It’s 28°C, there’s not a cloud in sight, and you’ve got a 10K run planned. Your legs say go, your lungs disagree after the first kilometre, and by halfway you’re pouring water over your head just to keep going. Running in heat is a different sport — your body works harder, loses more fluid, and overheats faster than you’d expect, even if you’re fit.
After bonking badly at a summer 10K where I’d barely drunk anything that morning, I learned the hard way that heat running needs proper planning. My finishing time was eight minutes slower than usual and I spent the next hour lying on the grass feeling like I’d run a marathon. A few simple adjustments to timing, hydration, and pacing would have prevented all of it.
In This Article
- Why Heat Affects Running Performance
- Hydration: Before, During and After
- Electrolytes: Do You Need Them?
- When to Run: Timing Your Sessions
- Adjusting Your Pace for Heat
- What to Wear
- Route Planning for Hot Days
- Heat Acclimatisation
- Recognising Heat-Related Illness
- Cooling Strategies That Work
- Nutrition in the Heat
- Frequently Asked Questions
Why Heat Affects Running Performance
The Thermoregulation Problem
Your body generates heat when you run — roughly 15-20 times your resting metabolic rate. In cool weather, sweat evaporates easily and blood flow to the skin dissipates excess heat. In hot weather, the cooling system struggles.
When air temperature approaches body temperature (37°C), sweating becomes less efficient because the temperature gradient between skin and air shrinks. Add humidity and evaporation slows further. Your body diverts more blood to the skin for cooling, which means less blood is available for working muscles. The result: higher heart rate, lower power output, and faster fatigue.
The Numbers
Research consistently shows that running performance degrades measurably above 15°C, with the biggest impact above 25°C:
- 15-20°C: Minimal impact on most runners
- 20-25°C: Pace drops 1-3% for the same effort
- 25-30°C: Pace drops 3-6%. Heart rate rises 5-10 bpm at the same pace
- 30°C+: Pace drops 6-10%+. Risk of heat illness rises sharply
Why Some People Handle Heat Better
Heat tolerance varies widely. Factors include acclimatisation (regular heat exposure), body size (smaller runners dissipate heat more easily), fitness level (fitter runners begin sweating earlier), hydration habits, and genetics. If you’ve always run better in winter, you’re not imagining it — your body may simply be less efficient at cooling.
Hydration: Before, During and After
Pre-Run Hydration
Start hydrating 2-3 hours before your run. Aim for about 500ml of water in the hours leading up to your session. Your urine should be pale straw colour before you head out — dark yellow means you’re already dehydrated.
Don’t overdo it. Drinking 2 litres in the hour before a run causes stomach discomfort and doesn’t improve hydration — your kidneys just excrete the excess.
During Your Run
- Under 45 minutes: Water is fine. 150-200ml every 15-20 minutes if you can
- 45-90 minutes: Water plus a small amount of electrolyte (a single tab or pinch of salt)
- Over 90 minutes: Electrolyte drink or a mix of water and energy gels. You’ll need 400-800ml per hour depending on heat and sweat rate
Carrying water on a hot run isn’t optional — it’s essential. A handheld bottle, running belt, or hydration vest all work. Choose whichever you’ll actually use.
Post-Run Recovery
Weigh yourself before and after your run. Every kilogram lost equals roughly 1 litre of sweat. Replace 150% of the fluid lost — so if you lost 1kg, drink 1.5 litres over the next 2-3 hours. Include some sodium (an electrolyte tablet, salty snack, or recovery drink) to help your body retain the fluid.
Electrolytes: Do You Need Them?
What You Lose in Sweat
Sweat contains sodium, potassium, magnesium, and chloride. Sodium is the biggest loss — roughly 500-1,500mg per litre of sweat. In cool weather with short runs, you replace these through normal eating. In hot weather with longer runs, you lose more than food alone replaces.
When to Supplement
- Runs under 60 minutes in moderate heat: Water is sufficient. Eat normally afterward
- Runs over 60 minutes or in high heat (25°C+): Add an electrolyte tablet (about £0.30-0.50 each from brands like SiS, HIGH5, or ORS)
- Runs over 90 minutes in heat: Use an electrolyte drink throughout and consider salt capsules if you’re a heavy sweater
Signs of Electrolyte Imbalance
- Muscle cramps — particularly in calves and hamstrings during or after a hot run
- White salt stains on clothing — you’re a salty sweater and need more sodium
- Headache and nausea — could be dehydration or hyponatraemia (see below)
- Fatigue out of proportion to effort — electrolyte depletion reduces muscle function
Hyponatraemia Warning
Drinking too much plain water without electrolytes can dilute blood sodium to dangerous levels. This is called hyponatraemia and it’s more common than heat stroke in endurance events. Symptoms include confusion, nausea, and swelling. If you’re running for over 2 hours in heat, use electrolytes — don’t just drink water.
