Updated: Jan 29, 2020
D.C.'s climate has a bad reputation; there is a popular myth that the city was built on a swampwith the purpose of discouraging a large bureaucracy—after all, if no one wanted to live in D.C., then there wouldn't be too many bureaucrats. This is all untrue. Although what is now the National Mall was originally mudflats, there was no swamp. Even in the early 1800's, most of the city was comprised of tobacco and corn fields and apple orchards.
The weather is actually quite pleasant during the spring and fall. It's hard to beat spring in D.C. The northerly subtropical climate results in cool breezes, moderate temperatures, lush growth, flowers, budding trees, and, of course, the cherry blossoms. The most beautiful time of spring usually falls from April to mid-May. Domestic tourists know this, though, and you can expect the cherry blossom walk around the Tidal Basin to see (pedestrian) traffic jams that put the Beltway to sham, although truly savvy tourists can escape the crowds but still enjoy the cherry blossoms at the National Arboretum in Near Northeast. Fall rivals spring for perfect temperatures. It's also a lovely time for a walk in Rock Creek Park, where the dense forest bursts with multicolored confetti. Winter is a great time to visit due to the practically empty museums and the absence of the summer theatre vacations. Winter temperatures are relatively mild, with very sporadic snow. However, it's very hot and very humid during the summer, due to the miserable, impenetrable humidity. On a hot day in D.C. in July, you will sweat like a dog, the kids will complain incessantly, and you'll want to spend as much time indoors as possible. It is not the best time to visit.
It's worth considering the political climate as well. Before heading to D.C., research which events will coincide with your visit. Major international conferences, political events (elections/inaugurations), or protests can lead to road closures and additional security checks and also send lodging prices through the roof. There are also several weeks during the year, as well as most of August, when Congress is on recess. During these weeks, there are fewer official visitors, elected officials, and staff members; the Metro becomes less crowded and there are overall fewer people in the city.