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SunnyDays

ERA5 almanac — 8 years of real data

Boston in May

Boston in May: rain frequency data not yet backfilled for this month, 68°F average high, 17% of days historically clear (n=248, ERA5 2018–2025).

Boston in May is historically a poor month for sun: only 17% of days clear across 8 years (n=248, avg high 68°F). August is the clearest month for Boston historically. Historical average, not a forecast.

What you are reading: 8-year historical averages from the ERA5 reanalysis, not a forecast. A month that is historically 90% clear can still be overcast on your specific trip. For this week’s actual forecast, see the live Boston page. Both can be true at once: “historically strong in this month; this particular week is not.” We label which is which.

Early vs. Late May

Clear-sky percentage for Boston in May, split by half-month. “n” is the number of days sampled across 8 years (2018–2025).
Period % Clear days Sample (n) Avg high
Early May
1st–15th		 9% n=120 68°F
Late May
16th–end		 25% n=128 68°F
Full month average 17% n=248 68°F

Bottom line

Boston is a poor choice for sun in May: just 17% of days clear historically (n=248). August is the historically strongest month for sun here. The 8-year record is consistent — this is not a fluke bad stretch.

→ See all destinations in August — historically the best month for sun in Boston.

Every month in Boston

Compare across all months with data. Numbers are historical clear-sky %.

Historical clear-sky % for Boston, all months, ERA5 2018–2025.
Month % Clear Sample (n)
January 0% n=248 All destinations →
February 0% n=226 All destinations →
March 0% n=248 All destinations →
April 2% n=240 All destinations →
May ← this page 17% n=248 All destinations →
June 33% n=240 All destinations →
July 37% n=248 All destinations →
August 41% n=248 All destinations →
September 31% n=240 All destinations →
October 12% n=248 All destinations →
November 2% n=240 All destinations →
December 0% n=248 All destinations →

Frequently asked

Is Boston good for sun in May?

Not usually. Boston is historically overcast in May: only 17% of days are clear (avg high 68°F, n=248 days, ERA5 2018–2025). August is historically far better for sun here.

All percentages are historical averages from the ERA5 reanalysis via Open-Meteo, 8 full years (2018–2025). “Clear” means avg high ≥ 72 °F, precipitation < 1 mm, and a non-precipitation weather code. This is history, not a forecast. The live 7-day outlook for any destination is on its own page and may differ from the long-run average. Data from Open-Meteo.