The short answer
Marathon weather history for 2015 to 2024 shows the Comrades Marathon in South Africa as the hottest of four big races, reaching High heat stress on three of its eight race days. Boston swings most, from cold rain to warm sun. London is usually cool, and Berlin's late-September mornings stay mild.
Key takeaways
- Comrades reached High heat stress, a WBGT of 22.8 °C (73 °F) or more, on three of eight race days.
- Boston had two Tough cold-rain race days and four warm Fair ones in nine races.
- London rated Good on eight of nine race days; 2018 was the one warm exception.
- Berlin stayed at Low heat stress on six of nine race days for a four-hour runner.
- Slower finishers run deeper into the warm hours, so check the weather at your own finish time.
On this page
- What does marathon weather history show for these four races?
- Which of the four races is usually hottest?
- How did each race day compare, year by year?
- What is Boston Marathon weather usually like?
- What is London Marathon weather usually like?
- What is Berlin Marathon weather usually like?
- What is Comrades weather like on the up and down runs?
- Does your finish time change the heat you meet?
- How much does race-day heat slow marathon runners?
- Where do these numbers come from, and how far can you trust them?
- How should you use weather history to plan your race?
- Questions runners ask
- Sources and method
Words this guide uses
- ERA5
- The Copernicus Climate Change Service's global reanalysis, an hourly reconstruction of past weather on a grid about 31 km (19 miles) apart.
- WBGT (wet-bulb globe temperature)
- A heat-stress number that combines air temperature, humidity, sun and wind, used by race medical teams to set heat flags.
- Dew point
- The temperature at which the air would be saturated; the higher it is, the less your sweat can evaporate.
- Pace-timed forecast
- The weather for each stretch of your route at the time you'll run it.
What does marathon weather history show for these four races?
Marathon weather history shows real differences between big races. Across every race day from 2015 to 2024, the Comrades Marathon in South Africa ran hottest, the Boston Marathon in the United States swung most, the London Marathon in the UK stayed coolest and the Berlin Marathon in Germany stayed mild.
Each race has its own guide with a full year-by-year table: Boston Marathon weather history, London Marathon weather history, Berlin Marathon weather history and Comrades Marathon weather. This page puts the four side by side.
The numbers are our own. We took hourly ERA5 weather at each real race date and start time. Then our engine timed it to runners along each course, as a pace-timed forecast run backwards: the weather for each stretch of the course at the time you would have run it.
Three gaps matter. No race ran on its course in 2020. Boston ran on 11 October 2021, and London on 3 October 2021 and 2 October 2022. Comrades skipped 2020 and 2021, then ran on 28 August 2022. So Boston, London and Berlin have nine race days each, and Comrades has eight.
Which of the four races is usually hottest?
Comrades, by a distance. It doesn't start warm, but it lasts up to 12 hours and runs straight through the warmest part of the day. The table uses a four-hour finish for the marathons and an 11-hour finish for Comrades.
| Race | Usual race day | Start (local) | Race days, 2015–2024 | Average air at the start, °C (°F) | Warmest air, °C (°F) | Highest dew point, °C (°F) | Highest peak WBGT, °C (°F) | Good / Fair / Tough |
|---|---|---|---|---|---|---|---|---|
| Boston Marathon, USA | Third Monday of April | 10:00 | 9 | 12.8 (55) | 22.9 (73), 2017 | 15.7 (60), 2021 | 21.6 (71), 2021 | 3 / 4 / 2 |
| London Marathon, UK | Late April | 10:00 | 9 | 10.7 (51) | 21.6 (71), 2018 | 13.6 (56), 2018 | 20.6 (69), 2018 | 8 / 1 / 0 |
| Berlin Marathon, Germany | Late September | 09:15 | 9 | 12.7 (55) | 23.4 (74), 2021 | 14.9 (59), 2021 | 22.4 (72), 2021 | 6 / 3 / 0 |
| Comrades Marathon, South Africa | Late May or June | 05:30 | 8 | 11.3 (52) | 26.5 (80), 2017 | 16.0 (61), 2022 | 27.1 (81), 2015 | 0 / 5 / 3 |
The ratings are our engine's. Fair means Moderate heat stress at some point on the course. Tough means High heat stress, or cold rain. At the four-hour mark, London's average start was the coolest, at 10.7 °C (51 °F), and its highest dew point the lowest, at 13.6 °C (56 °F).
How did each race day compare, year by year?
