Running in 85°F heat at the same pace as a 55°F day requires substantially more physiological effort. Your body diverts blood from muscles to skin for cooling, your heart rate rises to compensate, and your sweat rate increases to maintain core temperature. The result: the same pace feels significantly harder.
This isn’t a fitness problem. It’s thermodynamics.
How Much Heat Costs You
Pacewright adjusts pace based on dew point — a better indicator of heat stress than air temperature alone, because dew point accounts for humidity:
| Dew Point | Pace Adjustment |
|---|---|
| Below 55°F | 0% — ideal conditions |
| 55-60°F | +1% slower |
| 60-65°F | +2.5% slower |
| 65-70°F | +4.5% slower |
| 70-75°F | +6.5% slower |
| Above 75°F | +9% slower |
At a dew point of 72°F, your 9:00/mile easy pace should become approximately 9:35/mile for the same physiological effort. Running at your cool-weather pace in heat means running at a higher effort level — which turns easy runs into moderate runs and moderate runs into hard runs.
Gear
Light colors. Dark fabrics absorb more solar radiation. White or light-colored shirts reflect it. The difference in perceived heat can be significant on sunny days.
Moisture-wicking fabrics. Cotton absorbs sweat and holds it against your skin, reducing evaporative cooling. Technical fabrics wick moisture to the surface where it can evaporate. This is the single most important clothing choice for hot-weather running.
Minimal coverage. Singlets over t-shirts. Shorts over tights. Less fabric means more skin exposed for evaporative cooling. Some runners perform better in just a sports bra/shorts combination when conditions are extreme.
Hat or visor. A light-colored hat shields your face from direct sun. A visor provides shade while allowing heat to escape from the top of your head. Either works — the key is sun protection without heat trapping.
Sunscreen. Non-negotiable for runs over 30 minutes in direct sun. Choose a sport-specific formula that won’t run into your eyes with sweat.
Hydration
Pre-hydrate. Good hydration starts hours before the run, not at the water fountain. Drink 16-20 oz of water 2-3 hours before running. A good indicator: urine should be pale yellow, not clear (overhydrated) or dark (dehydrated).
During runs over 60 minutes. 4-8 oz every 15-20 minutes. This replaces sweat losses without overwhelming your stomach. If running over 90 minutes in heat, add electrolytes (sodium, potassium) to prevent hyponatremia.
After running. Replace what you lost. A rough guide: drink 16-24 oz of fluid for every pound of body weight lost during the run. Weigh yourself before and after a hot run to calibrate your sweat rate.
Don’t overdrink. Hyponatremia (dangerously low blood sodium from excessive water intake) is a real risk, particularly for slower runners in heat who drink at every aid station. Thirst is a reasonable guide — drink when thirsty, not on a forced schedule.
Heat Acclimatization
Your body adapts to heat exposure over 10-14 days:
- Sweating starts earlier and increases in volume
- Sweat becomes more dilute (conserving electrolytes)
- Heart rate at a given effort decreases
- Core temperature stabilizes at a lower point
- Perceived effort at a given effort drops
You don’t need to train hard in heat to acclimatize — easy running in hot conditions is sufficient. Schedule quality sessions for cooler times (early morning or evening) and let the easy runs in heat do the acclimatization work.
Safety
Stop for these symptoms: Dizziness, confusion, nausea, cessation of sweating, chills, or rapid heartbeat that doesn’t correspond to effort. These are signs of heat exhaustion or heat stroke. Get to shade, stop running, cool down, and seek help.
Time of day matters. 10 AM to 4 PM is typically the hottest window. Early morning (before 7 AM) and evening (after 6 PM) are cooler, but evening runs may have higher humidity. Dawn runs are usually the best option in summer.
Know the dew point. Check it before you run. Above 70°F dew point, even experienced, acclimated runners should reduce intensity and duration. Above 75°F, consider moving the run indoors or taking a rest day.