Check if weather suits a run.
Distance runners perform best around 10-12 C, and performance degrades far faster above that than below because heat dissipation, not warmth, is the limiting factor. Humidity only matters once it is warm enough to be sweating heavily, which is why its penalty is conditional. The pace slowdown reflects the well-documented relationship between heat and endurance performance, and fluid need scales with the same variables.
Running conditions score
Score = 100 - |temp - 11| x (3.2 above, 1.6 below) - max(0, RH - 50) x 0.35 when warm - wind x 0.5; pace slowdown = heat excess x 0.45 + humidity excess x 0.05
Exertion in extreme heat, cold or humidity carries real health risks including heat stroke and hypothermia. Adjust plans conservatively and seek medical advice if you have cardiovascular or respiratory conditions.
Around 10-12 C for sustained efforts. Marathon performance data show times deteriorating steadily above roughly 15 C.
Commonly 2-5% slower at 25 C than at 11 C, and more in high humidity. Running to effort rather than pace is the practical approach.
Roughly 400-800 ml an hour in moderate conditions, more in heat and humidity. Drinking to thirst works for runs under an hour.