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2026-05-24 · 4 min read


Training in heat and humidity — how not to die trying


Summer — anywhere tropical, subtropical, or in a US south — is one of the most underrated enemies of the runner. It's not just "you go slower": your body redirects blood to the skin to cool down, you lose electrolytes at the sweat rate, and if you cross the line, you end up in the ER. Here's how to avoid that and keep training usefully.


The physics of the problem


Your body has to hold core temperature near 98.6°F (37°C). When you run, muscle produces ~5x more heat than at rest. You have two pathways to dump it:


1. Convection (cool air moving against the skin).

2. Sweat evaporation (the main one — dissipates ~580 kcal per liter evaporated).


As ambient temperature rises, convection loses efficiency. Above 90-95°F (32-35°C), the only real path is to evaporate sweat. And here humidity hits: at high humidity, sweat doesn't evaporate well — it sits on your skin without cooling you. Result: you drench yourself, you lose water, but core temperature still rises.


How much slower you'll run


Practical rule: for every 9°F (5°C) above 54°F (12°C) of ambient temperature, your Z2-Z3 pace slows 2-3%. At high humidity (>70%), the effect doubles.


Example: if you normally run 8:00/mi in Z2 at 54°F:

  • 72°F / low humidity: ~8:13/mi at the same effort.
  • 82°F / high humidity: ~8:30/mi at the same effort.
  • 90°F / high humidity: ~8:48/mi at the same effort — and you're getting close to the limit.

    • [Vetta's heat/humidity calculator](https://vettatrainer.com/herramientas/temperatura) gives you the exact correction based on Galloway/ACSM. If you're running in summer, use it.


    When NOT to train


    Times when risk outweighs benefit:


  • Heat index >104°F (40°C) (temp + humidity combo). If your weather app says "feels like 108°F", the long run is canceled. Do short and easy early AM or treadmill.
  • No shade available + direct sun between 11am and 4pm. Sunburn + heat stroke are easy.
  • After 24h with <6h sleep or prior dehydration. Your thermoregulation is compromised.
  • Early symptoms: goosebumps without cold, headache, dizziness, sudden sweating stop. Stop and cool down.

    • How to acclimate to heat


    Your body DOES adapt to heat, but it needs gradual, consistent exposure. The process (Heat Acclimation):


  • Days 1-3: you start. HR high, you sweat fast. 30-40 min of Z1-Z2 in heat (not too hard).
  • Days 4-7: your body increases plasma volume (+5-10%). You sweat earlier and more diluted (less sodium lost per liter).
  • Days 8-14: deeper adaptation. Resting HR drops back to baseline. Pace in heat almost returns to normal pace.

    • Practical protocol for racing in hot conditions:

    1. 10-14 days pre-race, expose yourself to heat for 1h/day (training or not).

    2. Higher hydration (3-4 L/day with electrolytes).

    3. Sauna 20-30 min post-session accelerates adaptation (Scoon 2007 studies).


    Hydration: the detail people miss


    Practical rules:

  • Pre-session: 13-20 fl oz of water/electrolytes 2-3h before. 7 fl oz more 15 min before.
  • During (sessions >1h): 17-24 fl oz/h in temperate climates; 27-34 fl oz/h in extreme heat.
  • Electrolytes: if you sweat >1 L/h or more than 1.5h, water alone isn't enough. Sodium (500-700 mg/L), potassium, magnesium.
  • Post-session: replace 125% of weight lost. If you dropped 2.2 lb, drink 2.75 lb (1.25 L).

    • How to measure your sweat rate: weigh yourself naked before a 1h session in similar conditions. Weigh after. The difference + what you drank = your sweat rate.


    Common summer mistakes


    1. "I go out at noon because that's the only time I have". Change it: 6 AM or post-7 PM. The difference is brutal.

    2. Water only on long sessions. After 1.5h electrolytes crash and you start with cramps or headache.

    3. "I accept the heat, I'm tough". Thermoregulation isn't willpower — it's physiology. It can send you to the hospital.

    4. Not lowering intensity. If your plan says "5K pace intervals" and it's 86°F, adjust for temperature (not for stubbornness).

    5. Wrong clothing. Heavy cotton absorbs sweat without evaporating. Synthetic technical + light colors + visor are basics.


    The terrain factor


    If you run trail in summer, add:

  • Asphalt radiates a lot of heat: trail with dirt/shade is 5-9°F cooler than the road.
  • Forest > open field: 80% of UV blocked under trees.
  • Altitude lowers temperature (~3.5°F per 1000 ft). Climb if you can.

    • Vetta and heat


    Vetta's engine has a temperature/humidity correction that adjusts the week's pace targets to your location and expected climate. If you declare you live in a hot area (Mendoza, San Juan in summer, or anywhere in the US south), quality sessions are scheduled in viable windows and target TSS adjusts. To see it in action, [connect Strava here](https://vettatrainer.com/signup).


    Reading


  • *Heat Acclimation in Athletes* — Périard et al. 2015.
  • *Hydration and Performance* — McDermott et al., ACSM position stand.