Vaisala IceCast Thermal Mapping

Vaisala IceCast Thermal Mapping

On a typical winter night the differences in temperature across a road or runway network can vary by as much as 10ºC/18ºF. Consequently some sections may be below freezing while others may not.  

Thermal Mapping is a process by which the spatial variation of minimum night-time road surface temperature is measured, using a high resolution infrared thermometer.  Vaisala Thermal Mapping is the only proven and established technique to determine surface temperature relationships likely to occur across an entire road or runway network.  It is a technique, which has been utilised worldwide, to enhance the information available to both highway authorities and supporting forecast providers.  Thermal Mapping is an integral part of an effective Ice Prediction system as it provides a mechanism for extending point specific sensor site information between individual weather stations and across a road network.

Thermal Mapping identifies patterns of temperature variation, by undertaking accurate measurements of winter night time surface temperatures across pre-defined sections of a highway or runway network under a range of different weather conditions.  This pattern and distribution of warm and cold sections is determined by local environmental factors and prevailing weather conditions.  The occurrence of frost or ice is determined by the balance of energy a surface receives and loses in conjunction with the amount of available moisture. This is influenced by the complex relationship between a number of factors including:

  • Prevailing weather conditions.
  • Sky view factor (exposure). This will be dictated by features such as vegetation, buildings and tunnels etc. A low sky view indicates features overhanging or close to the road or runway. Such features will inhibit night-time cooling and may help the surface to retain some of its heat. A high sky view will relate to an open environment that will exhibit cold night-time road temperatures, as there is nothing to prevent the surface cooling.
  • Altitude (temperatures decrease with height).
  • Proximity to the coast and major water bodies.
  • Urban heat island effect.
  • Cuttings, embankments, elevated sections. These are localized features that will affect the energy flux of the road/runway at that point.
  • Road/runway construction (surface material e.g. concrete/asphalt/open asphalt, and depth of construction).
  • Traffic volume and flow.

The combination of these factors generates a unique temperature fingerprint for each road/runway. Thermal Mapping establishes the relationship between these variables and how they interact under different weather scenarios.

Thermal Mapping can identify localised sections of highway that are prone to frost and ice.

The surveys produce Thermal Fingerprints  - temperature profiles unique to a road or runway.  Thermal Maps are constructed from analysis of the Fingerprints, in conjunction with information on the specific survey weather conditions.  The Thermal Map for each weather condition identifies the pattern and magnitude of surface temperature variations, indicating the relative differences as colour-coded temperature categories.

Vaisala has over 20 years experience in the provision and development of Thermal Mapping services.  Vaisala expertise is utilised throughout an international client base of more than 1000 users, operating in a variety of different winter climates including; Western Europe, North America, Japan, Eastern Europe, Scandinavia, and New Zealand.  Thermal Mapping is now established Best Practice for many highway authorities throughout the world.


 

 

Thermal Mapping Weather Conditions

Under similar weather conditions a highway or runway will exhibit a constant and predictable pattern of surface temperature variations. The degree of temperature variation will alter between different weather conditions. To establish the relationship between local environmental factors and prevailing weather, Thermal Mapping uses three distinct weather categories to identify the changes in surface temperature variation.

Extreme Weather Conditions:

Calm and clear conditions, usually associated with anticyclonic systems (high pressure), or high pressure ridges between cyclonic systems (low pressure). The lack of cloud or wind allows the surface to cool very rapidly at night and is likely to produce the greatest degree of variation in night-time surface temperatures across an area.

Damped Weather Conditions:

Associated with cyclonic systems (low pressure), total low level cloud with moderate winds. The cloud cover acts as a blanket, reflecting energy back to the surface at night, whilst the wind promotes the mixing of surface air layers. This combination of factors results in the smallest degree of variation in night-time surface temperatures.

Intermediate Weather Conditions:

Arise from various meteorological features such as transient ridges or frontal approaches commonly associated with medium/high level cloud and moderate winds. Surface temperature variations will be greater than under Damped conditions but not as pronounced as under Extreme conditions.

