London Marathon Weather: What to Wear and Pace Tips

In 2018, a runner who had trained all winter for a 3:30:00 London Marathon crossed the line in 4:07:00. She was not injured. She was not undertrained. The temperature hit 24 degrees Celsius and she was wearing a long-sleeve base layer because the forecast she had checked on Tuesday said 14 degrees. By the time she reached Tower Bridge, she was overheating, under-hydrated, and 40 seconds per kilometre off target. The weather did not ruin her race. Her response to the weather did.

The London Marathon takes place in late April, and April in London is unpredictable. Race-day temperatures have ranged from 5 degrees in cold, wet years to 24 degrees in the infamous 2018 edition. The difference between those extremes is not just comfort – it is minutes on your finish time. This guide breaks down what to wear and when to adjust your pace for every temperature bracket you might face on the 2026 London Marathon course.

How temperature changes your marathon performance

Research by Matthew Ely and colleagues, published in Medicine and Science in Sports and Exercise, found that marathon performance begins to decline when temperatures exceed 10 to 12 degrees Celsius. Above 15 degrees, the decline accelerates. At 25 degrees, even well-trained runners lose between 3 and 10 % of their performance compared to optimal conditions.

The mechanism is straightforward. Your body generates significant heat during a marathon – roughly 15 to 20 times your resting metabolic rate. In cool conditions, the air absorbs that heat efficiently. As the temperature rises, your body diverts more blood to the skin for cooling, which means less blood is available for your working muscles. Your heart rate climbs at the same pace, your perceived effort increases, and if you do not slow down, you hit a wall earlier and harder.

This is not about being tough. It is physiology. The runners who handle warm London Marathons best are the ones who adjust their targets before the gun fires.

What to wear and how to pace by temperature

Cold conditions: below 10 degrees Celsius

Cold starts are more common at London than most runners expect. Early morning temperatures in late April can sit around 6 to 8 degrees, especially if it is overcast or raining.

What to wear: A light long-sleeve top or arm sleeves you can push down as you warm up. Lightweight gloves – your hands lose heat fastest and cold fingers make it harder to open gel packets. A vest or singlet underneath if you plan to strip the long sleeve later. Avoid heavy layers. You will warm up within 15 to 20 minutes of running. A bin bag over your kit while you wait at the start is cheap and effective.

Pace adjustment: None needed. Cold conditions are close to ideal for marathon running. If anything, your pace targets should be slightly more ambitious than usual – provided you warm up properly. Use your pace calculator targets as planned.

Ideal conditions: 10 to 15 degrees Celsius

This is the sweet spot. Most elite marathon world records have been set in the 10 to 15 degree range. The 2019 London Marathon (12 degrees, overcast) produced fast times across the field.

What to wear: Singlet or short-sleeve top. Shorts. No gloves. Consider a cap if the sun is out – direct sun on your head raises core temperature faster than air temperature alone. Light-coloured kit reflects more heat than dark colours.

Pace adjustment: None. Run your target splits. This is the weather you trained for.

Warm conditions: 15 to 20 degrees Celsius

This is where most runners underestimate the impact. Fifteen degrees does not feel warm when you step outside. But 3 hours of sustained effort at 15 degrees or above generates a thermal load that accumulates. The Ely study data shows a measurable performance drop starting right at this threshold.

What to wear: Lightest singlet you own. Shorts, not tights. A white or light-coloured cap. Consider a buff or bandana you can soak at aid stations and drape around your neck. Absolutely no long sleeves.

Pace adjustment: Add 5 to 15 seconds per kilometre to your target pace. At 15 degrees, aim for the lower end. At 18 to 20 degrees, add closer to 15 seconds. Recalculate your split targets the morning of the race using the adjusted pace. A 3:30:00 target becomes roughly 3:35:00 to 3:40:00 at 18 degrees – that is not a failure, that is smart racing.

Hot conditions: above 20 degrees Celsius

London has hit these temperatures on race day. The 2018 edition saw 24 degrees. The 2007 Flora London Marathon reached similar temperatures and organisers issued heat warnings. If you see a forecast above 20 degrees for race morning, your goal changes fundamentally.

What to wear: The minimum possible. Lightest singlet or crop top, shortest shorts. White cap, essential. Sunscreen on any exposed skin – sunburn increases core temperature and fluid loss. Carry a small sponge or plan to use the sponge stations on course.

