RWIS Weather Data
Surface Temperature
Sensors embedded in the highway measure the surface temperature of the road or bridge. The surface temperatures are a truer indicator than air temperature of the effect that snow and ice will have on the roadway. Surface temperatures can be much warmer or much colder than air temperature.  Regardless of the air temperature, if the surface temperature is below freezing, snow and ice can accumulate on the roads.

Pavement Sensor
Each site includes two to four surface sensors. Typically, about half the sensors will be located on bridge decks and the remaining sensors on the regular roadway. Temperatures at both types of locations are important. Pavement temperatures typically respond relatively slowly to changing air temperatures. The thermal energy stored in the earth transfers to the pavement and often slows the response of the pavement to changing air temperatures. In the case of bridges, the bridge deck surface temperature is more likely to respond quickly to changing air temperatures, because bridges lack the moderating influence of the earth's mass has on roadways.
More about surface temperature
Subsurface Temperature
The subsurface temperature is measured by a probe approximately 18 inches below the roadway surface. Subsoil temperatures normally change much slower than air temperatures. In the fall, subsurface temperatures may remain quite high until an extended period of cold weather occurs. In the spring, the subsurface temperatures may remain colder than the air temperature for quite some time. The accumulated heat or cold of the subsurface have a direct affect on the roadway temperatures.

Sub Surface Sensor
Atmospheric Weather Information

A tower along the roadside is equipped with weather instruments that record the weather conditions at the tower site.

  • Air temperature is the temperature of the air measured approximately eight feet above the ground.
  • Relative humidity is a measure of the amount of moisture present in the atmosphere, expressed in a percentage. A relative humidity of 0% means that the air contains no moisture, and 100% means that the air is completely saturated and cannot absorb more moisture. Relative humidity is used to calculate the dew point temperature.
  • Dew point temperature is the temperature at which the air is fully saturated with moisture. The dew point is a more useful measure of the moisture present, because it takes into consideration the air temperature. If you remember from science class, the amount of moisture air can hold decreases when it is cooled. As the air temperature drops there is a point at which the moisture in the air begins to condense and form fog, dew, or frost. That point is the dew point.
  • Wind speed is the average wind speed within a one-minute period. Wind speed is expressed in miles per hour.
  • Wind gust is the maximum wind speed within a one-minute period. Wind gust is expressed in miles per hour.
  • Wind direction is the average wind direction within a one-minute period. Wind direction is expressed as the direction the wind is blowing from.
  • Precipitation at RWIS sites may be expressed in two different ways. Many sites report either Yes, meaning the presence of some type of precipitation, or No, meaning precipitation is not occurring. A number of sites around the state have more sophisticated precipitation sensors that indicate the type of precipitation , such as light drizzle, moderate drizzle, light rain, rain, heavy rain, light snow, moderate snow, and heavy snow.
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