The start of the autumn season is defined by two distinct systems: the astronomical and the meteorological. This difference often causes confusion, especially in locations like Boston where the weather experience may not align with the traditional calendar date. Understanding these two definitions clarifies why the feeling of fall in New England often arrives earlier than the official date.
The Calendar Defined Start
The official, calendar-based start of fall is determined by the Autumnal Equinox, an astronomical event. This equinox marks the moment the sun crosses the celestial equator, resulting in nearly equal periods of daylight and darkness across the globe. The word “equinox” is derived from the Latin for “equal night.” In the Northern Hemisphere, this event typically occurs between September 21st and September 24th. The exact date shifts slightly each year because the Earth takes approximately 365.25 days to orbit the sun, necessitating a leap year.
The Weather Defined Start
Meteorologists use a different, more consistent method to define the seasons for record-keeping and comparison. This approach, known as meteorological fall, groups seasons into three full-month blocks based on the annual temperature cycle. This standardization allows for easier calculation of seasonal statistics and reliable year-to-year comparisons. Meteorological fall for the Northern Hemisphere always begins on September 1st and concludes on November 30th. This period aligns more closely with the typical shift in weather patterns, making it a practical definition for forecasting.
Local Climate Markers in Boston
While the calendar and meteorological dates provide a framework, the tangible experience of fall in Boston is defined by specific local climate markers. Boston’s average daily high temperature decreases significantly throughout the season, falling from around 73°F in early September to approximately 57°F by the start of October. This sustained drop in temperature is what most residents recognize as the true start of the season.
The city’s coastal location plays a unique role in delaying the onset of colder temperatures compared to inland areas. The Atlantic Ocean and Boston Harbor retain heat longer into the year, causing the temperature drop to be more gradual near the coast. This effect helps moderate the climate, especially in September, which remains generally warm with an average temperature of 66°F.
Foliage timing, a highly anticipated marker of fall, peaks later in the city than in New England’s inland regions and mountains. While color may begin to appear as early as mid-September, the most vibrant display in the Boston area typically occurs in the latter half of October. Peak foliage often hits the urban canopy, including parks like the Boston Common and Public Garden, around the third week of October.