Ideally, Thermal Mapping surveys should be conducted under all three weather conditions. However, the Extreme and/or Intermediate scenarios can be used for locating weather stations or for Route Optimization, depending on the local situation.

 

Thermal Mapping Operational facts

  • Vaisala Thermal Mapping surveys are undertaken by a fleet of specifically equipped vehicles, each mounted with a high resolution infrared thermometer connected to an automatic GPS data logger.
  • Thermal Mapping surveys are normally carried out between the hours of 23:00 and sunrise during the months of November through to April (in the northern hemisphere).
  • Vaisala Thermal Mapping surveys are carried out under normal road speeds, enabling surveys to be undertaken across different network types and environments including urban, rural, motorway, and airports.
  • Thermal Mapping measurements are normally made at an interval of every five metres.  It is possible to change the resolution of data collection depending on network requirements.
  • During Thermal Mapping surveys detailed observations are undertaken of prevailing meteorological conditions and road surface state.
  • On sections of Dual Carriageways surveys are normally carried out on the inside lane.  However, if there are three lanes or more then surveys are normally undertaken in the centre lane.
  • Before Thermal Mapping surveys are carried out a road/runway network is planned into smaller more manageable sections, by dividing it into c.30-50km Routes.  Routes will allow weather conditions to be continuously assessed throughout the survey.  The area one vehicle can cover during a survey night is referred to as a Sector. Therefore each Sector is comprised of a number of Routes.
  • Sectors and Routes always follow a set order and are planned to incorporate a series of overlapping sections.  Overlaps allow all the data to be compared and eventually related together (i.e. as one survey).
  • Thermal Maps are produced using collected data to provide a representation of the relative spatial variation of minimum road surface temperature under different weather conditions, indicating the relative differences as colour coded temperature categories.

Extreme Thermal Map - Wigan Council

Extreme Thermal Map, Wigan Council, UK

  • Thermal Maps can be produced in hard copy or electronic image formats. Thermal Maps are created in a GIS environment, geo-referenced and can be exported for inclusion in GIS platforms if required.
  • Under an on-going programme of research and development, Vaisala continues to advance the Thermal Mapping process.  A key part of this is the regular testing and examination of the accuracy and repeatability of Thermal Mapping, the results of which are endorsed in international scientific journals. 
  • Vaisala has established statistical procedures to validate the quality of data, and quantify the similarity between temperature patterns collected during different Thermal Mapping surveys.  Such processes demonstrate Vaisala’s Thermal Mapping process is reliable and the temperature variation along a road can always be reproduced under similar weather conditions.

 

 

Key Benefits of Thermal Mapping

  • Identification of potentially dangerous sections.  Thermal Mapping provides the temperature relationship across a whole network identifying those sections of the road which are likely to freeze first.  This may not be apparent from using road weather stations alone.
  • Enables selective anti-icing strategies. Thermal Mapping can identify and allow targeting of those areas in need of treatment, along with providing key input data for Route Optimization.
  • Identifies the optimum location and number of road weather stations.   Thermal Mapping identifies sections of road with stable thermal characteristics which are representative of a network and/or Climatic Domain, thus identifying optimum locations for weather stations.
  • Extends ice prediction from site specific weather stations to an entire road network.  Thermal Mapping enables the provision of Forecast Thermal Maps in an Ice Prediction System.   Thermal Maps provide information on the relationship of road surface temperatures between weather stations as it enables the extrapolation of minimum temperatures across the whole network.
  • Provides quantitative reference data.

The power of Thermal Mapping as a decision-making tool is maximised when integrated into the Vaisala's Ice Prediction System.  Site specific forecast data from a road weather station can be extended to all roads within a network using Thermal Mapping. 

Forecast Thermal Map display in IceView

Forecast Thermal Map, as shown in Vaisala's IceCast IceView display software.

Thermal Mapping is an integral part of Vaisala's Ice Prediction System providing users with forecast minimum temperature maps across an entire network.  Thermal Mapping also proves invaluable when considering the siting of weather stations.  If weather stations are only located in the coldest sections of a network this may lead to a pessimistic winter maintenance strategy.  This can result in unnecessary treatment operations and expense.