How much slower in the heat?

You do not overheat because the air is warm. You overheat because you cannot get rid of the heat you are making, and at running intensity almost all of that is shed by evaporating sweat off your skin. If the air is already close to saturated, the sweat has nowhere to go. It runs off you instead of evaporating, you lose the fluid without getting the cooling, and your core temperature climbs anyway.

Relative humidity is a poor way to describe that, because it is relative — it tells you how close the air is to saturation at its current temperature, not how much water it is actually carrying. Eighty per cent humidity on a 5°C British morning is nothing; eighty per cent at 30°C is oppressive. The same number, two completely different runs.

Dew point is absolute. It is the temperature at which the air would have to be cooled for its moisture to condense, so it maps directly onto how much water is in the air and therefore how well evaporation will work. A dew point below 10°C is dry air and you will barely notice it. Above 20°C you will feel it in the first mile. Above 24°C, evaporative cooling is severely limited no matter how fit you are.

The gap is bigger than most runners expect. At 06:00 in August, the archive gives Kingston an air temperature of 24.9°C against a dew point of 23.1°C — nearly saturated. London at the same hour is 14.3°C air, 12.6°C dew. That is an index of 150 against 112: the difference between very hard and noticeable, before you have taken a step.

Heat acclimatisation is one of the best-established adaptations in endurance sport, and it happens surprisingly fast. Ten to fourteen days of training in the heat produces measurable physiological change: plasma volume expands, you begin sweating earlier and more freely, your sweat becomes more dilute so you lose less salt, and your heart rate at a given effort comes down.

The practical consequence is that the same conditions cost an acclimatised runner less than a visitor who flew in three days ago. That is why the calculator has a box for it — and why ticking that box moves you to the better half of the same band rather than removing the penalty. Adaptation reduces the cost of heat. It does not repeal it.

If you are travelling to race somewhere hot, arriving two weeks early is the single most effective thing you can do. If you cannot, the next best is to train in the heat at home — run in the warmest part of the day, overdress deliberately, and accept slower paces for a fortnight in exchange for a better race.