Saturday, 2 August 2025

The Brutal Truth About Running in 28°C and 90% Humidity


On 2nd August 2025, Swati Mittal laced up before sunrise in Chandigarh, but even at 4:27 AM, the weather had other plans. With temperature at 28°C and humidity at 90%, her morning run turned into a physiological battlefield.

Environmental Stress: How Heat and Humidity Hijack Your Run

Here’s what most runners underestimate: evaporation, not sweating, cools your body. At 90% humidity, sweat doesn’t evaporate, it drips. This means zero cooling, and your core body temperature rises sharply, sometimes by 2–3°C within minutes.

To prevent overheating, your heart goes into overdrive, pumping more blood to the skin for cooling, which in turn elevates heart rate and leaves less blood (and oxygen) available for the working leg muscles. This raises heart rate, drops efficiency, and makes even your easy pace feel like a tempo.  This means that in high heat/humidity, even a moderate pace will send your heart rate much higher than in cool conditions. The “air feels thick” because the body is struggling to cool itself; breathing can feel more labored and every kilometer takes a greater toll.

For every 5°C above ~10°C, endurance running performance degrades by roughly 1–3%, and the slowdown is even greater when high humidity is added. For example, a 10°F (5.5°C) rise above 55°F (13°C) can add on the order of 20–30 seconds per mile to your pace. Real-world data back this up: in one analysis of millions of runs, at 80°F and 90% humidity the average training pace dropped to about 11:07 per mile (~6:53/km), versus ~9:19/mi (5:48/km) on a day with the same temperature but low humidity (runnersworld.com). That is roughly a 16% slowdown simply due to oppressive humidity.

It’s not the distance, it’s the climate that crushes you.

 Swati's Data: What the Numbers Reveal

Metric             Value

Distance             8.06 km

Avg Pace             5:33/km

Avg HR             170 bpm

Max HR             183 bpm

Moving Time     44:44

Elevation Gain    6 m

Calories Burned    450 Cal


Now let’s get TECHIE. Swati usually runs 5:15/km at threshold HR (~165–170 bpm). But here, heart rate reached 176–177 bpm in later kms even though pace slowed to 5:44–5:56/km.

This isn’t loss of fitness. It’s cardiovascular drift – your HR climbs while pace stays same or slows, because your body’s fighting heat.

Performance Degradation: Why Slowed

Running physiology shows:

  1. For every 5°C rise above 10°C, performance drops by 1–3%
  2. Add humidity, and you lose another 15–20% efficiency
  3. Heart rate climbs ~10–15 bpm at the same pace in hot-humid conditions
Real-World Example from Swati’s Splits:

KM     Pace        HR (bpm)         Comment
1 5:25                140                 Low HR, good start
2 5:15                 169                 Already rising
4 5:56                 171                 HR stays high despite slowdown
6–8 ~5:25–5:44 176–177         Red-zone HR at aerobic pace

This is heat’s invisible effect. You feel like your engine is revving at 7,000 rpm, but you're barely cruising.

Why 64+ km/week Feels Like 80+

Swati’s training volume is ~64.5 km/week – an ideal base. But in this heat, that volume feels like 80–90 km/week in cooler weather. Why?
  1. Cumulative dehydration
  2. Higher core body temperature
  3. Recovery delay due to thermal stress
  4. Increased perceived effort (RPE)
Unless the plan adapts, fatigue builds sneakily.

Psychological Fatigue and “The Wall”

That moment when your mind says, “Bas, ab aur nahi.” - Jo ki Swati ne 2-4 baar bola hoga.. He he he he...
It isn’t lack of willpower – it’s your body’s central governor system kicking in to protect vital organs. Once your core temp hits critical, performance tanks to prevent collapse.

That’s what Swati experienced mid-run: HR rising, pace dipping – despite no terrain change. It’s the physiology hitting the brakes.


How to Adapt: Smart Strategies for the Heat

1. Train by Heart Rate, Not Pace
  • Use Zone 2 heart rate as anchor
  • On humid days, run slower but keep HR in the safe aerobic range
  • E.g. Instead of 5:15/km, settle for 5:40–5:50/km if HR hits >170 bpm too early
2. Run Early, But Not Blindly
  • Even at 4:27 AM, dew point can crush recovery
  • Choose shaded, breezy routes or even treadmill runs for intervals
3. Cooling Hacks
  • Wet your hat, use ice bandanas, or pour water mid-run
  • Pre-run: drink ice slurry or chilled water to lower core temp
4. Adjust Workout Types
  • Replace tempo runs with intervals with long rest
  • Long runs: consider cutting volume or adding walk breaks
  • Don’t chase PBs in heat – use it for aerobic base building
Hydration + Electrolyte Protocol

Running in 90% humidity = sweating without cooling. You lose water + sodium.

Practical Rule:
  • Pre-run: 500 mL sports drink or water + salt
  • During: 150–200 mL every 20 mins (add salt tabs or electrolytes)
  • Post-run: weigh yourself and rehydrate 1.5× fluid loss
Use oral rehydration salts (ORS) if run >60 min in heat
Click here for LEAP Salt tabs - Discount Code: Amarjit15
Long-Term Advantage: Training in Heat Builds Champions

Though brutal, running in hot, humid conditions forces plasma volume expansion, better thermoregulation, and mental resilience.

“Train hard in the heat, race strong in the cold.” – Heat-acclimation proverb
Once the weather cools, the same pace will feel like flying. So if you’re grinding now, you’re investing in future performance.

Swati’s Run in Context
  • Despite extreme humidity and 28°C:
  • She held consistent effort (avg 170 bpm)
  • She showed smart pacing: slowed slightly as HR climbed
  • Her weekly volume shows strong aerobic base
  • She should now hydrate more, recover better, and adjust paces for weather
Run by Effort, Not Ego
Let this be a reminder for every runner chasing watch metrics:
“Your Garmin doesn’t know the weather. Your heart does.” – Technical Muscle (Mammy Kasam ye mera apna quote hai, google se copy paste nahi mara... :)
In the heat, respect the elements. Let your heart rate, breathing, and sweat tell the story—not just your splits.

Sources:
Che Muhamed et al., Temperature (Austin) – 2016 study on humidity’s impact: high RH (60–70%) at 31°C raised core temp and heart rate, and reduced run time to exhaustion

Runners Connect – Summary of heat/humidity research: hot & humid conditions caused double the performance drop of hot & dry in one test; dew point >75°F can slow pace by 12–15%

Runner’s World (Raskin, 2025) – Training data from Strava: at 80°F, pace averaged 9:19/mi at 30% RH vs 11:07/mi at >90% RH (dramatic slowing)

Precision Hydration (Andy Blow) – Explains why humidity impairs cooling (sweat can’t evaporate) and, highlights importance of pacing (e.g. Frodeno’s 15–20% power reduction in Kona)

Run & Become (personal account) – Notes that at 65% RH it’s “very hard” to lose heat, 75% RH “almost impossible”; heart rate can jump by ~10 bpm going from 50% to 90% humidity, and similarly +10 bpm from ~23°C to 32°C

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2 comments:

  1. Nandini2 August 2025 at 18:38

    It's such an eye opener with full explanation based on scientific criteria. All facts stated well with justified reasons. A well drawn comparison is also a good tactical phenomenon which has helped to draw towards the stated condition.
    Looking forward for more such information.

    ReplyDelete
    Replies
    1. Amarjit Singh2 August 2025 at 19:56

      Thanks a lot. Keep reading keep supporting. Do share this article with your fellow running buddies.

      Delete

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