In early June, a study released from Stanford University researchers suggested that climate change would result in "the permanent emergence of unprecedented summer heat" within the next 20 to 60 years. Climate models indicated that by the middle of this Century, even the coolest summers would be hotter than all prior summers for many locations in the Northern Hemisphere. Although this may seem counterintuitive, given that winter temperatures are subject to much larger swings in general, it actually makes senses when one considers that summertime weather patterns rarely vary significantly. Oftentimes, it's factors like drought and extremely dry soil moisture that contribute to the development of excessive heat. Dry soils more readily absorb heat from the sun, since less of the solar radiation energy is committed to evapotransporative processes.
We've heard a lot in recent winters about how cold and snowy it has been. In fact, recent winters have not been particularly cold in most locations -- although, they have had a number of extreme winter cyclones that have dumped significant accumulations of snow. The cold, snowy weather is largely attributable to the North Atlantic Oscillation (NAO). In recent winters, the NAO has persistently been in a negative state, which is characterized by the presence of an upper level ridge over Greenland and adjacent areas of North Atlantic. This spreads warm air over much of the Arctic and sub-Arctic regions, but is associated with large troughs in the Jet Stream outside of the area of blocking high pressure. These troughs can funnel cold air down into the lower Latitudes of North America and Eurasia and lead to bitter cold. Most of the coldest winters on record in the mid-latitudes occured during -NAO winters. Contrary to what some of the "professional confusionists" may want you to believe, recent winters have not been unusually cold averaged across the globe. The higher latitudes have seen very mild winters lately, with Hudson Bay, for instance, seeing its latest ice-out on record last winter.
At the same time, these same "confusionists" have been notably silent during the summertime. Last summer (2010) was a scorcher for many areas of the Northern Hemisphere. Much of Eurasia experienced one of the hottest summers on record. In Moscow, Russia, the heat and drought were unparalleled in history. All-time record high temperatures were smashed, as day after day of searing heat and thick smoke impacted the city. At the same time, North America was also experiencing intense heat. Indeed, the summer was the fifth warmest on record nationally. 12 states and the District of Columbia experienced their hottest summer on record.
This summer has been even worse. The summer started early for those in the South as extreme heat and dryness contributed to exceptional drought centered over Texas. June saw record breaking temperatures across the South. Extreme heat also occurred further north in spurts, albeit tempered from what was experienced in the South. Overall, June was the 25th warmest on record for the nation. But for the Southern Plains, it was one of the hottest on record. In July, the heat ridge spread north and east. Record-breaking temperatures were experienced from Texas to Minnesota and extended all the way to the East Coast. Most locations in TX and OK had their hottest July on record. Statewide in Oklahoma, this was the hottest July by a full degree over the previous record of 1954. The heat was not relegated to the south, however. Fort Wayne, IN; Detroit, MI; Columbus, OH; Philadelphia, PA; Baltimore, MD; Washington, DC all recorded their hottest months on record. Many other sites also experienced near-record breaking warmth during the month, with heat in the Midwest rivaled only by the Dust Bowl years of 1934 and 1936 and in the Great Lakes region by the scorching summers of 1921 and 1955.
In many locations, the month of July was the hottest in many years. What makes some of the records especially impressive is that the records broken occurred during an era when temperature recording was not nearly as quality-controlled as it is today. Prior to around 1940, most temperatures were taken at downtown sites often on rooftop locations. Today most readings are automated and sited in fields at expansive airport sites. In Baltimore, for instance, records are currently taken at BWI airport. However, before 1950, records were taken on the rooftop of the Commons House downtown. The effects of UHI contamination on the old temperature records are readily apparent. As Watts et al. have shown, rooftops can also contribute to a significant temperature bias (as much as 10 degrees under certain conditions). Although July was the hottest month on record in terms of mean and maximum temperature in the Baltimore region, it was only the 24th warmest with respect to overnight lows. Many of the years with warmer minima were from the pre-1950 era when records were taken downtown. Despite the obvious problems with the old records, Baltimore still set a new record (although it didn't shatter the previous record like Reagan National and Dulles Airports).
