Hottest Summers, Coldest Winters for Contiguous U.S.: A Few Years Loom Large

Hottest Summers, Coldest Winters for Contiguous U.S.: A Few Years Loom Large

Keeping track of all-time warmest/coldest daily maximum temperatures and all-time warmest/coldest months on record for any given site is a fairly easy task. However, very few NWS sites provide data concerning what their respective coldest climatological winters (December-February) or hottest climatological summers (June-August) have been. Researching 300 sites in the contiguous U.S. I have put together this summary for such. Below are the methods I used and some of the results, which proved quite interesting. For example, 73 of the 300 sites (almost 25%) measured their hottest summers on record during just the summers of 2010 and 2011.

Choosing the 300 Sites

In order to make an attempt to make this research of some use, I chose 300 sites across the contiguous U.S. that have long POR’s (periods of record) dating back at least 100 years or more and that are also more or less evenly spread across the U.S. geographically. The sites represent all of the country’s various climatic zones aside from high Alpine regions (where data for long POR’s is generally lacking). That being said, there are a handful of locations on the list that do not have very long POR’s but were the only sites I could find that represented large regions with very small populations. For example: Chisos Basin, Texas (in Great Bend National Park) with a POR only back to 1948. This site is in Brewster County that comprises some 6,200 sq. miles but with a population of only 9,300. Another example is Danner, Oregon with a POR back to only 1929. This site is located in Malheur County which is 9,300 sq. miles but with a population of just 30,500. These large regions simply have very few sites with climate data that span long periods of time. In any case, all but 10 of the sites have POR’s of 100 years or longer and 240 of the 300 have POR’s dating back to the 19th century. The list of all the sites, their data, and POR’s can be found in the links near the end of the blog.

Below are two maps showing the actual locations, one with the site names and one with only the locations.





Methodology

After identifying the 300 sites used in this analysis, I used several climate data sources to determine each site’s coldest winter and hottest summer of record. The most useful source was NOAA’s Online Weather Data (NOW Data) which has comprehensive statistics for thousands of locations across the U.S. For the most part, NOW data has been ‘Threadexed’ meaning that all the data available for each location includes such for all the various places within the town or city where official weather stations have been maintained. Virtually all the towns and cities in the U.S. have moved their respective weather stations from one location to another (within the city) at various times since the beginning of their POR’s. In the several cases where NOW data for a site was not ‘Threadexed’, I resorted to the climate summaries available from the Regional Climate Centers. These are outlined in the map below:



Unfortunately, the Northeast Regional Climate Center and Southern Regional Climate Center do not provide detailed historical climate summaries on their web sites. In the few cases where neither NOW Data nor the RRC’s provided the temperature data I needed it became necessary to dig into the actual published records contained in the USWB/NWS/NOAA Climatological Data by Sections and in the Department of Agriculture various state weather service monthly bulletins as well as the Monthly Weather Review for the early era of weather record-keeping. I have not included data from the Smithsonian observations (1849-1874) and only partially from the Signal Army Corps Service era (1870-1890). Much of this data is still currently recognized as ‘official’ by the NWS/ASOS/COOP sites and reflected in the NOW data collection including, for example, Lansing, Michigan, Minneapolis, Bismarck, Cheyenne, and New York City among many others. Of course, this raises the legitimate concern of comparing apples to oranges since the POR’s for all the sites vary. For instance, the winter of 1874-1875 is officially considered the coldest such on record for Minneapolis, Madison and Milwaukee in Wisconsin, and Lansing in Michigan but not for other sites in the region even though the data is available and the winter of 1874-1875 was likely their coldest as well. A more thorough analysis by another party with more time and resources at their disposal is warranted. This report is simply an initial ‘broad brush’ approach to the subject.