Pace adjustment: Add 20 to 40 seconds per kilometre depending on how far above 20 degrees the forecast sits. At 24 degrees, a 3:30:00 target realistically becomes 3:45:00 to 3:55:00. This is not giving up – it is the difference between finishing strong and ending up in the medical tent. Drink at every aid station. Pour water over your head and wrists. If your heart rate climbs 10 to 15 beats above normal at your target pace, slow down immediately.

How humidity makes warm days worse

Temperature alone does not tell the full story. A 16-degree day with 90 % humidity feels worse than a 20-degree day with 40 % humidity. High humidity impairs your body’s ability to cool itself through sweat evaporation – the sweat sits on your skin instead of evaporating, and your core temperature rises even though you are drenched. Research by Kenefick and Cheuvront, published in the American College of Sports Medicine’s position stand on exertional heat illness, confirms that humid conditions significantly increase the risk of heat-related performance decline even at moderate temperatures.

London in April tends toward moderate humidity (60 to 75 %), which is manageable in cool conditions but compounds the problem when temperatures climb. If the forecast shows both warmth and high humidity, lean toward the upper end of the pace adjustment range. Check running conditions before race day to understand the combined heat and humidity impact.

Rain and wind: what changes

Rain is the most likely weather scenario for a London Marathon. Light rain in cool conditions is almost ideal – it keeps you cool without any real downside. Heavy, sustained rain creates two problems: blisters and weight. Wet socks increase friction, and soaked clothing adds dead weight that costs energy over 42 km. If rain is forecast, apply anti-chafe balm generously (inner thighs, underarms, nipples, feet), wear thin synthetic socks that drain rather than absorb, and skip cotton entirely.

Wind is less of a factor at London than at exposed coastal marathons, but sections along the Thames Embankment and through the Docklands can catch a crosswind or headwind. On a windy day, tuck behind other runners when possible and accept that headwind sections will cost you 5 to 10 seconds per kilometre. Do not try to fight it – adjust your split expectations for those kilometres and make up time on sheltered sections.

When to check the forecast and how to act on it

Marathon weather forecasts are only useful within a 48 to 72 hour window. Anything earlier is too unreliable to base decisions on. Here is a simple timeline:

• Thursday (2 days before): Check the forecast. Note the predicted temperature range and precipitation probability. Start thinking about clothing choices.
• Friday (1 day before): Confirm your kit choice. If the forecast has shifted significantly, recalculate your pace targets using your race time predictor and the adjustment guidelines above. If you are still finalising your training plan, factor in a heat adaptation session. Lay out your race kit.
• Race morning: Final check. If conditions are warmer than Friday’s forecast predicted, downshift your target by another 5 seconds per kilometre. Better to adjust at the start than at 30 km.

The single best pre-race weather tool for runners is a race-specific forecast that combines temperature, humidity, wind, and precipitation into a single recommendation.

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Your questions about London Marathon weather

What is the ideal temperature for running a marathon?

Research consistently points to 10 to 12 degrees Celsius as the optimal range for marathon performance. At these temperatures, your body can dissipate heat efficiently without diverting excessive blood flow to the skin. Most marathon world records have been set in this window. London’s late April date means you have a reasonable chance of landing in this range, but it is far from guaranteed.

Should I change my finish time goal if the weather is warm?

Yes. Ignoring warm weather is one of the most common marathon mistakes. Above 15 degrees, add 5 to 15 seconds per kilometre. Above 20 degrees, add 20 to 40 seconds. A slower but controlled finish is always better than a collapse at 35 km. Adjust your targets and run the race the conditions allow.

What should I wear if rain is forecast for the London Marathon?

Light synthetic kit that drains rather than absorbs. Thin synthetic socks. Anti-chafe balm on all friction points – inner thighs, underarms, nipples, feet. A cap with a brim keeps rain out of your eyes. Skip cotton entirely. A disposable bin bag keeps you dry at the start but should be discarded before you cross the start line.

How does humidity affect marathon pace?

High humidity impairs sweat evaporation, which is your body’s primary cooling mechanism. A 16-degree day at 90 % humidity can feel worse than a 20-degree day at 40 % humidity. If humidity is above 75 % and temperatures are above 15 degrees, treat it as a warm-weather race and add pace accordingly. Monitor your heart rate – if it drifts upward at the same pace, you need to slow down.

This article is for informational purposes only. Weather-based pace adjustments are general guidelines. Individual responses to heat, humidity, and cold vary based on body composition, acclimatisation, hydration, and fitness. Consult a qualified coach or medical professional if you have concerns about racing in extreme conditions.

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