Peak heat stress tells the story best. The table shows the highest WBGT each race reached for a four-hour marathoner, or an 11-hour Comrades runner, with our rating.
| Year | Boston, peak WBGT °C (°F), rating | London, peak WBGT °C (°F), rating | Berlin, peak WBGT °C (°F), rating | Comrades, peak WBGT °C (°F), rating |
|---|---|---|---|---|
| 2015 | 8.3 (47), Tough | 10.0 (50), Good | 15.7 (60), Good | 27.1 (81), Tough |
| 2016 | 18.8 (66), Fair | 9.3 (49), Good | 19.5 (67), Fair | 22.9 (73), Tough |
| 2017 | 18.7 (66), Fair | 13.3 (56), Good | 16.0 (61), Good | 25.4 (78), Tough |
| 2018 | 8.6 (47), Tough | 20.6 (69), Fair | 19.4 (67), Fair | 20.3 (69), Fair |
| 2019 | 18.3 (65), Good | 12.6 (55), Good | 14.4 (58), Good | 20.2 (68), Fair |
| 2020 | not run | not run | not run | not run |
| 2021 | 21.6 (71), Fair (October) | 15.2 (59), Good (October) | 22.4 (72), Fair | not run |
| 2022 | 12.6 (55), Good | 17.8 (64), Good (October) | 18.3 (65), Good | 22.2 (72), Fair (August) |
| 2023 | 13.9 (57), Good | 14.5 (58), Good | 18.1 (65), Good | 22.2 (72), Fair |
| 2024 | 19.7 (67), Fair | 10.6 (51), Good | 14.0 (57), Good | 20.1 (68), Fair |
Boston's two Tough years, 2015 and 2018, were cold and wet, not hot. The 18.3 readings for Boston 2019 and Berlin 2022 rate Good because the unrounded values sit just under the Moderate line.
What is Boston Marathon weather usually like?
Changeable. Boston's mid-April race has delivered both the coldest and some of the warmest race days in this set, often with dry air.
In 2018 the air at the start was 3.2 °C (38 °F), and a four-hour runner met 12.4 mm (0.49 in) of rain, with a wet feels like of −3.2 °C (26 °F). A year earlier, the start was 21.1 °C (70 °F). In 2016 the dew point stayed at 3.1 °C (38 °F) while the air reached 21.1 °C (70 °F), so sweat evaporated easily. The Boston Marathon weather history guide has every year, and the downhill course explained.
What is London Marathon weather usually like?
Cool, and kind to runners. Eight of nine London race days rated Good for a four-hour finisher, with peak heat stress Low.
The exception was 2018, the hottest London Marathon on Met Office records. Our data puts the start at 18.9 °C (66 °F), with Moderate heat stress from the start line. The two October races, in 2021 and 2022, were mild and rated Good. The London Marathon weather history guide lists London Marathon temperatures by year.
What is Berlin Marathon weather usually like?
Mild. Berlin's late-September start averaged 12.7 °C (55 °F), and six of nine race days stayed at Low heat stress for a four-hour runner.
The course climbs only about 20 m (66 ft). Three world records fell there in this period, on mornings that started at 12.0 to 14.1 °C (54 to 57 °F). Warm days still happen: in 2021 the air reached 23.4 °C (74 °F) for a four-hour runner, and a five-hour runner met High heat stress near the finish. See Berlin Marathon weather history.
What is Comrades weather like on the up and down runs?
Cold at the 05:30 start and warm by the afternoon, whichever way it runs. The direction changes the start: down runs leave inland Pietermaritzburg, up runs leave coastal Durban.
Down-run starts averaged 7.4 °C (45 °F) in our data, and up-run starts 15.1 °C (59 °F). Between the gun and the warmest stretch, the air rose by 6 to 19 °C (11 to 34 °F). The three Tough years were 2015, 2016 and 2017, with High heat stress from km 33 to 60 (mile 21 to 37), depending on the year and finish time. The August 2022 race brought the highest dew point in the set, 16.0 °C (61 °F). Our Comrades Marathon weather guide goes race by race.
Does your finish time change the heat you meet?
Yes, on most warm days. A slower runner is out later, when the sun is higher, so the same race can rate Fair for one runner and Tough for another.
Berlin 2021 shows it clearly. Here is the same race day at three finish times.
| Finish time | Average pace | Warmest air, °C (°F) | Hottest feels like, °C (°F) | Peak WBGT, °C (°F) | Heat stress | Rating |
|---|---|---|---|---|---|---|
| 3:00 | 4:16 per km (6:52 per mile) | 22.1 (72) | 25.4 (78) | 21.6 (71) | Moderate from km 12 (mile 7.5) | Fair |
| 4:00 | 5:41 per km (9:09 per mile) | 23.4 (74) | 27.9 (82) | 22.4 (72) | Moderate from km 9 (mile 5.6) | Fair |
| 5:00 | 7:07 per km (11:27 per mile) | 24.1 (75) | 29.1 (84) | 22.9 (73) | Moderate from km 7, High from km 40 (mile 25) | Tough |
Comrades stretches the same effect over a whole day. In 2015, a nine-hour runner met High heat stress from km 42 (mile 26). An 11-hour runner met it from km 33 (mile 21).
