temperature measurements "pucks" could be causing measurements errors. Sample
infrared images of the same roadway temperature measurement sensor under these two
conditions are show in Fig. 12.1. The very fact that the roadway temperature
measurement sensors appear much different in gray-tone in the infrared image is an
indication that they either are at different temperatures, or they have different optical
properties, or both. To understand what sort
Daytime infrared image
Nighttime infrared image
Fig. 12.1. Infrared images of a roadway temperature sensor. (lighter grey-tones indicate
higher temperature, red and blue indicate over-range and under-range temperatures,
respectively)
of temperatures differences we might be witnessing, these images were analyzed with our
infrared image analysis software. Areas within the center of the "puck" as well as areas
of similar size in the pavement area adjacent to the puck were selected and their apparent
mean temperature was calculated. While assumptions used in the analysis, such as
surface emissivity and prevailing weather conditions will have some impact on absolute
temperatures inferred from the observed infrared signature, the impact on relative
temperatures will be much smaller.
Across all sites where we made the measurements, trends were very apparent as can be
seen from Fig. 12.2. It is apparent that during the nighttime clear sky condition that the
roadway temperature measurements appear to underestimate the adjacent pavement
temperatures. During the day the opposite impact was observed, the roadway
temperature measurements appear to be overestimating the adjacent pavement
temperatures. One interpretation of these data could be that the roadway temperature
sensors have a lower specific heat and/or higher thermal conductivity resulting in them
having a higher thermal diffusivity than the adjacent roadways. Another possible
interpretation would be that the thermal coupling between the roadway temperature
sensor and the roadway underneath it is inadequate for the sensor in the roadway to
emulate a solid, unbroken material. Of course, a possible combination of these effects
would be another reasonable interpretation. Unbeknownst to us, regretfully many of the
RWIS sites were not operational when we took these measurement so we have very little
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