D.C.’s hottest days so far this year have arrived, but fortunately we’re not quite near record territory. In fact, the first half of 2013 has been relatively quiet as far as extreme temperatures go – which makes it a good time to explore how the percentage of heat and cold records has changed over time.
We’ve talked a lot about record-breaking temperatures in D.C. these last few years, and usually along the lines of above-average warmth. Statistically, we would expect a comparable number of cold temperature records over time. Following the national trend, however, record warmth has far outpaced cold records in D.C. in recent decades.
In fact, daily heat records have outnumbered cold records in the nation’s capital by a 7 to 1 ratio since the year 2000 and by nearly 16 to 1 in the past 3.5 years.
Nearby Dulles Airport has seen a similar pattern, with new heat records outpacing record cold by a factor of three over the last decade.
The top graph shows the number of daily warmth records exceeding daily cold records at both Reagan National (DCA) and at Dulles Airport (IAD) since the 1980s. If we look at D.C. alone, record highs (both maxima and minima) have outnumbered record lows (max. and min.) in each of the last five decades, as seen below:
Most of D.C.’s daily cold records from the early 20th century are still intact today. By contrast, nearly half of our record high daily temperatures have occurred since 1980 or later. The 1990s stand out in particular, with 94 new daily record high temperatures recorded during the decade (compared with just 16 new daily record lows).
What if we consider only the number of record high maximum and record low minimum temperatures?
By this metric, new all-time record highs have outnumbered all-time record lows by about 17 to 1 since the year 2000.
When National Airport just missed tying a record low daily temperature in May 2013, it came to our attention that only three record low minimum temperatures have been recorded in the nation’s capital in the past 13 years (two of which tied record lows from previous years). Compare this with 50 daily high maximum records set in the same period, and we see that new cold temperature records are few and far between.
It’s interesting to note how the 1930s – which featured several record high maximum temperatures that still stand today – also included record low minimum temperatures by a nearly 1 to 1 ratio. In fact, if we compare all record highs and lows set in D.C. (Chart 2), cold records actually outnumber heat records during the 1930s.
A warming trend is apparent at Dulles Airport as well, where daily records for warmth outnumber record lows by about 3 to 1 since the start of 2000. Dulles, of course, has a much shorter climate record than D.C. proper, but the location is a good comparison point being removed from the downtown urban heat island.
Nearly 70 percent of Dulles’ daily cold records were set before 1990, whereas over 60 percent of the airport’s record warm temperatures have occurred since then. In the past 3½ years, Dulles has set six times as many high temperature records as it has record lows.
While new record low minimum temperatures at Dulles have occurred with greater frequency than at National Airport in recent years, the number of these records is also dwindling, as seen below:
Record high maxima have outnumbered record low overnight temperatures at Dulles by more than 5 to 1 since 2000 and by 15 to 1 in the current decade. Moreover, all three record low minima recorded since 2010 tied records from previous years.
What can be said about all these numbers? The frequency of new warm temperature records in both D.C. and its suburban counterpart – Dulles Airport – highlight the continued rise in local temperatures over the past 20 to 30 years.
While there continues to be debate on whether Reagan National Airport is representative of temperatures in the metro area – as well as the extent to which the urban heat island enhances warm temperature readings – heat records outperform cold temperature extremes by such a wide margin that urban warmth is unlikely solely responsible for the lack of cold temperature records.
The top graph in this post offers a revealing snapshot of how much warmth has outperformed cold in just one metropolitan area of the U.S. While the rise in warm temperature records at Dulles appears less dramatic than at D.C., it also reflects a shorter observation period. This means there are still more chances for new cold records to occur at weather stations with a shorter climate record.
Despite its shorter climate record, Dulles still sees warmth records trump cold records by 95 percent since 1990, and by about 280 percent since the start of the 21st century.
At Reagan National, warmth records have been over 6 times more frequent than cold records since 1990 and closer to 7 times more frequent since the year 2000. Urban heat island effects may enhance the proliferation of heat records at National Airport in recent years, but can likely only account for so much of the upward tick in new records for daily, monthly, and annual warmth.
When record warmth outpaces record cold by such a wide margin, it suggests that the dice are loaded for new warm temperature records to outnumber cold ones in the coming decades, especially as heat-trapping greenhouse gases build in the atmosphere and urban areas sprawl.
*Daily temperatures in D.C. were recorded at 24th and M Street NW prior to July 1945, when Reagan National Airport (DCA) became the city’s current weather station.