How much does race-day heat slow marathon runners?
Measurably, and slower runners lose more. Ely and colleagues (2007), writing in the American College of Sports Medicine's journal, studied seven marathons including Boston.
The top men ran 1.7% slower than the course record when the WBGT was 5–10 °C (41–50 °F). At 20.1–25 °C (68–77 °F), they ran 4.5% slower. The 25th to 300th finishers slowed more than the leaders as WBGT rose. The authors found a progressive slowing from 5 to 25 °C (41 to 77 °F).
Heat also brings risk. The ACSM's 2007 position stand warns that exertional heatstroke can strike seemingly healthy athletes even when it's relatively cool. Official race flags and medical advice override anything in this article, which is not medical advice. Our guides to WBGT for runners, marathon heat flags and the ideal marathon temperature go further, and running in heat and humidity covers pacing.
Where do these numbers come from, and how far can you trust them?
From ERA5, the reanalysis run by the Copernicus Climate Change Service at ECMWF. It is reanalysis, not a thermometer at the start line, so treat it as a consistent record rather than an official reading.
ERA5 gives hourly weather on a grid about 31 km (19 miles) apart. Each course crosses two to four grid points. Our engine adds sun and wind to work out WBGT with the method of Liljegren and colleagues (2008).
We checked it against race-day reports:
- Boston 2018: WGBH describes temperatures in the 30s and 40s °F with wind-driven rain. Our data runs from 3.2 to 8.0 °C (38 to 46 °F), with heavy rain.
- London 2018: the Met Office recorded 24.2 °C (76 °F) at St James's Park. Our warmest air for a four-hour runner is 21.6 °C (71 °F). A grid cell averages city and countryside, and a park's daily maximum can come after most runners finish.
- Comrades 2023: the race's own history gives a low of 5 °C (41 °F) in Pietermaritzburg. Our start reads 4.9 °C (41 °F).
RunFast is independent of all four races. We aren't official, and these aren't the organisers' figures.
How should you use weather history to plan your race?
Use it to set expectations and pack, not to forecast. History tells you the range a race can throw at you; the forecast in the final 10 days tells you which part of it you'll get.
- Look at the spread, not the average. Boston's starts ranged from 3.2 to 21.1 °C (38 to 70 °F).
- Check at your own finish time. A five-hour runner meets different weather from a three-hour runner.
- Pack for both ends. Bring a throwaway layer for a cold start and plan cooling for a warm finish. Our guide to what to wear for a marathon has the detail.
- Switch to the forecast in race week. Our guide to the race day weather forecast shows how to read it kilometre by kilometre.
- On the day, let your supporters follow. With live tracking for running events, they see you on the course, with the heat where you are.
Questions runners ask
Which big marathon has the hottest race-day weather?
Of the four races we analysed, the Comrades Marathon in South Africa ran hottest from 2015 to 2024. Its peak WBGT reached 27.1 °C (81 °F) in 2015, and three of its eight race days hit High heat stress. It isn't a hot climate; the race simply lasts up to 12 hours and runs through the warmest part of the day. Among standard marathons, Berlin 2021 peaked highest, at 22.4 °C (72 °F) WBGT for a four-hour runner.
Which of the four marathons has the most changeable weather?
Boston. From 2015 to 2024, the air at its 10:00 start ranged from 3.2 °C (38 °F) in 2018 to 21.1 °C (70 °F) in 2017. Two race days were rated Tough for cold rain and four were Fair for warmth. Mid-April in New England can bring either. If you run Boston, pack for both ends and decide on the morning, using the forecast rather than the history.
Why did some of these races take place in October?
After the 2020 races were cancelled or run virtually, some moved while pandemic restrictions eased. Boston ran on 11 October 2021, and London ran on 3 October 2021 and 2 October 2022, instead of in April. Comrades skipped 2020 and 2021 and ran on 28 August 2022, about 11 weeks later than usual. Our tables keep those years but flag them, because a different season means different weather.
Is ERA5 race weather the same as a weather station reading?
No. ERA5 is a reanalysis: a weather model combined with millions of past observations, giving hourly values on a grid about 31 km (19 miles) apart. It's consistent from year to year and covers every stretch of a course. But it smooths out local effects such as a sunny city park or a sea breeze, so a station near the course can read a few degrees warmer or cooler.
What WBGT counts as High heat stress in a marathon?
