Wednesday, March 5, 2014

Wind Chill Differences – Pre vs. Post 2001

As most readers of this blog know, the formula for calculating wind chill changed in 2001 (a good description of the new formula and the rationale behind the change cane be found here). The Old method was based on a study in 1945 that measured how long it took for water to freeze in various temperature and wind conditions in Antarctica. The study was not intended to determine equivalent temperatures but it was used for that purpose nonetheless and thus the Wind Chill Index (WCI) was born. Due to some glaring shortcomings with the application of the 1945 study to human perception, a series of wind tunnel experiments were conducted and a New Wind Chill Index was formulated and implemented by the NWS in 2001.

The New wind chill formula is actually an equation that was derived from a table with approximately 800 cells based on the wind tunnel experiments noted earlier. The air temperature in the original table ranged from +45°F to -40°F. The equation, however, can be used for any temperature and any wind speed. Clearly temperatures lower than -40°F occur frequently in Alaska. A comparison between the two formulas at 5°F is shown in Figure 1. In general, the New method's values are warmer than the Old values for most wind speeds. However, for low wind speeds, the New method produces slightly colder values since it assumes that the observer is walking at 1.5 m/s. The New method also takes into account the attenuation of wind from the standard measurement height to the space that people occupy.

Fig. 1. Old vs. New wind chill comparison.

Unfortunately many people in the general public do not know that this change was implemented and confuse Old and New values. For example, the football game in Cincinnati in January 1982 famously had a wind chill of -59°F at kickoff. However, few people know that using the New wind chill calculation, the kickoff value was 'only' -37°F. Figure 2 was produced by the Green Bay, WI, NWS office comparing the 1982 game with the 1967 Ice Bowl game. Conveniently, both Old and New wind chill values are given.

Figure 2. Football game comparisons. Source: http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=grb&storyid=99351 

The Howard Pass event of mid-February possibly had a minimum wind chill of -97 (see Figure 3). The -97°F value is 1°F lower than a -96°F wind chill at Prudhoe Bay in January 1989 (Richard has done some excellent analysis and modeling of the Howard Pass event here and here). To confuse matters even more, the Prudhoe Bay wind chill value was originally -120°F since it occurred before the New wind chill formula was developed and implemented. In fact, there were several stations in January 1989 that had a wind chill of -120°F; including Barter Island and Cantwell. However, when using the New formula, the Prudhoe Bay value of -96°F bested the Barter Island and Cantwell readings.

Fig 3. Howard Pass RAWS data between Feb. 11 and Feb. 15, 2014. Air temperature, wind, and wind gust values are displayed. In addition, the New and Old windchill values are displayed. 

January 1989:

I did some digging around at the NOAA hourly observation site and decided to download all data for stations on the North Slope. None of the North Slope stations have ever recorded a wind chill lower than the -96°F in January 1989 at Prudhoe Bay. Of course the completeness of the data leaves a lot to be desired.

It was apparent that most North Slope stations recorded their lowest ever wind chill in January 1989. Therefore I also downloaded all data for stations across Alaska with hourly observations in January 1989.

Figure 4 shows the monthly minimum wind chill based on those observations using the Old formula in effect at the time and Figure 5 shows the monthly minimum wind chill based on the New wind chill formula. It is important to remember that the calculations were often, but not always, based on the same observation. These are minimum monthly values.
Fig.4. Minimum wind chills in January 1989 for all stations in Alaska that reported hourly observations using the Old formula. In many cases observations are missing so a few data point should be considered suspect.

Fig.5. Minimum wind chills in January 1989 for all stations in Alaska that reported hourly observations using the New formula. In many cases observations are missing so a few data point should be considered suspect.

As you can see, no portion of the state escaped the brutally cold wind chills. The color scale on both maps is identical. What stands out on on Figure 5 as compared to Figure 4 is, 1) the overall lower values, and 2) the evenness of the distribution. The -100s along the North Slope and in Bristol Bay were replaced with -80s and -90s. Also, the -70s in the Interior were replaced with -80s in many cases. As stated earlier, at low wind speeds, the New wind chill values are actually colder than the Old wind chill values.

Figure 6 is a map that shows the difference for the minimum wind chills using the two methods. Those areas in red saw their minimum wind chill values rise (warmer) with the New wind chill formula and those areas in blue saw their minimum wind chill values fall (cooler). Areas exposed to more wind generally saw their wind chill values rise with the implementation of the new formula and places that are less windy saw their wind chill values fall.
Fig. 6. Difference in the lowest monthly wind chill value in January 1989 using the Old and the New methods.

