Running in high humidity

Humidity is the most underrated factor in running. Two days at the same temperature can feel completely different depending on how much moisture is in the air — and the number that captures it is the dew point. The dew point is the temperature at which air becomes saturated; the higher it is, the less your sweat can evaporate, and evaporation is how you cool down.

A rough scale: dew points below 55°F feel comfortable, the 60s start to feel sticky, and anything above about 70°F is oppressive — the air is so wet that sweat just drips instead of cooling you. At that point your core temperature climbs, your heart rate drifts up, and holding your normal pace costs far more effort. Relative humidity percentages are misleading because they depend on temperature; dew point is the honest signal, which is why RunWeather scores it directly.

When the dew point is high, treat pace as the variable and effort as the constant: ease off, run by feel or heart rate, pick the time of day with the lowest dew point (often just before dawn), and hydrate aggressively. The month-by-month breakdowns on each city page show exactly when the sticky season hits where you run.