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Road Weather Information System
Environmental Sensor Station
Siting Guidelines
April 2005

Appendix C - Road Weather Observation Requirements

Table C1 contains a list of weather elements for DOT consideration during the analysis of road weather requirements. This list was adapted from Appendices B-1, B-1.1, and B-5 in the report on surface transportation national needs by the Office of the Federal Coordinator for Meteorology - Services and Supporting Research.25 A weather element is a particular weather condition or a consequence of weather conditions affecting road operations or the safety, economic value, or efficiency of transportation or road maintenance activities. Table C1 also indicates the thresholds at which the weather element affects road operations and maintenance activities. In some cases any occurrence of a weather element has impact and may require action. For other elements, the thresholds are variable based on the specific needs of users.

The shaded weather elements in Table C1 deserve primary attention during the requirements analysis and the development of ESS plans. Not all the highlighted weather elements can be measured by an automated ESS; however, they all provide insight into important ESS deployment considerations. For example, an ESS cannot directly measure roadway ice accumulation in inches. If roadway ice accumulation is an important consideration, other sensors discussed in Section 2.1 should be selected that will indicate icing conditions. Identifying the highlighted elements for primary consideration is only a recommendation. The actual elements considered when planning an ESS installation must reflect the DOT's operational requirements for road weather information.

Table C1. Weather Element List
Weather Element
(Shaded weather elements are recommended for primary consideration during ESS requirements analyses.)
Freezing Precipitation Occurrence Any
Structure Ice Accumulation (inches) Any
Pavement Ice Accumulation (inches) Any
Frozen Precipitation Occurrence Any
Snow Accumulation Observation (inches) Any
Snow Drift Level Observation (inches) Any
Roadway Snow Depth Observation (inches) Any
Roadway Snow Pack Depth Observation (inches) Any
Adjacent Snow Depth Observation (inches) Any
Snow/Ice Bonding Observation (inches) Any
Liquid Precipitation (inches) Any
Precipitable Water Vapor Observation (inches) (amount of moisture in a vertical column of air) Any
Air Temperature (degrees Fahrenheit (°F)) Variable, based on impact criteria
Air Temperature Change Rate (°F per 24 hours) Variable, based on impact criteria
Air Temperature and Time Values (heating/cooling degree days) Variable
24 hour
Dewpoint Temperature (°F)
(Also referred to the frost point when this temperature is below 32 °F)
Variable, based on temperature and impact criteria
Wet Bulb Temperature (°F) (used to measure moisture in the air, similar to Relative Humidity and Dewpoint) Variable, based on temperature and impact criteria
Relative Humidity (percent)
(Can be derived from air temperature and either dewpoint or wet bulb temperatures.)
Variable, based on impact criteria
Wind Chill (°F) ≤ 32 °F
≤ 20 °F to 32 °F
Heat Index (°F) Variable, based on impact criteria
Atmospheric Pressure Variable, based on impact criteria
Total Sunlight (hours) All
Total hours
Cloud Cover Scattered, broken, overcast or tenths or octal of sky covered
Subsurface Temperature (°F) Variable, based on other contributing factors such as wind, shade, sun
Soil Moisture Saturated, unsaturated
Soil Temperature (°F) Variable
Pavement Freeze Point Temperature (°F) ≤ 32 °F with moisture (observation and forecast)
Pavement Temperature (°F) Variable, based on impact criteria
Pavement Condition Wet/dry, snow, snow pack, slush, ice
Occurrence of Road or Bridge Frost Any
Chemical Concentration Variable, based on application, residue
Visibility, Including Restricting Conditions such as Precipitation, Fog, Haze, Dust, Smoke (statute miles) < 1/4 to ≤ 1/2 mile
Sun Glare Any
Wind Speed to include Gusts (miles per hour) Variable, based on impact criteria
Surface Wind Direction (degrees) Any
Thunderstorms with Lightning (proximity to route or operational area in miles) ≤ 5 miles
Thunderstorms with Hail (hail size, proximity to route or operational area in miles) Any size, ≤ 5 miles
Thunderstorms with Tornado or Waterspout (proximity to route or operational area in miles) ≤ 10 miles and ≤ 5 miles
Blizzard - sustained wind or frequent gusts greater than or equal to 35 miles per hour accompanied by falling and/or blowing snow, frequently reducing visibility to less than mile for 3 hours or more. (proximity to route or operational area in miles) ≤ 50 miles
Flooding (The occurrence of flooding or a measurement of a water level above a flood stage) Any
Water Course Flow Volume (cubic meters per second) Variable, based on impact criteria
Water Body Depth (feet) Variable, based on impact criteria
Hurricane Storm Surge Any
Air Stability (determination of vertical motion or mixing in the atmosphere) Stable/Unstable

