Records for daily high temperatures are now outpacing record lows by an 8 to 1 ratio since the year 2000, and more than 16 to 1 since the year 2010. These ratios are far higher than any decade before the 21st century — including the warm 1930s.
Statistically speaking, we would expect a comparable number of warm and cold temperature records over time. Instead, rising temperatures caused by greenhouse gas emissions and urbanization are greatly tipping the scales toward record warmth — a pattern we’ve seen not just locally, but nationally as well.
Since 2010, Reagan National Airport — D.C.’s official weather station — has logged 115 records for either warmest daily (afternoon) high or (overnight) low temperature. That’s 16.4 times the number of daily cold records (defined as either the coldest maximum or minimum temperature).
If we consider only the record high maximum and record low minimum temperatures, the ratio more than doubles (35 to 1), as shown in the chart below. Notably, in February 2015, D.C. finally set a daily record low temperature for the first time this decade. Apart from this, most of D.C.’s daily cold records from the early 20th century remain intact.
To be sure, increased urbanization in downtown Washington has partly contributed to the proliferation of heat records in recent years — especially in overnight low temperatures. (It’s also worth mentioning that D.C. has seen fewer record low minimum temperatures since the city’s weather station moved from downtown to Reagan National Airport in the 1940s.)
That said, it is striking that the number of new daily heat records in D.C. has almost doubled in the three years since I last wrote about this topic. In mid-2013, D.C. had set 62 warm records (including record high maxima and minima) since 2010. That number has since jumped to 115. If we disregard the warm overnight lows, the number of daily record highs set since 2010 has also nearly doubled — from 21 in 2013 to 35 today.
Even at Dulles International Airport, a station which may have a cold bias, the number of warm temperature records has risen from 109 to 173 – a 50 percent jump since 2013. Similar to Reagan National Airport, Dulles has seen record warm overnight temperatures jump dramatically. This is consistent with scientific studies that show a warmer atmosphere locks in moisture, which keeps temperatures from falling as much at night.
The chart above shows Dulles has now set more daily warmth records in the 2010s than in any of the previous five decades since records began in 1962. This has happened even as the location also logged several record lows in the same period. While the ratio of daily record highs to record lows isn’t as impressive as we see at Reagan National, it’s still far from balanced. Warmth records at Dulles have outnumbered cold records by 2.8 to 1 in the current decade.
If we look at only daily record high maximum and record low minimum temperatures, the ratio is only slightly lower, nearly 2.6 to 1. (Note that Dulles has broken or tied record highs on four straight days this week alone.)
The bottom line is that while cold records still happen, they are few and far between compared to the high temperature records we’ve seen in recent years. As the region’s climate continues to warm, we can expect this imbalance to continue and likely expand.