This report describes the typical weather at the Spirit of St. Louis Airport (St. Louis, Missouri, United States) weather station over the course of an average year. It is based on the historical records from 1987 to 2012. Earlier records are either unavailable or unreliable.
St. Louis, Missouri has a humid continental climate with hot summers and no dry season. The area within 25 mi of this station is covered by forests (46%), croplands (19%), grasslands (18%), built-up areas (14%), and lakes and rivers (4%).
Over the course of a year, the temperature typically varies from 23°F to 89°F and is rarely below 8°F or above 97°F.
The warm season lasts from May 26 to September 17 with an average daily high temperature above 79°F. The hottest day of the year is July 25, with an average high of 89°F and low of 69°F.
The cold season lasts from November 29 to February 27 with an average daily high temperature below 50°F. The coldest day of the year is January 18, with an average low of 23°F and high of 40°F.
The length of the day varies significantly over the course of the year. The shortest day is December 21 with 9:28 hours of daylight; the longest day is June 20 with 14:52 hours of daylight.
The earliest sunrise is at 5:37am on June 13 and the latest sunset is at 8:32pm on June 28. The latest sunrise is at 7:33am on November 3 and the earliest sunset is at 4:41pm on December 9.
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 28% (mostly clear) to 80% (mostly cloudy). The sky is cloudiest on January 17 and clearest on September 14. The clearer part of the year begins around May 20. The cloudier part of the year begins around November 11.
On September 14, the clearest day of the year, the sky is clear, mostly clear, or partly cloudy 66% of the time, and overcast or mostly cloudy 31% of the time.
On January 17, the cloudiest day of the year, the sky is overcast, mostly cloudy, or partly cloudy 61% of the time, and clear or mostly clear 38% of the time.
The probability that precipitation will be observed at this location varies throughout the year. Precipitation is most likely around May 13, occurring in 50% of days. Precipitation is least likely around September 13, occurring in 34% of days.
Over the entire year, the most common forms of precipitation are thunderstorms, light rain, moderate rain, and light snow.
Thunderstorms are the most severe precipitation observed during 34% of those days with precipitation. They are most likely around June 11, when it is observed during 29% of all days.
Light rain is the most severe precipitation observed during 26% of those days with precipitation. It is most likely around April 4, when it is observed during 16% of all days.
Moderate rain is the most severe precipitation observed during 15% of those days with precipitation. It is most likely around April 16, when it is observed during 10% of all days.
Light snow is the most severe precipitation observed during 13% of those days with precipitation. It is most likely around January 16, when it is observed during 18% of all days.
During the warm season, which lasts from May 26 to September 17, there is a 40% 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 (62% of days with precipitation have at worst thunderstorms), light rain (22%), and moderate rain (11%).
During the cold season, which lasts from November 29 to February 27, there is a 45% 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 snow (34% of days with precipitation have at worst light snow), light rain (22%), moderate rain (16%), and moderate snow (9%).
The likelihood of snow falling is highest around January 16, occurring in 26% of days. The season in which it is relatively likely for snow to fall spans from November 16 to March 24.
Either snow rarely accumulates at this location or snow depth measurements are unavailable or unreliable.
The relative humidity typically ranges from 40% (comfortable) to 95% (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 April 9, at which time the relative humidity drops below 49% (comfortable) three days out of four; it is most humid around August 22, exceeding 93% (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 78°F (oppressive).
There are two periods in the year that are most comfortable: The first is between April 18 and June 4 and the second is between September 9 and October 19. The air feels neither too dry nor too muggy during these periods.
Over the course of the year typical wind speeds vary from 0 mph to 16 mph (calm to moderate breeze), rarely exceeding 24 mph (fresh breeze).
The highest average wind speed of 9 mph (gentle breeze) occurs around April 2, at which time the average daily maximum wind speed is 16 mph (moderate breeze).
The lowest average wind speed of 5 mph (light breeze) occurs around September 2, at which time the average daily maximum wind speed is 11 mph (gentle breeze).