Temperatures in Washington, D.C., can range from a chilly 30-44 F/-1 to 7 C in late January and February to a hot, humid 80s-90s F/27-33 C in summer. Springtime brings excellent touring weather, cherry blossoms and lots of tourists. In April and May, temperatures reach daytime highs of 67-76 F/19-24 C, with comfortable humidity.
Summer means, hot weather and even more tourists. (Congress only recesses for August—not the entire summer; that's the only month when you might notice fewer residents.) Because D.C. was built on a swamp, the humidity can be very high and uncomfortable in the summer. Thunderstorms can cause sudden heavy rain in the summer months—but they usually offer a brief respite from the heat.
Fall is the ideal time to see the nation's capital—temperatures are slightly warmer than in spring, and there aren't quite as many tourists. Winters are generally mild (and not a bad time to visit), but be prepared for snow and ice storms. Many tours also do not operate in winter months, so you may want to do more of your own sight planning in advance. Also, days can start and end fairly cold and get pretty warm by midday, so take along a warm coat or layer clothing. Those looking for a bargain will find cheaper hotel rates in late fall and winter.