For a full guide to fuelling and hydration for running, our nutrition and hydration guide covers everything from gels to recovery meals.
When to Run: Timing Your Sessions
The Best Times
- Before 8am: The coolest part of the day in UK summer. Air temperature is typically 5-10°C lower than peak afternoon heat
- After 7pm: Temperature drops but the ground may still radiate stored heat. Good for evening runners
- Avoid 11am-4pm: Peak UV, peak temperature, peak misery
The UV Factor
The NHS advises staying active regularly, but in summer that means thinking about UV exposure. UV radiation peaks between 11am and 3pm. If you’re running during peak UV, wear SPF 30+ sunscreen (sport-formula, sweat-resistant), sunglasses, and a cap. Sunburn impairs your body’s ability to cool itself through the damaged skin.
Wind and Humidity
A light breeze makes a huge difference to cooling. Check the forecast for wind direction and plan your outward leg into the wind — you’ll appreciate the tailwind on the way back when you’re hottest. High humidity (above 60%) is the bigger enemy. In humid conditions, sweat doesn’t evaporate properly, so your cooling system fails even if the air temperature isn’t extreme.
Adjusting Your Pace for Heat
The Honest Answer: Slow Down
Accept that your pace will be slower. Fighting it leads to overheating, dehydration, and potentially dangerous situations. A hot weather run is not the time for PBs.
How Much to Slow Down
A rough guide based on temperature above 15°C:
- 20°C: Slow down 10-20 seconds per kilometre
- 25°C: Slow down 20-40 seconds per kilometre
- 30°C: Slow down 40-60 seconds per kilometre or consider skipping the run entirely
Run by Effort, Not Pace
Switch to heart rate or perceived effort instead of pace. If your easy pace normally puts your heart rate at 140 bpm, run at 140 bpm regardless of how slow the pace is. In heat, that might mean running 30-60 seconds per km slower — and that’s perfectly fine.
Walk Breaks Are Smart, Not Weak
Structured walk breaks (30 seconds walking every 10 minutes) during hot runs help manage core temperature and heart rate without meaningfully affecting total time. Many experienced heat runners use this strategy.
What to Wear
Fabric
- Polyester/nylon blend with mesh panels — most running-specific gear. Wicks sweat and dries fast
- Merino wool — temperature-regulating and odour-resistant, but slower to dry
- Cotton — never. It absorbs sweat, becomes heavy, and causes chafing
Colour
Light colours reflect heat. Dark colours absorb it. White or pastel running gear in summer isn’t just aesthetic — it makes a measurable difference. A black t-shirt in direct sun can be 10-15°C hotter than a white one.
Essentials for Hot Weather Running
- Lightweight singlet or vest — less fabric means more cooling
- Shorts with a brief liner — avoid long tights in heat
- Cap or visor — shields your face from direct sun. A visor allows heat to escape from the top of your head
- Sunglasses — reduce squinting and UV exposure to your eyes
- Sunscreen — SPF 30+, sport formula. Apply 20 minutes before running
Route Planning for Hot Days
Seek Shade
Plan routes through tree-lined paths, parks, and canals. A shaded route can be 3-5°C cooler than an exposed road. The Thames Path, canal towpaths, and urban park loops are all good options.
Water Access
Plan routes that pass water fountains, shops, or public facilities where you can refill bottles. In urban areas, cemeteries and churches often have accessible taps. Some runners stash a water bottle at a midpoint on their route before starting.
Surface Temperature
Tarmac and concrete radiate stored heat. Grass, trail, and gravel are cooler underfoot. If you have a choice, run on natural surfaces in summer.
Loops vs Out-and-Back
Short loops (2-3km circuits) near your start point are safer than long out-and-back routes. If you overheat, you’re never far from home or the car. A 15km out-and-back in 30°C heat means being 7.5km from safety when you’re at your hottest.
Heat Acclimatisation
How It Works
Your body adapts to heat exposure over 7-14 days. Adaptations include earlier onset of sweating, higher sweat rate, lower heart rate at the same effort, and better blood plasma volume.
