This report describes the typical weather at the Ronald Reagan Washington National Airport (Arlington County, District of Columbia, United States) weather station over the course of an average year. It is based on the historical records from 1974 to 2012. Earlier records are either unavailable or unreliable.
Arlington County, District of Columbia has a warm humid temperate climate with hot summers and no dry season. The area within 25 mi of this station is covered by forests (63%), built-up areas (24%), croplands (9%), and lakes and rivers (3%).
Over the course of a year, the temperature typically varies from 29°F to 88°F and is rarely below 17°F or above 96°F.
The warm season lasts from May 29 to September 16 with an average daily high temperature above 79°F. The hottest day of the year is July 14, with an average high of 88°F and low of 72°F.
The cold season lasts from December 2 to March 1 with an average daily high temperature below 51°F. The coldest day of the year is January 19, with an average low of 29°F and high of 42°F.
The length of the day varies significantly over the course of the year. The shortest day is December 21 with 9:25 hours of daylight; the longest day is June 20 with 14:55 hours of daylight.
The earliest sunrise is at 5:42am on June 11 and the latest sunset is at 8:38pm on June 30. The latest sunrise is at 7:38am on November 3 and the earliest sunset is at 4:46pm on December 8.
Daylight savings time (DST) is observed in this location during 2012, starting in the spring on March 11 and ending in the fall on November 4.
The median cloud cover ranges from 71% (partly cloudy) to 88% (mostly cloudy). The sky is cloudiest on January 7 and clearest on October 1. The clearer part of the year begins around August 3. The cloudier part of the year begins around November 16.
On October 1, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 49% of the time, and overcast or mostly cloudy 51% of the time.
On January 7, the cloudiest day of the year, the sky is overcast, mostly cloudy, or partly cloudy 75% of the time, and clear or mostly clear 25% of the time.
The probability that precipitation will be observed at this location varies throughout the year. Precipitation is most likely around April 28, occurring in 48% of days. Precipitation is least likely around September 18, occurring in 35% of days.
Over the entire year, the most common forms of precipitation are light rain, moderate rain, and thunderstorms.
Light rain is the most severe precipitation observed during 35% of those days with precipitation. It is most likely around April 26, when it is observed during 20% of all days.
Moderate rain is the most severe precipitation observed during 23% of those days with precipitation. It is most likely around April 4, when it is observed during 13% of all days.
Thunderstorms are the most severe precipitation observed during 20% of those days with precipitation. They are most likely around July 12, when it is observed during 22% of all days.
During the warm season, which lasts from May 29 to September 16, there is a 42% average chance that precipitation will be observed at some point during a given day. When precipitation does occur it is most often in the form of thunderstorms (42% of days with precipitation have at worst thunderstorms), light rain (31%), and moderate rain (21%).
During the cold season, which lasts from December 2 to March 1, there is a 43% average chance that precipitation will be observed at some point during a given day. When precipitation does occur it is most often in the form of light rain (32% of days with precipitation have at worst light rain), moderate rain (19%), light snow (19%), and heavy snow (12%).
The likelihood of snow falling is highest around January 17, occurring in 20% of days. The season in which it is relatively likely for snow to fall spans from November 28 to March 27.
Either snow rarely accumulates at this location or snow depth measurements are unavailable or unreliable.
The relative humidity typically ranges from 40% (comfortable) to 91% (very humid) over the course of the year, rarely dropping below 23% (dry) and reaching as high as 100% (very humid).
The air is driest around March 17, at which time the relative humidity drops below 47% (comfortable) three days out of four; it is most humid around September 23, exceeding 86% (very humid) three days out of four.
Dew point is often a better measure of how comfortable a person will find the weather than relative humidity because it more directly relates to whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid.
Over the course of a year, the dew point typically varies from 16°F (dry) to 72°F (very muggy) and is rarely below -0°F (dry) or above 77°F (oppressive).
There are two periods in the year that are most comfortable: The first is between April 22 and June 7 and the second is between September 14 and October 28. The air feels neither too dry nor too muggy during these periods.
Over the course of the year typical wind speeds vary from 2 mph to 18 mph (light air to fresh breeze), rarely exceeding 26 mph (strong breeze).
The highest average wind speed of 11 mph (gentle breeze) occurs around March 12, at which time the average daily maximum wind speed is 18 mph (fresh breeze).
The lowest average wind speed of 8 mph (gentle breeze) occurs around August 7, at which time the average daily maximum wind speed is 13 mph (moderate breeze).