New England in mid-April doesn't commit to a season, and the Boston Marathon's long history is basically a record of that unpredictability. The race has been run in extreme heat, in driving cold rain, and in nearly every condition between — sometimes within a few years of each other. This guide covers the patterns and the most well-documented extremes; the race's own climate data on this page has the specific seasonal averages worth checking as race day approaches.

Why Boston weather is genuinely unpredictable

Patriots' Day sits at a genuinely transitional point in the New England calendar, and the course's point-to-point layout adds a wrinkle most marathons don't have to think about: because the route runs largely west to east, wind direction has an outsized effect on how the day plays out. A tailwind can meaningfully help times across the whole field; a headwind off the same geography can turn a manageable day into a grind, especially through the exposed stretch around the Newton hills.

The hot years

Boston's most notorious hot-weather race came in 2012, when temperatures climbed well into the 80s Fahrenheit by early afternoon. The BAA took the unusual step of offering runners a deferral to the following year rather than starting in that heat, and thousands of participants who did run needed medical treatment for heat-related illness along the course. It wasn't the first time Boston had run hot — earlier hot-weather editions go back decades — but 2012 is the year most commonly cited as the modern benchmark for how difficult a warm Boston can get.

The cold and wet years

On the opposite end, 2018 stands out as one of the most physically brutal weather days in the race's modern history: temperatures sat in the 30s and 40s Fahrenheit for most of the day, with driving rain and sustained headwinds strong enough to affect even elite competitors. Conditions were difficult enough that hypothermia became a genuine medical concern rather than a background risk, and that year's race is still referenced as a benchmark for how tough a cold, wet Boston can be — including in the context of one of the race's most memorable American victories, which came in exactly those conditions.

What the variability means for your prep

• Don't lock in a single race-day outfit months in advance. Boston's range of historical conditions is wide enough that flexibility matters more than a fixed plan.
• Pay attention to wind direction as forecasts firm up, not just temperature — a strong headwind changes the day more than a few degrees either way.
• If a hot forecast develops, adjust your goal pace rather than your effort. Boston's history shows that heat slows the whole field, not just the underprepared.
• Pack for both extremes until race week actually narrows the forecast, especially for the early-morning wait at the start in Hopkinton, which tends to run colder than the finish.

Checking the numbers

This page's climate data reflects the race's typical seasonal averages, which is the number worth planning around for most editions. The extreme years covered here are exactly that — extremes, not the typical day — but Boston's history shows they're not rare enough to ignore, either. The practical takeaway is the same one experienced Boston runners repeat every year: trust the week-of forecast over anything decided months earlier, and build a plan flexible enough for more than one kind of weather.