Summers in D.C. can be miserable enough based on temperature alone. Highs commonly climb into the 90s, and sometimes even the triple digits. But what really makes the season notorious is the city’s trademark humidity.
When thick, moisture-laden air settles in, the heat index — which combines temperature and humidity — can make it feel 10 to 20 degrees hotter. Those conditions can quickly turn steamy afternoons into something far more uncomfortable and, at times, dangerous.
Maximum heat index by year
The average annual peak heat index in D.C. is around 110 degrees, and that figure has remained fairly steady over time. But the extremes can vary widely from year to year.
A recent high came in 2011, when the heat index reached 121 degrees. Just two years earlier, in 2009, the yearly peak was only 100 degrees. Going farther back, it felt like 124 degrees in 1946, one of the highest values on record.
These oppressive conditions usually occur in midsummer — especially July — when the season’s highest temperatures overlap with its most humid air. D.C.’s location along the Potomac River can amplify the moisture, particularly at Reagan National Airport, where dew points occasionally climb above 80 degrees. Dew points are a measure of humidity; values over 70 are indicative of rather sticky air while 80 degrees or more is suffocating.
How often D.C. feels like the 90s, 100s – and beyond
In a typical year, D.C. experiences about 10 full days — roughly 240 hours — when it feels at least 92 degrees. Conditions that feel like 95 degrees occur for about six days, or 145 hours.
Triple-digit heat indexes are less frequent but still show up most summers. On average, D.C. sees about 48 hours when it feels like 100 degrees.Conditions reaching a 110-degree heat index are rare, averaging only about one hour per year.
Values near 120 degrees are exceptionally unusual, occurring roughly once a decade.
Data from the National Weather Service, via Iowa Environmental Mesonet.
