This report describes the typical weather at the Logan International Airport (Boston, Massachusetts, 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.
Boston, Massachusetts has a humid continental climate with hot summers and no dry season. The area within 25 mi of this station is covered by forests (51%), oceans and seas (26%), and built-up areas (21%).
The month of September is characterized by rapidly falling daily high temperatures, with daily highs decreasing from 76°F to 67°F over the course of the month, exceeding 85°F or dropping below 58°F only one day in ten.
Over the course of September, the length of the day is decreasing. From the start to the end of the month, the length of the day decreases by 1:22 hours, implying an average day-over-day decrease of 2.7 minutes.
The shortest day of the month is September 30 with 11:46 hours of daylight; the longest day is September 1 with 13:08 hours of daylight.
The earliest sunrise is at 6:10am on September 1; the latest sunset is at 7:18pm on September 1; the latest sunrise is at 6:41am on September 30; the earliest sunset is at 6:27pm 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 59% (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 39% of the time, partly cloudy 20% of the time, and mostly cloudy or overcast 42% of the time.
The average probability that some form of precipitation will be observed in a given day is 38%, with little variation over the course of the month.
Throughout September, the most common forms of precipitation are light rain, moderate rain, heavy rain, and thunderstorms.
Light rain is the most severe precipitation observed during 43% of those days with precipitation. It is most likely around September 26, when it is observed during 17% of all days.
Moderate rain is the most severe precipitation observed during 25% of those days with precipitation. It is most likely around September 21, when it is observed during 10% of all days.
Heavy rain is the most severe precipitation observed during 15% of those days with precipitation. It is most likely around September 28, when it is observed during 6% of all days.
Thunderstorms are the most severe precipitation observed during 11% of those days with precipitation. They are most likely around September 1, when it is observed during 7% 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 52% (mildly humid) to 89% (very humid) over the course of a typical September, rarely dropping below 33% (comfortable) and reaching as high as 100% (very humid).
The air is driest around September 1, at which time the relative humidity drops below 63% (mildly humid) three days out of four; it is most humid around September 23, rising above 83% (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 44°F (dry) to 63°F (mildy humid) and is rarely below 31°F (dry) or above 71°F (muggy).
The time of the month between September 1 and September 30 is the most comfortable, with dew points that are neither too dry nor too muggy.
Over the course of September typical wind speeds vary from 5 mph to 17 mph (light breeze to moderate breeze), rarely exceeding 24 mph (fresh breeze).
The highest average wind speed of 11 mph (gentle breeze) occurs around September 30, at which time the average daily maximum wind speed is 17 mph (moderate breeze).
The lowest average wind speed of 11 mph (gentle breeze) occurs around September 5, at which time the average daily maximum wind speed is 16 mph (moderate breeze).