Updated June 2026

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This week, the UK has a Red Extreme Heat Warning in place, with temperatures forecast to hit 38 to 40°C and overnight lows barely dropping below 20°C. It is the kind of heat that genuinely changes the calculation around whether you should be running at all, and it is a useful prompt to revisit the basics properly rather than just toughing it out.

As a UKA-qualified running coach, it’s something I am often asked, not just from runners training for summer ultras but from anyone whose long run happens to fall on the one genuinely hot week of a British summer. Heat does not just slow you down; it changes how your body handles effort entirely, and respecting that difference is what keeps a hot run enjoyable rather than dangerous.

When Is It Too Hot to Run?

There is no single number that applies to everyone, but there are useful guidelines. The American College of Sports Medicine advises caution when exercising outdoors once temperatures exceed 27°C, and the NHS recommends extra precautions during any UK heatwave regardless of how acclimatised you think you are. Humidity matters as much as the raw temperature. High humidity stops sweat evaporating efficiently, which is the main way your body cools itself during exercise, so a humid 28°C can feel and behave like a dry 34°C.

During genuine extreme heat events, the kind that trigger a red warning from the Met Office, the honest advice is not to run outdoors during the hottest part of the day at all – why run at all if it’s not necessary? A red heat alert specifically warns of risk to life for the otherwise healthy population, which is a different category of risk from ordinary summer discomfort. If you are tempted to push through a session purely because it is in the training plan, missing one run on an extremely hot day will not undo months of training. A heat-related medical emergency might undo considerably more.

Run Early or Not at All

The single most effective change you can make in hot weather is timing. Starting at 6 am or earlier, before the sun has had time to heat the ground and the air, can mean the difference between a comfortable run and a genuinely unpleasant one. This will not fully prepare you for a summer ultra where you will be out during the heat of the day regardless, so if you are training for an event in hot conditions, it is worth deliberately including a couple of long runs in the heat you expect to race in, so your body has some exposure before it matters.

If early starts are not possible, the alternative is accepting a significantly reduced session rather than forcing your usual mileage or pace into conditions your body is not built to handle. Heat slows everyone down. Trying to hit your normal pace in genuinely hot weather is one of the more common ways people end up in trouble.

Hydration and Electrolytes

Water alone is not always enough once you are sweating heavily for an extended period. As you sweat, you lose sodium, potassium, calcium and magnesium, and a deficit in any of these can contribute to cramping, dizziness and poor performance well before you would describe yourself as dehydrated.

Electrolyte tablets are a simple and effective way to manage this, particularly for anything over an hour in genuine heat. I tend to use them to rehydrate after a hot run and before a race, rather than during easy training, where plain water generally does the job. SIS is the one I reach for most, though Nuun and High5 are both well regarded and worth trying in training rather than leaving it to race day to discover you do not get on with a particular brand or flavour.

For longer efforts, a dedicated endurance drink mix like Tailwind is also worth considering to combine fuel and electrolytes in one place, particularly for ultra distances where eating solid food becomes harder as the day goes on.

What to Carry

For anything beyond a short run in heat, plan your hydration properly rather than hoping you will be fine. A handheld water flask is a genuinely underrated piece of kit for shorter hot-weather runs where a full vest feels like overkill. It is small enough to disappear in your hand once it has been squeezed down, has room for a key and a card in the strap, and means you are never caught out on a route without water simply because you did not want to wear a vest for a five-mile run.

For longer runs and anything off-road where you cannot guarantee a refill point, a hydration vest is the better option. Look for one with room for a bladder or soft flasks, and consider carrying more capacity than you think you need on a genuinely hot day. See our best trail running vests guide for current recommendations across distances and budgets.

A cooling running buff is a small addition that makes a real difference. Soaked in cold water and worn around the neck or wrist, it gives you a genuine drop in perceived effort for a surprisingly long stretch of a run, and if you pass a stream, tap or water station, you can simply re-soak it and carry on.

Our Kit Suggestions

Cooling Down on the Move

If you are running off-road and pass a stream, a church with an outside tap, or any natural water source, use it. You do not need to drink it, but soaking a buff, a cap or simply splashing your arms and the back of your neck gives you real relief. The back of the neck and the wrists are both useful spots because blood vessels sit close to the skin there, so cooling them has a more noticeable effect on how you feel than you would expect.

At organised aid stations, the same principle applies. Take the water you are offered, drink some, and do not be precious about tipping the rest over your head or down your back. Nobody finishing a hot race looks elegant, and that is fine.

Dress for the Conditions

Light, breathable, moisture-wicking fabrics matter more in heat than at any other time of year. Pale colours reflect rather than absorb heat. A cap or visor with some ventilation protects your head and face from direct sun without trapping heat the way a heavier hat does. Sunscreen is not optional, and it is worth choosing one specifically designed for sport so it does not run into your eyes the moment you start sweating properly.

Recognising When Something Is Wrong

Heat exhaustion and heat stroke exist on a spectrum, and it is worth knowing the difference because the correct response is different too.

Heat exhaustion typically presents as heavy sweating, dizziness, nausea, headache and a fast but weak pulse. Stop, get into shade, remove excess clothing, sip water and cool the skin with water or a wet cloth. Most people recover within thirty minutes with this approach.

Heat stroke is more serious. Signs include a high body temperature, confusion, a lack of sweating despite the heat, and a rapid strong pulse. This is a medical emergency. Call for help immediately and begin cooling the person while you wait.
If you are running with others, agree before you set off that anyone showing these signs stops immediately, no exceptions and no pushing on to finish a session. Heat does not respect training plans.

Frequently Asked Questions

At what temperature should I stop running outdoors?

There is no universal cut-off, but most sports medicine guidance suggests caution above 27°C, with humidity making a significant difference to how that temperature actually feels and behaves on your body. During a red extreme heat warning, avoiding outdoor exercise entirely during the hottest part of the day is the safest approach.

How can I tell the difference between heat exhaustion and heat stroke?

Heat exhaustion involves heavy sweating, dizziness and nausea, and usually improves with rest, shade and fluids. Heat stroke involves confusion, a lack of sweating and a very high body temperature, and is a medical emergency requiring immediate help.

Do I need electrolytes for every run in hot weather?

Not for short, easy runs where plain water is generally sufficient. For anything over an hour in genuine heat, or for racing, electrolytes help prevent the cramping and fatigue that come from losing sodium and other minerals through heavy sweating.

Is it safe to train for a summer ultra in the UK without proper heat exposure?

It is risky to arrive at a hot ultra having only ever trained in mild UK conditions. Deliberately including some long runs during the warmer parts of the day in the weeks before a hot race gives your body some adaptation, even if it is far from a full heat acclimatisation protocol.