D.C. may be home to world-class weather complainers, but it’s actually a pretty good weather town. The region averages a healthy number of days that most people would consider just about perfect — and they can occur nearly any time of year. But two stretches stand out above the rest.
What qualifies as a “nice day”? For us to place a Nice Day Stamp in the forecast, several conditions typically need to be met:
- At least 50 percent sunshine
- Less than a 20 percent chance of rain
- High temperatures between 65 and 85 degrees
- Dew points no higher than 65 degrees
- Wind gusts generally below 25 mph
To simplify the analysis, I examined Washington’s weather history and identified days that finished with high temperatures between 65 and 85 degrees and no rainfall.
Average nice days by month
In a place with four distinct seasons, it’s not surprising that the transition seasons produce the most days that qualify as especially pleasant.
In Washington, the spring “nice day” season runs roughly from April through June, each averaging more than seven such days per month over the most recent 30 years examined. The spring peak comes in May, with an average of 10.7 nice days.
Fall’s nice day season arrives quickly after the long, hot summer. Early and mid-autumn in the region are often sunny, dry and relatively stable, allowing the nice days to pile up. September averages 13.1 nice days, while October averages 13.7, making them the most pleasant months of the year by this measure.
Percent of time a day in history has been nice
The chart of monthly averages reflects today’s climate, while the chart showing probabilities by date draws on the entire historical record, dating back to 1872.
With only a few exceptions, most days from December through early February have just a 1 to 2 percent chance of qualifying as nice in the long-term data.
By late February, the odds begin to climb — slowly at first, then sharply through the spring. The spring peak occurs around mid-May, when the chance of a nice day rises to about 40 to 50 percent.
Nice days become less common during the heat and humidity of midsummer before rebounding heading into fall. The single most favorable date is Sept. 24, which has been nice about 65 percent of the time in Washington’s roughly 150 years of weather records.
Nice days by year
D.C. is something of a nice-day haven compared with many places. Since around 1900, the city has averaged roughly 90 nice days per year, and that number has remained fairly steady over time.
Earlier, colder decades in the late 1800s tended to produce somewhat fewer nice days. In recent years there are hints the total may be slipping again, perhaps as a warming climate squeezes these ideal conditions on the edges of spring and fall.
It’s also worth noting that the fewest nice days in a year with reliable data occurred in 2018, when just 69 qualified. That same year was also D.C.’s wettest on record.
