This report describes the typical weather at the San Antonio International Airport (San Antonio, Texas, United States) weather station over the course of an average September. It is based on the historical records from 1974 to 2012. Earlier records are either unavailable or unreliable.
San Antonio, Texas has a warm humid temperate climate with hot summers and no dry season. The area within 25 mi of this station is covered by grasslands (56%), forests (20%), croplands (14%), and built-up areas (9%).
The month of September is characterized by falling daily high temperatures, with daily highs decreasing from 92°F to 86°F over the course of the month, exceeding 98°F or dropping below 78°F only one day in ten.
Over the course of September, the length of the day is gradually decreasing. From the start to the end of the month, the length of the day decreases by 51 minutes, implying an average day-over-day decrease of 1.7 minutes.
The shortest day of the month is September 30 with 11:54 hours of daylight; the longest day is September 1 with 12:44 hours of daylight.
The earliest sunrise is at 7:11am on September 1; the latest sunset is at 7:55pm on September 1; the latest sunrise is at 7:27am on September 30; the earliest sunset is at 7:20pm on September 30.
Daylight savings time is observed in 2012 but it neither starts nor ends in September, so the entire month is in daylight time.
The median cloud cover is 54% (partly cloudy) and does not vary substantially over the course of the month.
On September 15, a representative day, the sky is clear or mostly clear 40% of the time, partly cloudy 23% of the time, and mostly cloudy or overcast 37% of the time.
The average probability that some form of precipitation will be observed in a given day is 28%, with little variation over the course of the month.
Throughout September, the most common forms of precipitation are thunderstorms, light rain, and moderate rain.
Thunderstorms are the most severe precipitation observed during 43% of those days with precipitation. They are most likely around September 1, when it is observed during 13% of all days.
Light rain is the most severe precipitation observed during 30% of those days with precipitation. It is most likely around September 30, when it is observed during 10% of all days.
Moderate rain is the most severe precipitation observed during 19% of those days with precipitation. It is most likely around September 26, when it is observed during 6% of all days.
Either snow is exceptionally unlikely to fall at this location in September or this station does not reliably report precipitation types.
The relative humidity typically ranges from 41% (comfortable) to 90% (very humid) over the course of a typical September, rarely dropping below 25% (dry) and reaching as high as 99% (very humid).
The air is driest around September 25, at which time the relative humidity drops below 50% (comfortable) three days out of four; it is most humid around September 11, rising above 87% (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 September, the dew point typically varies from 56°F (comfortable) to 72°F (very muggy) and is rarely below 42°F (dry) or above 75°F (very muggy).
Over the course of September typical wind speeds vary from 1 mph to 14 mph (light air to moderate breeze), rarely exceeding 19 mph (fresh breeze).
The highest average wind speed of 8 mph (gentle breeze) occurs around September 28, at which time the average daily maximum wind speed is 14 mph (moderate breeze).
The lowest average wind speed of 8 mph (light breeze) occurs around September 4, at which time the average daily maximum wind speed is 14 mph (moderate breeze).