Saturday, February 28, 2015

Lowest Wind Chills in Winter 2014-2015

No matter where you live. it probably felt really, really cold at some point this winter. But the question is, how cold did it feel? There is a very straightforward way to assess this. There are several thousand Automated Surface Observing Stations (ASOS) across the U.S. that record a wide range of meteorological conditions every hour of the day (sometimes more frequently). To compute a wind chill value, all we need are the temperature and wind speed values. The good folks at Iowa State generously provide an archive of all ASOS observations and some tools to extract the data (thank you Daryl Herzmann!).

There are 2,242 ASOS stations with sufficient data to generate a meaningful map. Figure 1 shows all of the stations and color codes them based on (generally) 10°F categories of wind chill. Wow! Look at the extremely low wind chills experienced by the vast majority of U.S. residents. Over 70% of the U.S. had a wind chill below 0°F at some point this winter. The "winner" was the Mount Washington Observatory with a low wind chill of -87°F! For non–mountain top stations, Deadhorse, Alaska, came in with a low wind chill of -86°F. In the contiguous U.S., Pinedale and Laramie, Wyoming, had the low (non–mountain top) value at -55°F. A word of caution though. Sometimes bad values are reported by the ASOS. Therefore, some readings may not be accurate. (Note: to read about a -100°F wind chill, see this blog post).

Figure 1. Lowest wind chill at all ASOS stations during this winter (2014-2015). Al stations are shown as dots.

Showing a bunch of dots on a map isn't always easy on the eyes. Therefore, we can use a little computational magic to turn those dots into a continuous color scheme. To do this, I used ArcGIS's inverse distance weighted surfacing function to make the map shown in Figure 2.

Figure 2. Lowest wind chill during this winter (2014-2015). Colors derived from inverse distance weighted surfacing algorithm using all 2,242 ASOS stations.

The surfacing technique looks at a series of points to determine natural boundaries. This is often referred to as a "smoothing" technique. Dots from Figure 1 do not always line up with the color bands in Figure 2. This is a necessary compromise to make a map that is not too chaotic. Imagine five nearby stations with four stations having values in the mid -20°s and one station with a value of -30°F. The dots (see Figure 1) will reflect the true values but the all five stations will be assigned to the -20°s category in Figure 2 since those color bands are based on weighted values of a station and their neighbors.

In Figure 2, the mean grid cell value for the entire U.S. was -17°F. For Alaska, the average lowest wind chill value was -43°F and for the contiguous U.S. it was -12°F!

Selected Values

With over 2,000 values in the data set, there are too many to list in a table. However,this table has some selected low wind chill values for major airports around the U.S.

Table 1. List of lowest wind chill values at selected major airports around the U.S. during this winter (2014-2015).

** Supplemental Figure **

For reference, this map (Figure S1) shows the lowest observed air temperature based on daily climate reports for all stations in the U.S.
Figure S1. Lowest observed actual air temperature this winter. Data obtained through xmACIS2.


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  2. Your article helped me to understand the categories of wind chill. You have clearly explained it by using color bands and maps. It is really helpful to get good idea. To know about writing research proposal writing service is the best choice


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