McGrath:

If you looked closely at Figure 5, you might have noticed the January 1989 minimum value for McGrath was -100°F. That is not a mistake. At 6 a.m. on January 27th 1989, the air temperature was a chilly -72°F with a wind speed of 7 miles per hour. According to the New wind chill formula, it felt like -100°F. Using the old formula, it felt like -94°F. Therefore, when the formula changed, McGrath went from middle of the pack to 1st place. Not just first place in January 1989 but possible first place all time. In addition to January 1989, I also downloaded hourly observations for any station that recorded an air temperature of -65°F or colder in any year (sub -100°F wind chills can occur with temperatures warmer than -65°F but the amount of data was far too large so I cut it off at -65°F). There was one other instance of a sub -100°F wind chill (-106°F to be precise) but its reliability is in doubt. Rick is doing some checking on it since that station does not have any other wind chills lower than -92°F. That will probably be the subject of another blog post.

Figures 7 and 8 show the hourly observations for McGrath and Prudhoe Bay respectively on January 27th and January 28th, 1989.


Fig 7. Hourly temperature and wind speed for McGrath, Alaska, on January 27 and January 28, 1989. Both the New and Old wind chill values are displayed. The -100°F value using the New formula is shown.

Fig. 8. Hourly temperature and wind speed for Prudhoe Bay, Alaska, on January 27 and January 28, 1989. Both the New and Old wind chill values are displayed. The -96°F value using the New formula is shown.

Is the -100°F at McGrath a populated station record?

That is an interesting question. For starters, it is worth noting that the original research that went into developing the new wind chill formula did not use air temperatures lower than -40°F and that the fitting of a formula to the observational data is the only reason it can be extended backward. Nevertheless, one of the pioneers of the New wind chill research, Randall Osczevski, discusses the New formula (which was actually developed in the 1990s) in this paper and uses values as low as -100°F with air temperatures as low as -60°F. He also discusses how the New formula is more realistic at extreme low temperatures. Also, since NOAA calculates wind chill values for any temperature and any wind speed, we can assume an implicit endorsement of the formula at extreme low temperatures.

The Alaska NWS offices do not issue Wind Chill Advisories or Wind Chill Warnings unless the sustained winds are, or are forecasted to be, 15 mph or greater for at least three hours. Maps of all Alaska advisory criteria can be found HERE. So, can a wind chill be a record if it wouldn't even qualify for a Wind Chill Advisory? Should there be a minimum wind speed criteria?

Several NWS offices have lists of statewide wind chill records. When the Minnesota Climatological Working Group discussed extreme wind chills, several examples they gave used wind speeds of 6 or 7 mph. The Montana Climate Atlas uses 10 mph as a threshold for their monthly probability maps. However, it is unclear whether the record lowest wind chills (see Fig. 9) also used the 10 mph threshold. Since it was unstated, my guess is that they did not.

Fig. 9. A table from the Montana Climate Atlas showing statewide lowest wind chills.

The NWS office in Lacross has a climatology of wind chills in the Northern U.S (Lower 48). They use a 10 mph filter do develop their probability maps. In their report, they state, "[t]his speed was chosen as it is the minimum threshold currently used throughout most of the NWS for the issuance of Wind Chill Advisories or Warnings." Since their report is looking at climatological probabilities, it makes sense that they have a wind speed cutoff. Their report does not contain a list of extreme wind chills.

The NCDC's State Climate Extremes Committee does not keep track of wind chill records and as far as I can tell, no standards exist. Wind chill is a human construct and is therefore not directly measurable with a calibrated instrument. My guess is that wind chill records will always remain unofficial.

To my knowledge, no one has questioned the measurements in McGrath. It is a first-order station and the NWS had an observer in McGrath at the time. Therefore, we can assume that the measurements and calculations are correct. In my opinion, any list of extreme wind chills should have all valid calculations included, regardless of the wind speed. If the wind chill made the temperature feel like -100°F, then that should be on the list ahead of the -96°F at Prudhoe Bay and the -97°F at Howard Pass (if verified).

11 comments:

  1. Well, well... another day, another potential wind chill record! I'll be curious to learn more about the -106F report.

    Does the new formula assume the unlucky individual is always walking into the wind, or does the 1.5 m/s simply represent a wind speed cutoff on the basis that calm conditions actually translate to 1.5 m/s while walking?

