A Snowpro Plus+ Implementation of Flags (Lemons)

By Gary Sims, Gasman Industries Ltd. Oct 2005

At ISSW 2002 in Penticton, B.C. Canada, Ian McCammon and Jürg Schweizer presented a paper titled “A field method for identifying structural weaknesses in the snowpack” [1] and a poster where they developed a simple method for analysing snow profiles and flagging certain characteristics associated with instability of the interfaces between adjacent snow layers. Coined, “Lemons”, they proposed that the lemons were a good indicator of instability and especially useful for novices learning how to interpret snow profiles.

At the 2004 ISSW, Jürg Schewizer and others developed a set of critical layer and interface properties similar to the lemons. Using profiles from the Columbia Mountains, the methods was simplified by Bruce Jamieson and Jürg Schweizer in their March 2005 paper titled “ Using a checklist to assess manual snow profiles” [2]. In this paper they identified 3 layer properties and 3 interface properties that would be tested against critical ranges and would be used to “flag” instabilities in the snow profile. The paper also provided a method for determining the flags.

The layer properties are Average Grain Size, Hardness, and Grain Form..

The interface properties are Difference in Grain Size, Different in Hardness, and Depth.

The method is to first look at each layer and “Flag” the layer when the properties meet the critical range. For example in their example [2] Page 2, the layer is flagged 1 to 3 times if average grain size is greater that 1 mm, or hardness is less than One-Finger (1F) or if the primary grain form is a persistent type.

The next step is to take each interface between adjacent layers and “Flag” the interface where the properties meet the critical range. For example the interface is flagged up to 3 times if the difference in average grain size between two layers is greater than 0.5 mm. or difference in hardness is greater than 1.0 (each hand hardness has a specific value set) or interface depth is between 20 and 85 cm.

Once the flags are drawn on the profile the number flags at the interface layer are added to the maximum flag count in the layer above or below to arrive at a total count (0-6). The predicted failure interfaces are those with the maximum number of flags. More than one interface can have the same number of flags. The maximum number of flags for any interface is the structural instability index of the profile.

This method makes it very easy for novices to recognize the unstable layers. However, as noted by Jamieson and Schweizer, “its value in making decisions about avalanche risk is unclear, especially for experienced avalanche practitioners.”

Our company is the developer Snowpro Plus+ for Windows which is a software product designed to provide high quality plots of Snow Cover Profile information according to the International Classification for Seasonal Snow on the Ground (ICSI). It was developed in conjunction with the Snow Avalanche Programs of the B.C. Ministry of Transportation (MOT) in Victoria, B.C., Canada about 10 years ago. Since then we have updated the software on a frequent basis and the software is used world wide in the snow industry and by ski hills and snow recreational operators.

Recently Ted Weick of the B.C. Ministry of Transportation expressed an interest in Snowpro Plus+ incorporating the “Yellow Flag” feature. We agreed it was a good idea and have used the Jamieson and Schweizer paper [2] as a template to the implementation of this feature.

Figure 1 - Flag Preferences Screen

We have made the critical range values for the layer and interface properties user configurable so that these values can be tailored for a particular areas snow types and conditions. Figure 1 shows the Layer Flags preferences screen. We have included the ability to set the colour of the flags and to set a test so that if more than a certain number of flags are set at the interface that the flags there are marked a different colour. In Figure 1 it shows that the layer flags are yellow and if there are more than 4 flags at the interface then set the interface flags colour to red.

The initial or default list of persistent layers as used by McCammon, Schweizer and Jamieson [1], [2] are based on current grain forms available in Snowpro Plus+. Any of the grain forms can be disabled by unchecking them and others can easily be added to a text file in the software directory which then show up in the list.

Figure 2 - Flag Panel Displayed

Figure 2 shows a sample Snowpro Plus+ profile with the yellow flags on the layers and the red flags on the interfaces and the total count to the right. This screen can be toggled between the regular profile data with the simple click of a button on the bottom of the profile.

Our firm appreciates feedback and suggestions from the snow community to make our software a better product and we implement those things which we feel benefit the majority of the Snowpro Plus+ user community. Further information on this or Snowpro Plus+ is available by contacting Gary Sims. 

I would like to acknowledge Bruce Jamieson for his help in answering a number of my questions on his method when implementing the “Yellow Flag” feature.

References:

[1] McCammon, I., Schweizer, J. 2002. A field method for identifying structural weaknesses in the snowpack. In: J.R. Stevens (editor), Proceedings ISSW 2002 International Snow Science Workshop, Penticton, BC, Canada, 29 September-4 October 2002. International Snow Science Workshop Canada Inc., BC Ministry of Transportation, Snow Avalanche Programs, Victoria, BC, Canada, pp. 477-481. http://www.snowpit.com/articles/lemons%20reprint%20copy.pdf

[2] Jamieson, B. and J. Schweizer. 2005. Using a checklist to assess manual snow profiles. Avalanche News 72, Canadian Avalanche Association, Revelstoke, BC., 72-61.
http://www.eng.ucalgary.ca/Civil/Avalanche/Papers/ChecklistProfiles.pdf