Some weather elements in Table C1 can be determined using the sensors or a combination of ESS sensors. For example:

  • Winds associated with blizzards are observations of wind speed and gusts under these high wind conditions.
  • Wind chill temperatures are numerically derived from air temperature and wind speeds.
  • Heat index is numerically derived from air temperature and relative humidity.
  • Air temperature change rate is derived from air temperature.
  • Wet bulb temperatures can be derived from dry bulb and dewpoint temperatures.
  • Relative humidity can be derived from air temperature and either dewpoint or wet bulb temperatures.

This appendix presents two additional analyses of road weather observation requirements. These analyses supplement the data presented above and are included to help state DOTs evaluate their needs for road weather observations. The first analysis, based on a Strategic Highway Research Program (SHRP) RWIS implementation guide,26 identifies the road weather requirements of highway agencies, as well as those of meteorological organizations and the traveling public. The results are presented in Table C2. A "Y" indicates the users identified a requirement for the associated observation. Requirements are presented for both snow and ice control activities and for other road maintenance activities.

Table C2. Road Weather Information Systems Weather Observation Requirements
Types of Observation and Associated Weather Elements Road Agencies Meteorological Organizations Travelers
Snow Other Snow General Alert General
Pavement Temperature Surface Y Y Y N N N
Subsurface Y Y Y N N N
Surface condition Y Y Y N Y Y
Chemical concentration Y N N N N N
Weather Wind Speed Y Y Y Y Y Y
Direction Y Y Y Y N N
Temperature Air Y Y Y Y N Y
Dewpoint/Humidity Y Y Y Y N N
Solar radiation Y Y Y N N N
Pressure N N N Y N N
Visibility Y Y Y Y Y N
Present weather Y Y N Y N Y
Precipitation Amount Y Y Y Y N N
Rate Y Y Y Y Y N
Begin/end times Y Y Y Y Y N
Type Y Y Y Y Y Y
Snow = Snow and ice control activities.
Other = Road maintenance activities other than snow and ice control.
General = Weather and/or road condition information used for other than snow and ice control or severe conditions alert.

Table C3 presents another list of road weather observation elements adapted from an Ohio University study.27 These data are based on responses from users and administrators from three state DOTs and a European country. Study participants were asked to evaluate the importance of sensors by comparing them in pairs using a scale of 1 to 10, such that the sum of the rankings of the two sensors totaled 10. If two sensors were equally important, they would each be given a score of 5. The evaluator might give a very useful sensor a score of 9 and a sensor not useful to that evaluator a score of 1.

The inputs for all comparisons were used to compute the relative importance of the types of measurements. The results shown in Table C3 provide a prioritized list of the sensor measurements, with the precipitation type considered the most important and wind gusts the least important. While this table may help state DOTs prioritize their sensor requirements, it should be noted that the results were based on a single study with a limited sample size. Table C3 should be considered just another tool in evaluating sensor requirements. The final decision for sensor selections should reflect DOT operational requirements for road weather information.

Table C3. RWIS Sensor Measurement Rankings
Sensor Measurement
Precipitation Type
Surface Temperature
Surface Status (dry/wet)
Precipitation Rate or Intensity
Precipitation Accumulation
Chemical Percentage or Factor
Air Temperature
Ice Percentage
Freezing Point Temperature
Depth of Water Layer
Wind Speed
Relative Humidity
Wind Direction
Barometric Pressure
Subsurface Temperature
Wind Gusts

25 Office of the Federal Coordinator for Meteorology - Services and Supporting Research. Weather Information for Surface Transportation, National Needs Assessment Report. FCM-R18-2002. December 2002. Back to Footnote.

26 Boselly, S.E., and D.D. Ernst. Road Weather Information Systems Volume 2, Implementation Guide. Strategic Highway Research Program Publication- SHRP-H-351, National Research Council, Washington, D.C., 1993. Back to Footnote.

27 Zwahlen, H.T., et al. Evaluation of ODOT Roadway/Weather Sensor Systems for Snow and Ice Removal Operations Part I: RWIS. Human Factors and Ergonomics Laboratory, Ohio University, June 2003. Back to Footnote.

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