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?
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.