How to Acclimatise
- Run in the warmest part of the day (safely — start short and easy) for 7-14 days
- Start with 20-30 minutes at easy effort and build gradually
- Expect to feel terrible for the first 3-5 days — this is normal
- By day 10-14, you’ll notice a significant improvement in heat tolerance
- Maintain with 2-3 heat sessions per week
When to Acclimatise
If you have a summer race coming (a July 10K or August half-marathon), start heat training 2 weeks before. Even partial acclimatisation helps.
Recognising Heat-Related Illness
Heat Cramps
- Symptoms: Painful muscle spasms, usually in legs or abdomen, during or after exercise
- Cause: Electrolyte imbalance and dehydration
- Action: Stop running, stretch gently, drink electrolyte solution, move to shade
Heat Exhaustion
- Symptoms: Heavy sweating, pale skin, nausea, dizziness, headache, rapid weak pulse, temperature up to 40°C
- Cause: Prolonged heat exposure with inadequate hydration
- Action: Stop running immediately, move to cool area, remove excess clothing, apply cold water to neck and armpits, sip cool fluids. If symptoms don’t improve within 30 minutes, seek medical help
Heat Stroke (Medical Emergency)
- Symptoms: Confusion, altered consciousness, hot and dry skin (sweating may stop), temperature above 40°C, seizures
- Cause: Core body temperature rises to dangerous levels
- Action: Call 999 immediately. Cool the person rapidly — immerse in cold water if possible, apply ice packs to neck, armpits, and groin. This is life-threatening
When to Stop a Run
Stop immediately if you experience any of these during a hot run:
- Confusion or disorientation
- Cessation of sweating despite high effort
- Nausea or vomiting
- Severe headache
- Chills or goosebumps in hot conditions (paradoxical sign of core temperature dysregulation)
Cooling Strategies That Work
Pre-Cooling
Drinking an ice slurry (crushed ice in water) 30 minutes before running lowers core temperature and delays the onset of heat-related performance decline. Even ice-cold water helps. Keep a drink in the freezer and sip it on the way to your start point.
Mid-Run Cooling
- Water on the wrists and neck — major blood vessels run close to the surface here, so cooling these areas lowers blood temperature
- Wet cap or bandana — evaporative cooling on your head
- Ice in a buff or neck gaiter — stays cold for 10-15 minutes during a run
- Dousing your head at water stops — effective but dries quickly
Post-Run Cooling
A cold shower or bath after a hot run brings core temperature down quickly. If you have access to cold water immersion (even a paddling pool), 10 minutes at 10-15°C is the gold standard for rapid cooling.
Nutrition in the Heat
Before Your Run
Eat a light meal 2-3 hours before — toast with peanut butter, porridge with banana, or a rice cake with honey. Heavy meals divert blood to digestion, competing with your cooling system.
During Your Run
Gels and chews work but can cause stomach issues in heat because blood flow to the gut is reduced. If you use gels, take them with water and consider halving the dose. Real food options (dates, banana pieces, jelly babies) may sit better.
Recovery Nutrition
Replace glycogen and protein within 30-60 minutes. A recovery shake, chocolate milk, or a meal with carbs and protein all work. Include something salty — crisps, miso soup, or an electrolyte drink — to replace sodium.
Frequently Asked Questions
Is it safe to run in 30°C heat? It can be, with precautions — run early or late, hydrate well, slow your pace, and listen to your body. If you’re not acclimatised, consider shortening your run or switching to a shaded trail. Above 35°C, most experts recommend cross-training indoors instead.
How much water should I drink before a hot run? About 500ml in the 2-3 hours before your run, sipped gradually. Your urine should be pale straw colour at the start. Don’t chug a litre right before running — it’ll slosh in your stomach and your kidneys will just flush the excess.
Should I wear a hat or visor when running in the sun? A visor is better than a cap in extreme heat because it shades your face while allowing heat to escape from the top of your head. A cap traps heat but provides more sun protection. Either is better than nothing — sunburn on your scalp and face is painful and impairs cooling.
How do I know if I’m dehydrated during a run? Thirst is the first sign, but by the time you feel thirsty you’re already mildly dehydrated. Other signs include dark urine, dry mouth, headache, and a sudden decline in pace or energy. Weigh yourself before and after runs — anything over 2% body weight loss indicates concerning dehydration.
Can I still do speed work in hot weather? Yes, but adjust expectations. Do intervals at the same effort level rather than the same pace — they’ll be slower, and that’s fine. Consider doing speed work early in the morning when it’s coolest, or on a treadmill with air conditioning if the heat is extreme.