Will I be cold?
Will I be cold in Boston in September?
Verdict
No — No, comfortable.
Lows stay near 59°F — you won't be reaching for the puffer.
Day vs. night
Day
74°F
Warm
Breathable fabrics; sunscreen matters more than a jacket.
Night
59°F
Comfortable
Comfortable outside without an extra layer.
Why
Boston in September averages highs of 74°F, lows of 59°F, 8 rainy days.
You can pack for the daytime temps and not worry about the evening dip.