On the bands our engine uses, a WBGT of 22.8–27.8 °C (73–82 °F) is High and anything above is Extreme. Moderate starts at 18.3 °C (65 °F). Those are our engine's bands for planning; each race's medical team sets its own flags and rules. Always follow the official flags on the day, which override anything you read here.
Does starting in a later wave mean hotter conditions?
Usually, yes, on a warming day. Our race pages start everyone at the race's main start time. If your wave goes off 30 or 60 minutes later, you reach each kilometre later in the morning, when the sun is higher and the air is often warmer. To see the effect, set a slower finish time on the race page: it moves you into later hours in the same way.
Which of these races has the best weather for a fast time?
On our numbers, London and Berlin. London rated Good on eight of nine race days, and Berlin's start averaged 12.7 °C (55 °F) with Low heat stress on six of nine. Both are flat, too. Ely and colleagues (2007) found marathon times slow progressively as WBGT rises above 5–10 °C (41–50 °F), so cool starts help. One race day is still one race day: check the forecast.
How often did rain affect these races from 2015 to 2024?
Rarely, and mostly at Boston. Boston 2018 brought 12.4 mm (0.49 in) of rain for a four-hour runner, and Boston 2015 brought 2.4 mm (0.09 in), both cold enough to rate Tough. Berlin 2019 had 2.2 mm (0.09 in) and London 2023 had 1.5 mm (0.06 in). Comrades stayed almost dry in every year we have. Light rain on a cool day mostly matters for what you wear.
Can I check the weather for a race that isn't one of these four?
Yes, for upcoming days. Drop the race's GPX, FIT or TCX file into the free Map your run tool, pick a day up to 10 days ahead and set your pace. It shows the forecast heat stress, dew point and feels like for every kilometre, timed to when you'll get there. Ten years of race-day history is only available for Comrades, Berlin, London and Boston.
Your race, at your pace.
Pick Comrades, Berlin, London or Boston, set your finish time, and see the heat stress, feels like and dew point you’d have met at every landmark, on every race day from 2015 to 2024.
Sources and method
- Copernicus Climate Change Service (ECMWF) (2023). ERA5 hourly data on single levels from 1940 to present
- Quarterly Journal of the Royal Meteorological Society (2020). The ERA5 global reanalysis (Hersbach et al.)
- Journal of Occupational and Environmental Hygiene (2008). Modeling the wet bulb globe temperature using standard meteorological measurements (Liljegren et al.)
- Medicine and Science in Sports and Exercise (2007). Impact of weather on marathon-running performance (Ely, Cheuvront, Roberts and Montain)
- American College of Sports Medicine (2007). Position stand: exertional heat illness during training and competition (Armstrong et al.)
- Met Office (2025). London Marathon 2025: weather, records and event milestones
- WGBH (2026). Snow squalls and the 'Run for the Hoses': a brief history of Boston Marathon weather
- Comrades Marathon Association (2026). Histories (race-day weather, Pietermaritzburg and Durban)
- South African Journal of Science (2022). Increased risk of heat stress conditions during the 2022 Comrades Marathon (Havenga, Coetzee, Burger and Piketh)
Every race day from 2015 to 2024 on which each race was run on its course, at its real date and start time (Boston 10:00, 2021 at 09:00; London 10:00, 2019 at 10:10, 2021 at 09:30, 2022 at 09:40; Berlin 09:15; Comrades 05:30, in each year's direction). Weather is hourly ERA5 reanalysis (Copernicus Climate Change Service, ECMWF): air temperature and dew point at 2 m, sunshine, wind at 10 m and rain, at the 0.25° grid points each course crosses (Boston three, London two, Berlin two, Comrades four). Each stretch of the course takes its nearest grid point. Runners are timed along the course by the RunFast engine at an even effort for the stated finish time, with pace adjusted for gradient; marathons at 3:00, 4:00 and 5:00 finishes, Comrades at 9:00 and 11:00 (and 12:00 where stated). WBGT is computed with Liljegren et al. (2008), with cloud worked back from ERA5's sunshine; levels are Low below 18.3 °C (65 °F), Moderate to 22.8 °C (73 °F), High to 27.8 °C (82 °F), Extreme above, judged on unrounded values. Ratings: Fair at Moderate heat stress, Tough at High or Extreme, and Tough in cold rain (1 mm or more with a wet feels like of 5 °C, 41 °F, or less). The Comrades up run is modelled as the down course reversed. Averages are means across race days; Fahrenheit values are our conversions. Data file: content/blog/data/race-history.json. RunFast is independent of every race named here.
Wes Thomson
Wes Thomson builds RunFast, which times the heat, humidity and dew point to a runner's own pace along the route, and its sister Windline, live wind forecasts for cyclists. Both run on the same forecast engine. He writes these guides with one focus: what the weather will do to your run, kilometre by kilometre, and how to plan around it.