Running parkrun
in the heat

The honest starting point

Hot weather parkrun is not the same event you ran in April. The distance is identical, the course is identical, but your body is working significantly harder to keep cool. Your heart rate will be higher at the same pace, your perceived effort will be up, and your finish time is going to be slower. That is not a fitness problem. That is just physics.

The UK is not built for heat, and neither are most of us. We are conditioned to worry about rain and wind, not 27-degree Saturday mornings. When temperatures climb, the number of people who come unstuck at parkrun rises with them. Most of the time it is just a slow run and a headache. Occasionally it is something more serious.

This guide covers what actually helps.

What happens to your body in the heat

When you run, around 75% of the energy you produce becomes heat rather than movement. Your body needs to get rid of that heat. In normal conditions, sweating does the job. In hot weather, with high ambient temperatures and sometimes high humidity, that system has to work harder.

Your heart rate increases to pump blood to the skin for cooling. This means less blood going to your working muscles, which is why the same pace feels harder. Your core temperature rises faster. You lose more fluid and electrolytes through sweat. All of this compounds across a 5k.

At 9am in a UK heatwave, you might already be starting warm after a poor night's sleep in a hot bedroom. You are already one step behind before the klaxon goes.

Expect to be slower

There is a rough rule of thumb that works well enough for parkrun distances. For every 5 degrees Celsius above 15 degrees, expect to run around 30 seconds per mile slower than your usual pace. At 25 degrees, that is about a minute per mile off your normal time. At 30 degrees, closer to 90 seconds per mile.

On a 5k, that translates to roughly 1 to 2 minutes added to your finish time in warm conditions, and 2 to 4 minutes in genuine heat.

These are rough guides, not guarantees. Humidity matters as much as temperature. A dry 28 degrees is more manageable than a humid 24. Effort-based running, rather than chasing a time, is the sensible approach on hot days.

Hydration

parkrun does not have water stations on course. Five kilometres does not usually require them in normal conditions. In the heat, that changes.

The key is arriving hydrated rather than drinking a lot in the hour before the run. Your kidneys cannot process large amounts of fluid quickly, so flooding your system immediately before a race is not very effective. Drink steadily in the 24 hours beforehand, around 500ml of water two hours before is a reasonable guide, and cut back on alcohol the night before if you can.

After you finish, drink before you feel thirsty. Thirst is a slightly delayed signal. Cold water is fine. Sports drinks with electrolytes are genuinely useful after a hard run in the heat, particularly if you are a heavy sweater.

What to wear

This is simpler than it sometimes gets made out to be. The goal is to keep as much of your body's heat-management system working as possible, and to avoid adding friction or extra heat.

Fabric: lightweight, moisture-wicking technical fabric only. Modern running tops and shorts do an excellent job of moving sweat away from your skin. Cotton holds moisture, clings, and causes chafing. If you are still running in a cotton t-shirt, a hot parkrun is a good reason to change that.

Colour: light colours reflect heat more than dark ones. This is a marginal gain at 5k distances, but on a very hot day it is worth thinking about.

Cap: useful, not essential. The shade on your face reduces your perception of heat and keeps direct sun off your head. A lightweight running cap works well. A heavy baseball cap does not.

Suncream: non-negotiable if you burn easily. The back of the neck is often forgotten. Reapply before you go if you applied it an hour earlier in the house.

Socks: thin technical socks rather than thick cotton ones. Your feet swell slightly in the heat. A thinner sock in your usual shoe keeps things comfortable.

When to ease off completely

There are conditions where the sensible thing to do is to walk the whole thing, or not go at all. This is not defeatism. It is common sense.

If the temperature at 9am is already above 28 degrees, you are about to run in conditions that professional athletes treat with serious caution. If you are older, have a heart condition, are new to running, or are in a heavy training block with tired legs, the risk-reward calculation shifts significantly.

Walking a parkrun is always an option. You still get your token scanned. The time counts. Nobody thinks less of you for it. On a day where the conditions are extreme, a brisk walk might be the most sensible performance decision you make all year.

Before the run

A few things that make a real difference and cost almost nothing.

Start slower than you think you need to. The first kilometre in hot weather feels fine for almost everyone. Kilometres three, four and five are where you pay for going out too hard. Consciously ease back at the start and you will have more left when it matters.

Wet your wrists and neck. These are areas where blood vessels are close to the surface. Running cold water over them before the start, or carrying a small wet cloth, can take the edge off your core temperature meaningfully.

Know the course shade. If you run the same parkrun regularly, you probably know which sections are exposed and which are sheltered. Plan to ease back on the open sections and recover on the shaded ones.

Set a realistic target before you start. This is the most underrated tip on this list. Decide before the klaxon goes that today is not a PB day. Letting go of your normal time expectations removes a lot of the temptation to push harder than the conditions allow.

After the run

Get into shade as quickly as you can. Do not stand around in direct sun waiting for results. If there is a café or indoor space nearby, use it. Drink water steadily rather than in one large go.

Keep moving gently for five to ten minutes rather than sitting or lying down immediately. Blood pooling in your legs after stopping hard can make dizziness significantly worse.

The telltale sign of a good hot-weather run is that you feel fine an hour later. Headache, nausea or prolonged exhaustion after getting home are signs you pushed further than the conditions warranted. Make a note for next time.

Will the event be cancelled?

It can happen. Most events run in hot weather with extra marshals and a slightly adapted briefing. Occasionally, usually in genuine heatwaves rather than just warm days, an event director will decide conditions are unsafe and cancel at short notice.

This is more likely if temperatures are forecast to be extreme and the course has significant exposed sections. Woodland or shaded park courses tend to fare better than open field events.

The best thing you can do is sign up for free cancellation alerts so you know before you make the trip. SaturdayOff monitors UK parkrun cancellations and sends email alerts to subscribers before Saturday morning.

Does the heat get easier over time?

Yes, genuinely. Heat acclimatisation is well-studied. After two to three weeks of regular running in warm conditions, your body adapts: plasma volume increases, you start sweating earlier, your heart rate drops at the same effort, and your core temperature rises more slowly. The first hot parkrun of summer is always the hardest one.

If the hot weather is going to last, keep running in it. Short, easy runs are better than skipping completely. You will feel meaningfully better by week three.

Written by Azeem Ahmad  ·  June 2026.