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 April. 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%).
The month of April is characterized by rising daily high temperatures, with daily highs increasing from 63°F to 72°F over the course of the month, exceeding 83°F or dropping below 48°F only one day in ten.
Over the course of April, the length of the day is increasing. From the start to the end of the month, the length of the day increases by 1:08 hours, implying an average day-over-day increase of 2.3 minutes.
The shortest day of the month is April 1 with 12:40 hours of daylight; the longest day is April 30 with 13:48 hours of daylight.
The earliest sunrise is at 6:06am on April 30; the latest sunset is at 7:55pm on April 30; the latest sunrise is at 6:47am on April 1; the earliest sunset is at 7:27pm on April 1.
Daylight savings time is observed in 2012 but it neither starts nor ends in April, so the entire month is in daylight time.
The median cloud cover ranges from 55% (partly cloudy) to 61% (partly cloudy). The trend is for the sky to be clearing with the median cloud cover decreasing by 6% from the start to the end of the month.
On April 15, a representative day, the sky is clear or mostly clear 43% of the time, partly cloudy 10% of the time, and mostly cloudy or overcast 47% of the time.
The average probability that some form of precipitation will be observed in a given day is 49%, with little variation over the course of the month.
Throughout April, the most common forms of precipitation are thunderstorms, light rain, and moderate rain.
Thunderstorms are the most severe precipitation observed during 37% of those days with precipitation. They are most likely around April 30, when it is observed during 21% of all days.
Light rain is the most severe precipitation observed during 33% 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 19% of those days with precipitation. It is most likely around April 16, when it is observed during 10% of all days.
The likelihood of snow falling is highest around April 1, occurring in 6% of days.
Either snow rarely accumulates at this location in April or snow depth measurements are unavailable or unreliable.
The relative humidity typically ranges from 40% (comfortable) to 91% (very humid) over the course of a typical April, 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 April 30, rising above 88% (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 typical April, the dew point typically varies from 32°F (dry) to 55°F (very comfortable) and is rarely below 19°F (dry) or above 65°F (mildy humid).
Over the course of April 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 8 mph (gentle breeze) occurs around April 30, at which time the average daily maximum wind speed is 15 mph (moderate breeze).