July 2012: hottest month in U.S. history

During the 1930s, a series of epic heat waves gripped the U.S., drying up the soil in an unprecedented drought that brought about the great Dust Bowl. The most intense heat hit during July 1936, which set a record for hottest month in U.S. history that stood for 76 years. That iconic record has now fallen, bested by 0.2°F during July 2012, which is now the hottest month in U.S. history, said NOAA's National Climatic Data Center (NCDC) this week. So far in 2012, we've had the warmest March on record, 3rd warmest April, 2nd warmest May, and warmest July. These remarkably warm months have helped push temperatures in the contiguous U.S. to the warmest on record for the year-to-date period of January - July, and for the 12-month period August 2011 - July 2012. Twenty-four states were record warm for that 12-month period, and an additional twenty states were top-ten warm. The past fourteen months have featured America's 2nd warmest summer (in 2011), 4th warmest winter, and warmest spring. The summer of 2012 is on pace to be a top-five warmest summer on record, and could beat the summer of 1936 as the warmest summer in U.S. history.


Figure 1. When the temperature peaked at an all-time high of 108° in Minneapolis, Minnesota on July 14, 1936, the want-ad staff at the 'St. Paul Daily News' was provided with 400 pounds of ice and two electric fans to cool the air in the press room. Photo from the Minnesota Historical Society.


Figure 2. Year-to-date temperature, by month, for 2012 (red), compared to the other 117 years on record for the contiguous U.S., with the five ultimately warmest years (orange) and five ultimately coolest years (blue) noted. The 2012 data are still preliminary. The year-to-date period of January - July was the warmest on record by a huge margin--1.0°F. Image credit: NOAA/NCDC.


Figure 3. For the fourth consecutive month, a new U.S. record for hottest 12-month period was set in July 2012. Five of the top-ten warmest 12-month periods in the contiguous U.S. since 1895 have occurred since April 2011. Image credit: NOAA/NCDC.

Most extreme January - July period on record
NOAA's U.S. Climate Extremes Index (CEI), which tracks the percentage area of the contiguous U.S. experiencing top-10% and bottom-10% extremes in temperature, precipitation, and drought, was 46% during the year-to-date January - July period. This is the highest value since CEI record-keeping began in 1910, and more than twice the average value. Remarkably, 83% of the contiguous U.S. had maximum temperatures that were in the warmest 10% historically during the first seven months of 2012, and 74% of the U.S. of the U.S. had warm minimum temperatures in the top 10%. The percentage area of the U.S. experiencing top-10% drought conditions was 20%, which was the 16th greatest since 1910. Extremes in 1-day heavy precipitation events were the 24th greatest in the 103-year record.


Figure 4. NOAA's U.S. Climate Extremes Index (CEI) for January - July shows that 2012 has had the most extreme first seven months of the year on record, with 46% of the contiguous U.S. experiencing top-10% extreme weather.

Little change to the U.S. drought during the past week
The great U.S. drought of 2012 remained about the same size and intensity over the past week, said NOAA in their weekly U.S. Drought Monitor report issued Thursday, August 9. The area of the contiguous U.S. covered by drought dropped slightly, from 63% to 62%, and the area covered by severe or greater drought stayed constant at 46%. The area of the country in the worst drought categories (extreme to exceptional drought) doubled from 10 percent last month to 22 percent this month. The extreme dryness and excessive heat devastated crops and livestock from the Great Plains to Midwest. During July, the area of the U.S. covered by moderate or greater drought was 57%, ranking as the 5th largest drought in U.S. history:

1) Jul 1934, 80%
2) Dec 1939, 60%
3) Jul 1954, 60%
4) Dec 1956, 58%
5) Jul 2012, 57%



Video 1. This is Not Cool: Peter Sinclair's July 2012 video from the Yale Forum on Climate Change and the Media couples historical footage with contemporary clips and news segments. In one of the latter, for instance, NBC anchor Brian Williams opens the network’s flagship news program with the words: “It’s now official. We are living in one of the worst droughts of the past 100 years.” NASA scientist James Hansen is featured testifying about risks of “extreme droughts” in the nation’s breadbasket, and I'm featured in a few clips talking about the drought of 2012.

I'll have a new post by noon Saturday.

Wunderground's weather historian Christopher C. Burt has a post looking back at the great Heat Wave of July 1936.

Jeff Masters