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Boston, MA UV Index Today

Modelled for · GMT-4

What is the UV index in Boston right now?

0.2 Low

EPA describes low UV as needing no protection for most people.

Today's peak is 5.5 (High) at about 12pm local time.

When is UV strongest in Boston today?

UV in Boston peaks at 5.5 (High) around 12pm local time (GMT-4). It does not reach the high band at any hour today. UV is 3 or above for 6 hours.

Local time, midnight to midnight. Bar colour is the EPA UV band, never restyled.

Boston today, in numbers

Latitude
42.4°N — 29 of 121 from the north
Today's peak
5.5 (High) — 98 of 121 nationally
Peak arrives
12pm local (GMT-4)
At or above 6
not today
At or above 3
6 hours, 10am to 4pm

Boston never reaches the high band today; the midday hours still warrant a hat. Nationally today the spread runs 11.4 in Hilo (20°N) to 1.3 in Fargo (47°N). Why latitude and the clock decide this →

A cold Gulf of Maine can cancel a June afternoon

The wind direction that decides a day here arrives from behind. From late spring into early summer a backdoor cold front can slide southwest out of the Canadian Maritimes and swing the wind around to the east-northeast, drawing air in off a Gulf of Maine that is still cold in May. What follows at the coast is low cloud, drizzle and sea fog, sometimes holding from morning to dark, at a point in the year when the sun is already near its annual high.

That produces a split most of the country never sees. Boston can spend a late-May afternoon under a gray marine deck while central Massachusetts, forty miles inland and out of reach of that flow, sits clear and hot. The two are under the same sun on the same day and are not receiving the same amount of it.

The pattern is confined to part of the year rather than running through it. Backdoor fronts are mainly a spring and early-summer feature; by midsummer the Gulf of Maine has warmed and the wind more often comes in from the southwest, over land, with nothing cold offshore to condense against. So the months when an east wind can hold the coast under cloud for a whole day are also the last months before the water stops being able to do it.

Nearest metros we model

UV changes with latitude, not with a city boundary, so a neighbour a hundred miles north reads differently for a reason.

New York, NY 190 mi · 40.7°N peak 5.9 Philadelphia, PA 271 mi · 40.0°N peak 6 Baltimore, MD 359 mi · 39.3°N peak 6.2

What does Boston’s UV index mean today?

0.2 is Low. EPA describes low UV as needing no protection for most people. Today Boston also reaches High at its peak — a different band from the one it is in right now, which is why the hour matters as much as the day.

The scale is open-ended: 11 is where the named bands stop, not the maximum. All five bands, with the EPA guidance for each →

Modelled, not measured

The UV figures for Boston are computed by the Copernicus atmosphere model, not read off an instrument here — unlike the AQI on our air-quality pages, which comes from a regulatory monitor. Cloud is where a UV model is weakest. What that means in practice →

Questions about UV in Boston

What is the UV index in Boston today?

0.2 right now (Low), peaking at 5.5 around 12pm local time. It does not reach the high band today. This is a modelled value, not a ground measurement.

When is UV strongest in Boston today?

Around 12pm local time (GMT-4), not at noon — solar noon drifts across a timezone and daylight saving moves it later on the clock.

Is this measured or modelled?

Modelled. The figures come from the Copernicus atmosphere model via Open-Meteo, unlike the AQI on our air-quality pages, which is read from an EPA regulatory monitor. Cloud cover is where a UV model is weakest.

Boston air qualityThe AQI, from a regulatory monitor rather than a model UV across the USWhere Boston sits against every metro we track

Updated