Recommended Weather Severity Index WSI

Original Message: Hello,

I am writing to inquire as to which WSI you have found to be the best and easiest for use.

Thank you for your help.

Happy Holidays everyone.

Mark Cornwell
Sustainable Salting Solutions, LLC
12415 North Holly Road
Holly, MI 48442
Phone: 248/634-0820
Mobile: 248/895-2888


LinkedIn: Mark Cornwell

Response Messages

Iowa DOT uses a few, each with a different purpose.

The longest standing index is an in-house index loosely based on one from Wisconsin DOT. It is designed to summarize a season or part of a season, not single events. It takes into account:

  • Inches of snow

  • Frequency of freezing rain, snow, mix, blowing snow and sleet

  • Duration of freezing rain, snow, mix, blowing snow and sleet

  • Average pavement temperature during the events

  • Wind speed during the events

This is relatively simple for us to run using information reported from RWIS and collected in our standard crew reports. Since its creation in ~2005 it has been a staple on our end-of-year reports. Prior to developing this we used the index that appeared in the SHRP-H-350 report.
This year we have a pair of new indices that specialize in more detailed summarization. These can describe very short time periods down to periods within a single storm. They are actually the base for larger initiatives here at the DOT – estimating the impact of weather on salt use and traffic mobility. As they are used now, they describe the weather impact for a day or for a certain amount of time (like a pay period or weekend; user-defined). These indices are much more complicated to calculate than the above index but are based on much the same kinds of information: RWIS pavement and atmospheric information and reported storm start/stop times and precipitation type that come standard in our daily crew reports. The salt index steps through the weather records 10 minutes at a time and estimates salt per lane mile use based on a chart in our Instructional Memorandums that is very similar to the salting guidelines found in NCHRP-526. The traffic index also steps through at this frequency, following an adaptation of the traffic index found in Iowa Highway Research Board Project TR-491. In both cases the 10 minute chunks can be added up to create storm, daily, or other summaries. The products based on these indices are turning out to be extremely useful for quick feedback whether our salt use and traffic outcomes are comparing well with expected. But they are not nearly as easy to compute as most other indices.
I can provide more info for any of these if needed.
Tina Greenfield
Iowa DOT RWIS Coordinator