"It's 25 degrees, is that too hot to run?" has no answer, and that is not pedantry — it is the whole problem. Twenty-five degrees in dry air and twenty-five degrees on a saturated morning are different runs, and the difference is larger than most of the training decisions people agonise over.
What follows is the rule coaches actually use, what each band of it costs you, and where the line sits between a slower run and a medical problem.
Why the thermometer is not enough
You do not slow down in the heat because the air is warm. You slow down because you cannot shed the heat you are producing, and at running intensity almost all of that is shed by evaporating sweat off your skin. If the air is already close to saturated, that sweat cannot evaporate — it runs off you instead, and you get the fluid loss with none of the cooling.
Dew point measures that directly. It is the temperature the air would have to cool to before its moisture condensed, so it maps straight onto how much water the air is already carrying. Below 10°C is dry air you will barely notice. Above 20°C you will feel in the first mile. Above 24°C, evaporative cooling is badly compromised regardless of how fit you are.
Relative humidity is the wrong number
80% humidity at 12°C gives a dew point of 8.7°C and an index of 101 — no meaningful penalty. The same 80% humidity at 25°C gives a dew point of 21.3°C and an index of 147, which costs 3-4.5% of your pace. Identical humidity reading, completely different run. That is why forecasts showing humidity are close to useless for a runner, and why the first thing to do is convert it.
The rule: temperature plus dew point
Add air temperature and dew point together, both in Fahrenheit. The resulting number maps onto bands that have been used in coaching for decades, and those bands are what the Verus heat-adjusted pace calculator runs on — so any number here can be checked against the tool.
| Index (°F) | What it means | Pace penalty | What to do |
|---|---|---|---|
| Under 100 | No real penalty | 0% | Race your goal pace. Conditions are not the limiting factor. |
| 100-110 | Barely noticeable | 0-0.5% | Goal pace, but drink to thirst and be honest in the last quarter. |
| 110-120 | Noticeable | 0.5-1% | Ease off slightly for the first few kilometres. You get it back by not blowing up. |
| 120-130 | Working against you | 1-2% | Start at the slow end of the adjusted range. Fluid at every station, not every other one. |
| 130-140 | Hard | 2-3% | The adjusted pace is the plan, not a fallback. Run to effort, not to pace. |
| 140-150 | Very hard | 3-4.5% | Drop the time goal. Use every sponge and ice point, and slow down early rather than late. |
| 150-165 | Severe | 4.5-6% | Finishing well is the goal. Walk the aid stations and keep drinking. |
| Over 165 | Dangerous | 6-10%+ | The range where races get shortened or cancelled. Move it to dawn, shorten it, or do not run it. |
Two things are worth saying about this table. Every entry is a range, deliberately — it is a coaching heuristic derived from race-day observation, not a validated physiological model, and two runners in the same race and the same conditions routinely differ by more than the width of a band. And the top band is open-ended on purpose: past about 165 the honest answer stops being a percentage.
What real days score
Abstract bands are hard to feel. These are worked examples, all computed with the same model.
| Conditions | Air / dew point | Index | Penalty |
|---|---|---|---|
| Cool British spring morning | 10°C / 6°C | 93 | None |
| Typical UK race day | 15°C / 10°C | 109 | 0-0.5% |
| Warm UK summer morning | 20°C / 14°C | 125 | 1-2% |
| Hot but dry | 32°C / 10°C | 140 | 2-3% |
| UK heatwave afternoon | 30°C / 16°C | 147 | 3-4.5% |
| Hot and humid | 32°C / 22°C | 161 | 4.5-6% |
| Tropical, saturated | 34°C / 26°C | 172 | Dangerous |
Compare rows four and six. Both are 32°C. One costs you 2-3%, the other 4.5-6% and puts you one band from the point where events get cancelled. Anyone who has run in both will recognise the difference immediately, and no thermometer would have told you.
How often does this actually bite in Britain?
Less often than the anxiety suggests, and that is worth knowing. From eleven years of hourly records for London, the dawn index runs from 74 in January to 113 in July — which puts every single month of the British year in the bottom three bands at 06:00.
| Month | London index at 06:00 | At 10:00 | Penalty at 10:00 |
|---|---|---|---|
| January | 74 | 76 | None |
| April | 81 | 92 | None |
| June | 107 | 116 | 0.5-1% |
| July | 113 | 122 | 0.5-1% |
| August | 112 | 122 | 0.5-1% |
| October | 98 | 104 | 0-0.5% |
So on an average British day, heat is not your problem — even a July mid-morning run costs under 1%. The British heat problem is not the average, it is the exception: the handful of days a year when the number jumps into the 130s and 140s, arriving in a country with no acclimatisation, no shade culture and races scheduled as though it will be 15°C.
That is a genuinely different risk profile from a hot country, and it cuts the wrong way. A runner in a permanently hot climate is acclimatised, starts at dawn by habit, and plans around it. A British runner meets 30°C four times a year with none of that, which is why UK summer race incidents cluster on exactly those days.
Acclimatisation changes the number
Ten to fourteen days of training in the heat produces measurable adaptation: plasma volume expands, you start sweating earlier and more freely, your sweat becomes more dilute so you lose less salt, and your heart rate at a given effort drops.
It reduces the penalty rather than removing it. In practice that means moving to the better half of the same band rather than dropping a band — which is why the heat calculator has an acclimatisation option that does exactly that, and why a fortnight of hot-weather training before a hot race is worth more than any session you could do instead.
Where it stops being about pace
Everything above concerns running slower. Exertional heat illness is a different thing, it develops faster than people expect, and it is a medical emergency rather than a pacing problem.
Stop and get help if
You get chills or goosebumps in the heat, you stop sweating, you become confused, disoriented or unusually irritable, you develop a headache with nausea, or your coordination goes. Get into shade, get cool, and get someone qualified to look at you. None of these are things to run through.
The practical thresholds most people should hold to, regardless of what the index says:
- Above about 165 on the index, move the session, shorten it, or cancel it. That is the range in which races themselves get shortened or called off.
- Any run where you feel worse at a slower pace than you did at a faster one is a run to stop.
- If you are not acclimatised and the index is over 140, treat it as a heat-tolerance session rather than a workout, and set the expectation before you start.
- Verus is a running site, not a medical one. If you have any condition affecting heat tolerance, that is a conversation for a doctor rather than a calculator.
Running well in it
- 1 Move the run to first light. This is the single largest lever available, and it is bigger than any adaptation you can make in a fortnight.
- 2 Convert the pace before you go, not after it goes wrong. Knowing the target is 3% slower makes it a plan; discovering it at eight miles makes it a bad day.
- 3 Run to effort. Your watch is reporting the weather at least as loudly as your fitness.
- 4 Drink before you are thirsty, and use electrolytes rather than plain water on long efforts.
- 5 Choose shade as a route input rather than a bonus. A tree-lined loop and an exposed seafront at the same hour are not the same run.
- 6 Take planned walk breaks on long efforts. In hard conditions they usually produce a faster overall time, not a slower one.