More than a chill factor

Ice, generally, is something I want in my beverage glass, not on my driveway or in my trees. I don’t, of course, always have a choice in this matter, and while I was looking for information about today’s storm, I wandered into this BatesLine piece and found the following:

The ice and wind would combine to result in a Sperry-Piltz Ice Accumulation Index of 3 or more for all of northeastern Oklahoma (except for northern Osage, northern Washington, Nowata, Craig, Ottawa, Pushmataha, and LeFlore counties). That value is defined as, “Numerous utility interruptions with some damage to main feeder lines and equipment expected. Tree limb damage is excessive. Outages lasting 1-5 days.”

Not being a Tulsan, I pondered: Who are Sperry and Piltz, and what do they know about ice?

The answer is plenty:

Sid Sperry, director of Public Relations and Communications and Research at the Oklahoma Association of Electric Cooperatives and Steve Piltz, National Weather Service meteorologist, have combined their efforts to produce “The Sperry-Piltz Ice Accumulation Index.”

This scale runs from 1 to 5, where 4 means “Prolonged & widespread utility interruptions with extensive damage to main distribution feeder lines & some high voltage transmission lines/structures. Outages lasting 5-10 days.” You don’t even want to imagine a 5.

The SPIA Index was formulated apparently in Tulsa in 2008 in 2007. (We have, therefore, no figures for the December 2007 ice blast, which seems to be on the high side of 4-ish, at least here in OKC; OG&E had most everyone back on by day 10.) The methodology [pdf] is downright elegant:

Utility systems may be able to handle moderate ice accumulations, but stressed lines under wind forces are more likely to break. Therefore, one inch of ice may be a Level 2 or Level 3 ice event, but if wind speed exceeds 25 mph, it becomes a Level 5 event.

If sustained winds are 15 mph or less, up to half an inch of ice presents the threat of only a 1; the 0.4 to 0.8 inch predicted for Oklahoma City (which we didn’t get), given the 25-mph-plus winds most of the day, would have fallen into the 3-4 range.

Tulsans seem to have embraced Sperry-Piltz; I have no idea how I managed to avoid finding out about this earlier.



  1. Jeffro »

    28 January 2010 · 9:19 pm

    Ice. A little dab will do ya.

  2. McGehee »

    29 January 2010 · 7:36 am

    You don’t even want to imagine a 5.

    That’s the one where the leading edge of the North American ice sheet looms over Fort Worth — but not to worry, it’ll still be proof of global warming.

  3. fillyjonk »

    29 January 2010 · 9:15 am

    On the other hand, the Parry-Splitz index measures how much your arms flail around and the kind of awkward and painful positions you fall into when trying to walk on ice…

  4. CGHill »

    29 January 2010 · 12:06 pm

    Already went through that this morning to fetch the paper, which was planted on the edge of the driveway; I crunched down a few steps, and it oozed half a foot away from me. I wound up retrieving it with a snow shovel (cf. previous experience with a garden rake).

  5. fillyjonk »

    29 January 2010 · 1:54 pm

    My (fortunately plastic-wrapped) paper was merrily floating down the gutter last night when I ran out to retrieve it. Who knows where it might have ended up if I hadn’t gone after it?

    I think we had about 3″ of rain yesterday.

  6. Sidney K. "Sid" Sperry »

    29 January 2010 · 11:52 pm

    Just a comment to make one slight correction to the writer’s original post. The “Sperry-Piltz Ice Accumulation Index,” or SPIA Index for short, was first formulated in early January, 2007, following back-to-back ice storms in the Oklahoma panhandle in December, 2006. I worked with officials from the Oklahoma Climatological Survey office in Norman, OK, and they recommended that I visit with Steve Piltz and his team of top meteorologists at the Tulsa NWS forecast office. Steve and his team were eager to help, and within a matter of hours had an algorithm put together using the parameters that I gave them for radial ice accumulation, wind speeds and directions, and temperatures during an icing event. The SPIA Index is now copyrighted, with myself and Steve Piltz as principle authors. The use of the Index is growing in popularity rapidly among emergency managers and other disaster preparedness agencies. We hope it proves to be a useful tool to the citizens of Oklahoma, and to all those who are impacted by ice storm disasters! – Sid Sperry, Director of Public Relations, Communications & Research, Oklahoma Association of Electric Cooperatives, Oklahoma City, OK.

  7. Fishersville Mike »

    31 January 2010 · 10:36 am

    Find my jiggly, red avatar…

    Dustbury explains the Sperry-Piltz Ice Accumulation Index…

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