- Heat increases thermoregulatory and cardiovascular stress.
- Wind affects both heat loss and mechanical demand.
- Gradient changes the energetic cost of running substantially.
Heat changes internal effort
Running produces substantial metabolic heat. As temperature, humidity and solar load rise, the body must dissipate more heat. Skin blood flow and sweating increase, while dehydration can add cardiovascular strain. The same pace can therefore produce a higher heart rate and RPE.
Wind is more than headwind
Wind changes aerodynamic resistance and heat exchange. Its effect depends on direction, speed and temperature, which makes universal pace-adjustment formulas overly simplistic.
Gradient changes energy cost
Laboratory studies show clearly different energetic costs when running uphill and downhill. A rolling route can therefore produce the same average pace with a very different physiological load from a flat route.
Why pace still matters
Context should not become an excuse for every poor session. Pace remains a key performance marker in many quality workouts. The useful approach is to add environmental information without overriding clear fatigue, stop or safety signals.
Weather and route stay linked to the workout.
When available, PACE uses temperature, feels-like temperature, humidity, dew point, wind and a minimized route summary with elevation gain/loss, gradient and altitude. Easy and recovery analysis can lean more on HR and RPE; quality sessions can receive bounded context-aware pace tolerance. There is deliberately no universal seconds-per-kilometer correction.
Scientific sources
Bergeron (2014): Heat stress and thermal strain challenges in running.
Junge et al. (2016): Prolonged self-paced exercise in the heat.
Minetti et al. (2002): Energy cost of walking and running at extreme uphill and downhill slopes.