A look at weather extremes and the big-picture climate connections.

Heat Wave Continues, With Record Warm Overnight Lows

The heat wave that has been roasting much of the central and eastern United States during July has featured an unusually potent combination of high heat and humidity. One of the hallmarks of this heat event has been extremely warm overnight low temperatures, with hundreds of longstanding records either being broken or tied, particularly in the Midwest.

In general, warmer low temperatures are consistent with climate change caused in part by human emissions of greenhouse gases, such as carbon dioxide (CO2). In recent decades, while global average surface temperatures have increased, overnight low temperatures have increased faster than daytime highs

On Friday, the hottest temperatures will be focused on the major cities of the Mid-Atlantic and Northeast, from Washington to Boston. Washington, D.C. recorded its second-warmest overnight low temperature on record on Thursday night, when the temperature failed to fall below 83°F at Reagan National Airport, one degree shy of the city's all-time record. Heat indices on Thursday evening hovered near 100°F in the Washington and New York City metro areas through midnight, and are expected to do so again on Friday night. According to Ian Livingston of the Washington Post's Capital Weather Gang blog, there have never been more than two consecutive days with low temperatures of 80°F or above in the nation's capital since instrument records began. 

Kim Knowlton, a senior scientist with the Natural Resources Defense Council's health and environment program, says that the persistance of extreme heat and humidity into the overnight hours makes this heat wave especially dangerous from a public health perspective. "Hot, stagnant nights can prove even more harmful than daytime highs, because people are unable to cool down and get relief from the stress of daytime heat when it persists into the evening. The body benefits from nighttime cooling relief, at least a few hours daily to maintain health. So those record-breaking hot nights can be especially health-harming, especially for people most vulnerable to heat stress: young children, older Americans, economically disadvantaged families, [and] people without home without air conditioning."

To allow you to explore the recent record warm low temperatures, as well as trends going back at least a decade, Climate Central data journalist David Kroodsma created this interactive using data pulled from the National Climatic Data Center. You can click on each record location to see more details about that record. For more information, see the NCDC's surface temperature records lookup page (note that this can be a bit slow to load).

Record high minimum temperatures set through July 27, 2011. Use the drop down menus to see historical records for other months and years, dating back to 1999. Data credit: NCDC.

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