The Results

What immediately becomes clear is that just a handful of extremely cold winters and hot summers account for the vast majority of the records. For instance, the winter of 1948-1949 was the coldest climatological winter (December-February) on record for virtually every site in Washington, Oregon, California, Nevada, Arizona, and Idaho. The winter of 1917-1918 remains the coldest for almost all sites in New England, New York, New Jersey, and points south to and including Washington D.C. The back-to-back winters of 1976-77, 77-78, and 78-79 accounted for almost all the record cold winters in the Ohio Valley, Midwest, and Central Plains. These five winters alone account for two-thirds (199 out of 300) of all the coldest winters for all the sites used in my summary. Conversely, the climatological summers (June-August) of 1934, 1936, 1988, 2010, and 2011 account for 130 out of 300 of the hottest summers for all sites. The other interesting result of the study is the apparent ‘clustering’ of some of the coldest winters and hottest summers. The summers of 1934 and 1936 and 2010 and 2011 stand out, as do the winters on 1976-1977 through 1978-1979. It is also interesting that in the anomalous years (weather-wise) of 1936 and 1949 many sites across the U.S. endured both coldest winters and hottest summers. In fact, in 1936, a few individual sites measured BOTH their coldest winter and their hottest summer that same year (Glasgow, Montana, Aberdeen, South Dakota, and Grand Forks, North Dakota).





Above are graphics indicating the number of sites (out of 300) recording their coldest winters and hottest summers arranged by year (top graph) and by decade (bottom graph). In the annual graph the winters and summers that had at least 10 sites with record seasons are highlighted. The data for 2015 includes the winter of 2014-2015 (none of the sites recorded their coldest winter) but, obviously, there is no data yet for the summer of 2015. In 2014 two sites recorded their hottest summer on record (Portland, Oregon and Yakima, Washington) and during the winter of 2013-2014 three sites their coldest winter (Eau Claire, Wisconsin, and Ironwood and Marquette in Michigan).

It is hard to draw any macro-climatic conclusions from these events since the contiguous U.S. only comprises 2% of the Earth’s surface, but the results are interesting nevertheless. The other trend that becomes obvious is that it has become more common to observe ‘hottest summers on record’ versus ‘coldest winters on record’ over the past 25 years or so. This trend is confirmed by the NOAA trend analysis graphics (represented below) that include the entire GHCN database.





Average temperatures for climatologcial summer (top) table) and climatological winter (bottom table) for the U.S. since 1895. This data is for the entire contiguous U.S. and based on all reporting sites in the GHCN database. It is interesting to note that the top three warmest summers on record, 1936, 2010, and 2011 also equate with years with the most sites with warmest summers in my database. This also holds true for the coldest winter of 1978-1979. Note that in the NOAA data the winter of 1948-1949 does not stand out the way it does in my graphic. That is because all the sites that endured their coldest winter on record were confined to the western quarter of the nation. The Midwest and East had a fairly normal winter that season (1948-1949). Tables from NOAA.

The Data

For those interested in the actual data for each of the 300 individual sites used in my project please find the complete data set here. In this link I also included data for Hawaii and Alaska, although these records are not used in my analysis for obvious reasons (Hawaii and Alaska are beyond the geographical parameters of this project) so just a FYI. I have also sorted the data by year for the coldest winters here and hottest summers here. In addition, for those interested, find here a chronological list of the POR’s for each site. The dates indicate the beginning of temperature records for the site indicated. However, in many cases there are months and even a few years missing since the beginning of the record for some sites. NOTE: The links provided above will load as Excel files.

Next Step

Hopefully, someday someone will produce a more professional survey of what I have done here and refine this initial attempt. Homogenizing the site POR’s would be an important first step.

KUDOS: Thanks to Mark Stroud of Moon Street Cartography for the maps and graphics reproduced above.

SOURCES USED:

NOW Data (NOAA Online Weather Data). This can be found on all the NWS weather web sites for each respective city/region.

U.S. Regional Climate Centers: Historical Climate Summaries U.S. Weather Bureau

Climatological Data by Sections U.S. Weather Bureau

Climatological Data (by states) NOAA

Climatology of the United States: Bulletin Q, U.S. Department of Agriculture, Weather Bureau, 1906

Monthly Weather Review, U.S. Department of Agriculture


Christopher C. Burt
Weather Historian