Scenario

Running in the heat

When is it too hot to run outside?

Add temperature and dew point in Fahrenheit. Under 100 there is nothing to adjust; from 130 expect to run 2–3% slower; above 180 the published guidance is not to run hard at all. Dew point matters more than temperature because it decides whether sweat can still evaporate.

The number to look at

Not the temperature. The dew point.

Relative humidity is a moving target: at dawn it might read 90% and by 3pm 45%, with exactly the same amount of water in the air. What changed was the temperature, not the moisture. Dew point expresses the moisture directly, in degrees, and it barely moves through the day — which makes it the right number for comparing this morning's run with last Tuesday's.

Dew point as runners experience it.
Dew point	Feels like
Below 10°C · 50°F	Dry. Nothing to think about.
10–16°C · 50–60°F	Noticeable on a hard session, fine easy.
16–20°C · 60–68°F	Heavy. Paces slip and effort rises.
20–24°C · 68–75°F	Oppressive. Quality sessions are compromised.
Above 24°C · 75°F	Sweat stops working properly. Easy running only.

The published coaching table adds temperature and dew point together in Fahrenheit, which combines both in one number. The pace calculator applies it.

What to change, in order

The target pace. Take the adjustment from the table and treat it as the new goal, not as a failure.
The recovery, upward. Longer recoveries in the heat, not shorter. The point of a session is the work, and incomplete recovery makes the later reps worse rather than harder in a useful way.
The time of day. Dawn is the only genuinely cool window in most climates, and the hottest part of the day explains why evening is not its mirror image.
The route. Shade, and away from asphalt where possible — a tree-lined path can be several degrees cooler than a road, and the road is radiating at you as well as heating the air.
The clothing. Light, loose, pale. Anything that traps moisture against the skin is working against the only cooling mechanism available.
The session itself, last. Swapping a threshold run for an easy hour is a legitimate coaching decision, not a concession.

Fluids, briefly and honestly

Sweat rates during hard running in heat commonly reach 1 to 2 litres an hour, which cannot be replaced while running. What is achievable is starting hydrated, drinking to thirst during, and replacing afterwards with fluid that contains sodium — because heavy sweating loses salt as well as water.

Two things worth avoiding: arriving dehydrated from yesterday, and drinking large volumes of plain water on a long hot run, which dilutes sodium further.

The fortnight that changes everything

Heat adaptation is real, fast and large. Within 10 to 14 days of regular exposure, plasma volume expands, sweating starts at a lower core temperature and at a higher rate, and sweat becomes more dilute. The same run at the same conditions gets measurably easier.

It also disappears in about the same time. A fortnight indoors or in a cool spell puts you back near the start. Heat acclimatization has the mechanism and the schedule.

When to stop

Not "when it feels hard" — running in heat always feels hard. Stop when the body starts giving signals that are qualitatively different from effort:

Chills, goosebumps or shivering while hot.
Sweating stopping.
Skin turning cold and clammy.
Nausea with headache.
Confusion about pace, route or how far you have gone.

Those belong to the range that ends in heat stroke, and in fit runners they can arrive without the intermediate stages. The session is not worth it.

Common questions

Why does humidity ruin a run more than heat?

Because running produces a great deal of heat and nearly all of it leaves by sweat evaporating. Humid air accepts less water vapour, so the sweat sits on the skin doing nothing. The heat has to go somewhere, so core temperature rises and heart rate climbs to move blood to the skin — which is why the same pace feels harder for no gain in speed.

How much slower should I run?

Use the temperature-plus-dew-point table. Broadly, a sum of 130°F suggests 2–3% slower, 150°F suggests 4.5–6%, and 170°F suggests 8–10%. On a five-minute mile pace, 8% is around 24 seconds. The pace calculator does the arithmetic.

Should I still do intervals in the heat?

You can, with two changes: adjust the target pace and lengthen the recovery rather than shortening it. What does not work is holding both the pace and the recovery, which converts a controlled session into a race and produces the classic mid-session collapse.

Is it better to run in the morning or the evening?

Morning, in most climates. The dew point barely falls overnight, but the temperature does, and surfaces have given up the previous day's heat. Evening air may be cooler than mid-afternoon but roads and pavements are still radiating, and the humidity is often higher.

When should I abandon a run?

If you stop sweating, get chills or goosebumps in the heat, feel your skin go clammy and cold, get a headache with nausea, or find yourself confused about the route or the pace. Those are not "push through" signals — they are the early part of the range that ends in heat stroke. Stop, get to shade, get fluids.

Sources

NIOSH (2016). Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments
Périard, J. D. et al. (2015). Adaptations and mechanisms of human heat acclimation — Scandinavian Journal of Medicine & Science in Sports 25(S1)
NWS Weather Prediction Center — The Heat Index Equation

Published 30 July 2026