Running in summer: the real problem
You have a training plan. You committed to running 4 times a week before September's race. And now it's 90°F out and the weather app is showing 10 days of heat advisory.
Do you skip the sessions? Do you push the evening runs without adjusting pace? Do you just grind through like usual?
The physiological reality: training in heat without adjusting your approach means you're either putting yourself at risk, adapting much more slowly than expected — or both. But skipping sessions because it's hot is also regression. There's a middle path, and that's what this guide covers.
Why heat tanks your splits
In the heat, your body has two simultaneous priorities: supplying your muscles with oxygen (to run) and bringing blood to your skin surface (to cool down). These two demands compete for the same blood volume.
The most widely used rule of thumb among running coaches: add 30–90 seconds per mile to your target pace for every 5°C above 15°C (roughly every 9°F above 60°F). If you were comfortably running 9:00/mile at 60°F, expect 9:30–10:30/mile at 80–90°F at the same perceived effort.
That's not regression. It's adaptive physiology. Your fitness hasn't changed — the thermal equation has.
When to run in summer
This is probably the highest-impact question. The 11am–5pm window in peak summer is a no-go for intense sessions. Direct sun, hot asphalt, and peak temperature create a combination that can make a 6-mile run genuinely dangerous.
Your options:
Before 8am. Temperature is often 9–18°F below the daily peak. Asphalt is still cool. You can often run close to your normal paces.
After 7pm. Temperature is dropping. Direct sunlight is reduced. The caveat: asphalt can stay hot long after sunset on big heat days — prioritize shaded routes or grass paths where possible.
Hydration: the practical formula
Under normal conditions, runners going under 45 minutes often don't need water mid-run. In the heat, that rule changes.
The baseline: 400–800ml per hour depending on intensity and ambient heat. For sessions under 60 minutes, water is sufficient. Beyond that, electrolytes — particularly sodium — become necessary. Sweat is salty: at an hour or more in heat, you're not only losing water.
Practical options: a lightweight running belt for 60–90 minute sessions. For long runs, plan resupply points or carry a soft flask. And if you're doing loops, stash a bottle at your starting point.
What to wear: the details that actually matter
A few non-negotiables for summer running:
Light colors. White and pastels reflect radiant heat. Black absorbs it. In a heatwave, the perceived temperature difference between a black kit and white can be 5–10°F.
Technical moisture-wicking fabrics. No cotton — it absorbs sweat and stays wet. Modern synthetics (polyester, nylon) or merino wool wick moisture away from the skin.
Sunscreen. SPF 50+ on all exposed areas, reapplied if your run goes over 90 minutes. UV doesn't care that you're training.
Signs that mean stop immediately
Heat can escalate to medical emergencies. These signs require stopping immediately and finding a cool area:
Cessation of sweating (paradoxically a warning sign in heat), confusion or disorientation, unsteady gait, intense nausea, or dry and burning skin sensation. These are signs of heat stroke — a medical emergency. Stop, cool down, and call for help if symptoms persist.
Sources
Runner's World, How Heat and Humidity Affect Your Running — read the article NIH/PubMed, Heat acclimatization and exercise performance — read the study