Washington is in the humid subtropical climate zone . The Trewartha classification is defined as an oceanic climate (Do). Winters are usually cool with light snow, and summers are hot and humid. The district is in plant hardiness zone 8a near downtown, and zone 7b elsewhere in the city, indicating a humid subtropical climate.
Spring and fall are mild to warm, while winter is cool with annual snowfall averaging 15.5 inches (39 cm). Winter temperatures average around 38 °F (3 °C) from mid-December to mid-February. However, winter temperatures in excess of 60 °F (16 °C) are not uncommon.
Summers are hot and humid with a July daily average of 79.8 °F (26.6 °C) and average daily relative humidity around 66%, which can cause moderate personal discomfort. Heat indices regularly approach 100 °F (38 °C) at the height of summer. The combination of heat and humidity in the summer brings very frequent thunderstorms, some of which occasionally produce tornadoes in the area.
Blizzards affect Washington, on average, once every four to six years. The most violent storms are called “nor’easters”, which often affect large sections of the East Coast. From January 27 to 28, 1922, the city officially received 28 inches (71 cm) of snowfall, the largest snowstorm since official measurements began in 1885. According to notes kept at the time, the city received between 30 and 36 inches (76 and 91 cm) from a snowstorm in January 1772.
Hurricanes (or their remnants) occasionally track through the area in late summer and early fall. However, they are often weak by the time they reach Washington, partly due to the city’s inland location. Flooding of the Potomac River, however, caused by a combination of high tide, storm surge, and runoff, has been known to cause extensive property damage in the neighborhood of Georgetown.
Precipitation occurs throughout the year.
Washington’s climate will grow warmer and rainfall will increase as the result of climate change.
The highest recorded temperature was 106 °F (41 °C) on August 6, 1918, and on July 20, 1930. while the lowest recorded temperature was −15 °F (−26 °C) on February 11, 1899, right before the Great Blizzard of 1899. During a typical year, the city averages about 37 days at or above 90 °F (32 °C) and 64 nights at or below the freezing mark (32 °F or 0 °C). On average, the first day with a minimum at or below freezing is November 18 and the last day is March 27.