[Air Temperature] [Dew Point] [Environmental Sensor Station (ESS)] [Pavement Sensor Location] [Precipitation Occurrence] [Relative Humidity] [Remote Processing Unit (RPU)] [Snow Depth] [Station Pressure] [Subsurface Temperature] [Surface Temperature] [Wind Speed] [Wind Direction] [Wind Speed Maximum] [Wind Direction of Maximum Speed]

Air Temperature [Top]

Temperature of the air measured at approximately eight feet above the ground.

Dew Point [Top]

Temperature at which the air is fully saturated with moisture as it cools. The dew point is calculated from the sensor reported relative humidity.

Environmental Sensor Station (ESS) [Top]

The collection component of the Road Weather Information System (RWIS) site. The ESS is composed of an array of three categories of environmental sensors: atmospheric, surface/sub-surface/, and water/snow level. Deployed sensors may include pavement temperature, pavement condition, pavement chemical composition, soil sub-surface temperature, air temperature, wind speed and direction, humidity, atmospheric pressure, solar radiation, visibility, snow depth, river water level, riverbed scour, and closed circuit camera images.

Pavement Sensor Location [Top]

Physical location of the sensor in regards to the lanes of travel. Normally the sensors are placed outside of the wheel travel in the lane nearest the shoulder. Mulitple sensors are often installed in case of sensor failure. The Pavement Sensor Location includes the general direction of travel and the approximate distance from the RPU.

Precipitation(PPT) Occurrence [Top]

Occurrence of precipitation(abbreviated as PPT) is measured with dual beam infrared (IR) sensors over a 150 mm distance. The sensor can detect droplets as small as 0.010" (0.10 mm) to large hail stones. Sampling timeframe is a sliding one minute window. Valid sensor output is YES or NO. OTHER indicates IR beam degradation from interferance such as a dirty lens, or rime ice.

Precipitation(PPT) Accumulation [Top]

The liquid water equivalent precipitation(abbreviated as PPT) in the time period immediately preceding the observation time. Rain guages consist of a rotating loop of small buckets, which hold 0.01" inches of liquid. An 8" or 12" funnel directs precipitation to a tipping bucket assembly. When these buckets are full, they tip their contents and rotate around the loop. The RPU keeps track of the number of buckets tipped in a given time periond. Hence the name precipitation tipping buckets. Some tipping buckets are heated, allowing snow/ice to melt and enables liquid water equivalent measurement during the winter. This is true in southern Alaska sites. In the interior, where snowfall is dry, there is little liquid water equivalent. Therefore, these tipping buckets are not heated. There are six (6) precipitation time intervals:

• 10 min
• 1 hour
• 3 hour
• 6 hour
• 12 hour
• 24 hour

Relative Humidity [Top]

Measure of the amount of moisture present in the atmosphere, expressed as a percentage.

Remote Processing Unit (RPU) [Top]

Remote Processing Unit (RPU) is the collection and processing component of the Road Weather Information System (RWIS) site. The RPU collects data from each of the RWIS sensors and provides the observation when the central server initiates a polling request. The RPU may process camera images, sum sensor data for the observation, or perform algorithm calculations for proprietary equipment.

Snow Depth [Top]

Snow depth is measured in centimeters with an ultrasonic pulse to and from the target surface. The RPU is baselined with the ground distance. The readings can be inaccurate during moderate snowfall and blowing snow episodes.

Station Pressure [Top]

The force per unit area exerted by the atmosphere, reported in millibars. The pressure is for RWIS site and has not been corrected for altitude or temperature. It should not be used for aviation purposes.

Subsurface Temperature [Top]

Temperature approximately 17" below the roadway surface. Not all RWIS sites have a subsurface temperature sensor. For those RWIS sites that do not have a subsurface temperature sensor, but do have a temperature data probe(TDP), the TDP temperature at a depth of 18" is displayed.

Surface Temperature [Top]

Measurement of the temperature on the roadway. The temperature sensor is approximately 3 mm (1/8 inch) below the roadway surface. Sometimes called the pavement temperature.

Wind Speed [Top]

The wind sensor is located approximately 10 meters (30 feet) above the ground. The remote processing unit (RPU) averages wind speeds over a 1 second (for the ESS-ESP RPU operating system) or a 3 second (for the Linux LX-RPU operating system) period. The reported wind speed is the mean of these average wind speeds over the previous 2 minutes.

Wind Direction [Top]

The direction from which the wind is blowing, reported in true north. The wind sensor is located approximately 10 meters (30 feet) above the ground. The remote processing unit (RPU) picks the last wind direction from the 1 second (for the ESS-ESP RPU operating system) or 3 second (for the Linux LX-RPU operating system) wind sampling period. The reported wind direction is the mode of these wind directions over the previous two minutes.

Wind Speed Maximum [Top]

The RPUs maintain wind speed/direction array of 10 one-minute cells. If the current wind speed is greater than the value in the most recent one-minute array, then the current wind speed and the associated wind direction replaces those values in that cell. When the current minute has expired, the 10 one-minute cell contents shift, and the oldest wind speed/direction values are dropped. The reported maximum wind speed is the highest wind speed from the 10 one-minute cells.

Wind Direction of Maximum Speed [Top]

The wind direction at the time of reported maximum wind speed over the last 10 minutes, as taken from the wind speed/direction array (see Wind Speed Maximum).