Running weather forecast: how to check it for your exact run

What is a running weather forecast, and how is it different from a normal one?

A running weather forecast is one timed to your route and pace: the weather where you'll be, when you'll be there. A normal forecast gives one place and the whole day. On a two-hour run, or a five-hour marathon, the weather moves while you do.

That gap is what this guide is about. We call the fix a pace-timed forecast: the weather for each stretch of your route at the time you'll run it. You set a start time and a pace, and each kilometre gets the forecast for its own place and its own hour.

The difference is easy to see on a real race day. These are three finish times on the same Berlin Marathon morning, 26 September 2021, from the ERA5 reanalysis of past weather.

Same race, same start, three different days out on the course. The five-hour runner met the worst of it, in the last two kilometres. You can see the same for your own route in Map your run, free and without an account.

This hub links to five detailed guides: reading a marathon race day weather forecast, getting the weather along your running route from a file, knowing when to trust a forecast 10 days out, finding the best time to run in hot weather, and why the feels like temperature for running differs from your app's.

What should you check in the forecast before a run?

Check five things for the hours you'll be out: air temperature, dew point, feels like at your pace, heat stress (WBGT), and wind and rain. Each tells you something the others don't.

Humidity as a percentage is the one number to treat with care. Relative humidity is highest at dawn because the air is coolest then, not because there is more water in it. Dew point measures the water itself, which is why the US National Weather Service uses it to judge how muggy the air feels.

The dew point bands above are RunFast's, from the engine that powers Map your run. The WBGT bands match the American College of Sports Medicine's 1996 race guidance: 18°C (65°F), 23°C (73°F) and 28°C (82°F).

How much does the weather change during a long run?

Often more than the day's forecast high suggests, because you run into the warmest hours. The longer you're out, the bigger the swing between your first and last kilometre.

These are real race days from RunFast's ERA5 race history, each at one finish time.

London 2018 shows why a reanalysis is a guide rather than a record. The Met Office measured 24.2°C (75.6°F) at St James's Park that day, the hottest London Marathon on record. The ERA5 grid, averaged over a wider area, reads a few degrees lower.

Comrades shows the other extreme. An 11-hour runner on the 2015 up run reached High heat stress from about km 33, while a 9-hour runner got there at km 42. Our race-day forecast guide shows how to read this for your own finish time.

How do you get the forecast along your own route?

Use your route as a file. Most running watches and mapping apps export a GPX, FIT or TCX file of a planned route or a past run, and a route-aware tool reads the forecast along it.

The tool needs three things from you: the route, the start time and your pace. From those it works out when you'll reach each point, slower up the hills and quicker down them, then fetches the forecast for that place and hour. Our guide to the weather along your running route explains where to find the files and what to look for.

How far ahead can you trust a running forecast?

Trust it more each day closer you get. NOAA says a five-day forecast is right about 90% of the time, a seven-day one about 80%, and a ten-day one only about half the time.

So plan in stages. Ten days out, a forecast tells you whether to expect a warm or cool spell, not the hour-by-hour detail. Inside five days, start planning kit and pace. From two days out, the hour-by-hour numbers for your route are worth acting on. Our guide on trusting a marathon forecast sets out what to check at each stage.

Before the forecast is useful, history is your best guide. Boston's start-line air has ranged from 3.2°C (38°F) in 2018 to 21.0°C (70°F) in 2017 in the ERA5 record. You can see every race day from 2015 to 2024 on the Boston race page.

What is the best time of day to run when it's hot?

Usually the last hour before sunrise. That is when the air is coolest and the sun hasn't started to heat you, and heat stress climbs fast once it does.

In Seville, Spain, on 10 July 2023, WBGT at the nearest ERA5 grid point was 16.9°C (62°F) at 07:00 and 23.9°C (75°F) by 09:00, from Low to High in two hours. But dawn isn't always the coolest hour. When a cool change arrives in the afternoon, as in Melbourne on 17 January 2024, the evening can be far kinder. Our guide to the best time to run in hot weather has the hour-by-hour numbers.

Why does your weather app's feels like look low?

Because most feels-like numbers are worked out for someone in the shade, not for a runner in the sun. The National Weather Service says its heat index assumes shade and light wind, and full sunshine can add up to 15°F (8°C).

On a run, two things change that. The sun on your skin adds heat. The air rushing past you takes some away, unless a tailwind cancels it. At 27°C (81°F) with 55% humidity, an app-style apparent temperature reads about 27.5°C (82°F). Running at 5:30 per km (8:51 per mile) in full sun with a light crosswind, the engine's feels like is 34.0°C (93°F). Our feels like guide explains the maths and the cold-day version.

How do you read the forecast for race day?

Read it for your finish time, not the start. Find the hour you expect to reach halfway and the last 10 km (6.2 miles), and check the heat stress there.

Then set your plan by the hottest stretch you'll run. If the forecast puts you in High heat stress for the last hour, start slower than your goal pace and drink to plan from the first station. Watch the organiser's flags on the day. The Chicago Marathon, for one, runs an Event Alert System with four levels, from green (Low) to black (Extreme). Those flags and the medical team's advice override anything a forecast or an article tells you.

When is it too hot to run?

There is no single cut-off, but the risk rises sharply with heat stress. The ACSM's 1996 race guidance advised postponing or cancelling races above a WBGT of 28°C (82°F), RunFast's Extreme level.

Heat illness can affect healthy, fit runners. The ACSM's 2007 position stand notes that exertional heatstroke can strike even when conditions seem relatively cool, and that being unacclimatised, dehydrated, recently ill or on some medications raises the risk. Early recognition and rapid cooling save lives. Our hub on running in heat and humidity covers what heat does to your pace, and how hot is too hot to run sets out the limits. This guide is not medical advice.

How do you check the weather for your run in five steps?

Get your route, set your start and pace, read the hottest stretch, then decide. It takes a few minutes and can change the whole plan.

1. Get the route as a file. Export it from your watch app or mapping app as GPX, FIT or TCX.
2. Set your start time and pace. Use the pace you'll really run, in minutes and seconds per km or per mile.
3. Find the hottest stretch. Look for where heat stress steps up a level, and when.
4. Check the dew point and wind there. A high dew point makes sweat work less well; a tailwind leaves you in still air.
5. Decide. Move the start, change the route, slow the pace or shorten the run.

Check it again the evening before. Forecasts change, and the last update is the one to plan on.