    The Canadian wind chill page lists a 1975 report from Nunavut with new-formula wind chill of -108F. I wonder if Alaska will ever match that.

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    1. I am not sure about whether or not the assumption is that the individual is walking into the wind is discussed. The context for the 1.5 m/s in the New formula is to counter the Old formula's assumption that wind chill is not 'felt' at wind speeds less than 4 mph. The New formula always assumes that a person is in the shade and is moving. Although that is somewhat difficult to reconcile with the fact that the wind chill is equal to the air temperature when the wind is calm.

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    2. Brian, according to my calculations the new formula gives nonsense results for zero wind speed, so I think it must be the case that the minimum input wind speed is 3 mph. The following link says one of the changes was to "lower the calm wind threshold to 3 mph":

      http://www.erh.noaa.gov/er/iln/tables.htm

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    3. You are correct Richard. The formula breaks down with a wind speed of 0 (as does the old formula). Looking back at the paper I linked to, it states "[t]hey should not be used at very low wind speeds, that is, less than 3 mph..." On their charts the wind chill and air temperature are the same for calm winds.

      This is probably an artifact of their using 5° and 5 mph increments in the original survey. Fitting the equation back to those charts probably cause asymptotic problems.

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  2. Excellent presentation Brian. I enjoyed reading about the subject that led to a review of the original work by Siple in Antarctica.

    I keep a copy of the old table on my remote cabin wall. The later NWS version seems puny by comparison. I let my dogs make the final judgement...an outdoor walk or remain close to the wood stove.

    As any follower of the cable show Deadliest Catch (Bering Sea fishers) can testify, water can affect the chill. But for dry landers the humidity is apparently not a significant factor.

    McGrath deserves a record. Friends I've spoken with that lived there and trapped and travelled in the nearby river drainages recount the frequent cold often accompanied by breeze in that region.

    Gary

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  3. I worked at Deadhorse, AK from 1987-1989 and took the Deadhorse official hourly weather observations. The windchill chart posted near our instruments went down to -180 degrees. I wish I could recall the date but we experienced a combination of temperatures and wind the put the windchill off the chart. I manually calculated it out to just under -200 degrees. We had a number of other days with windchill's below -100.

    I think it was Feb 25, 1989 that a monster storm engulfed the North Slope with winds up to 110 mph. I thought maybe this one produced the -200 windchill but now I don’t think so. From our four-story glass enclosed tower, I had the privilege of watching a calm morning under blue skies transform into a howling dangerous blizzard. The blowing snow on the surface began to rise as the wind increased. Over the next few hours the tops of the blowing snow climbed up to the third floor, just below me. I still had mostly blue skies to the east and an unofficial visibility of 75-100 miles (zero on the surface). Looking over the tops of the blowing snow just under my feet, it was a strange site, watching what appeared to be the surface skim by me at 60 knots now. As the wind increased, the tops of the blowing snow continued to rise engulfing the tower. All visibility, up, down, or sideways, ceased. This occurred before any snow actually started falling. Winds eventually reached 110 mph shutting the oil camps down for several days. It was an incredible experience.

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    Replies
    1. Mr. Batt, it is an honor to have your input in this discussion. I just looked at the hourly observations that you collected for the 1987-1989 time period and your estimate of the date for the blizzard you described was spot on. There were several hourly observations on 2/25/89 and 2/26/89 with sustained winds over 60 mph in Deadhorse with gusts over 90 mph!

      As far as the wind chills go, the January 1989 values were lowest during that, or any other, time period on record for Deadhorse between 1973 and the present day based on the hourly observations in the DS-3505 database. The wind chill formulas break down when wind speeds exceed 100 mph. With a temperature of -60°F, the pre-2001 formula cannot produce a wind chill lower than -150°F no matter how high the wind is. It is certainly possible that the accompanying chart had an alternate extrapolation technique for extreme values lower than -150°F.

      Again, thank you for sharing your first-person insights into these extreme observations. You are certainly part of a very small number of people to have experienced those conditions.

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    2. Thanks, Brian. The unofficial 110 mph wind was recorded over at the ARCO airport 3 miles away. By that time we had long abandoned the tower because our windows were only rated to 55 knots, but as much as they flexed inward, they held. It was a great storm.

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    3. Mick, do you know if there is any documentation of the observations from the ARCO site? That would be an important climatological resource.

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