -5° C pavement temperature. This condition can cause moisture to rapidly freeze and create a thin, transparent layer of ice on the roadway. This situation is very dangerous because it is not usually obvious to motorists who can mistake black ice as water on the road. The other fact about this phenomenon is that it does not have to be snowing or raining for it to occur. The only necessary condition is that the pavement temperature be below freezing.

Black Ice usually occurs when the dew point and air temperatures converge. At this point, the air can no longer hold the moisture, so it condenses onto the pavement. It is more prevalent on bridges since cold air passes under and above the bridge and make it freeze easily.