The "Soup" Season: What Really Happens When You Run in Humidity

There is a specific kind of dread known only to runners waking up for a summer morning long run, stepping outside, and realizing the air feels less like oxygen and more like warm bathwater.

Running in high humidity is a completely different beast than running in dry heat. When the air is saturated with moisture, your body's primary cooling mechanism breaks down. But if you handle it right, "soup season" can become your secret training weapon.

Here is what is actually happening to your body when you run through the humidity, how to adjust your expectations, and why it is shockingly similar to training at high altitudes.

The External Reality vs. Internal Chaos

To understand why a 75°F (24°C) day with 90% humidity feels significantly harder than a 90°F (32°C) day with 15% humidity, we have to look at how humans stay cool.

Externally: The Sweat Trap

When you run, your muscles generate massive amounts of heat. To dump that heat, you sweat. In dry air, that sweat evaporates off your skin, which pulls the heat away with it.

But when the relative humidity is high, the air is already holding as much water vapor as it can. Your sweat has nowhere to go. Instead of evaporating, it just pools on your skin and drips off. You look incredibly sweaty, but your body isn't actually cooling down.

Internally: The Cardiac Tug-of-War

Because evaporation fails, your core temperature starts to spike. Your brain triggers a high-stakes survival tactic: it redirects blood flow away from your working muscles and sends it to your skin in a desperate attempt to radiate heat outward.

This creates an intense internal bottleneck:

• Less oxygenated blood reaches your running muscles, meaning they have to work harder just to maintain your usual pace.

• Your stroke volume (the amount of blood pumped per beat) drops because you are losing fluid volume through heavy sweating.

• Your heart rate skyrockets to compensate for the lower stroke volume, trying to pump fewer reserves of blood at a much faster rate.

  
  

Why Humidity is the "Poor Man’s Altitude Training"

If you’ve ever trained at altitude, you know the feeling of gasping for air while running a pace that usually feels like a warm-up. Believe it or not, humidity triggers almost the exact same physiological stress. Legendary distance coaches often refer to summer heat and humidity training as "the poor man's altitude."

Here is how they stack up:

Feature	Altitude Training	Humidity Training
The Core Challenge	Less oxygen available in the air.	Body is starved of oxygen because blood is diverted to the skin for cooling.
Perceived Effort	Very high; standard paces feel unsustainable.	Very high; heart rate is 10–20 beats higher at normal paces.
The Biological Reward	Triggers an increase in red blood cells to carry more oxygen.	Expands your plasma volume (the liquid part of your blood) by up to 10% to better handle cooling and circulation.

When you spend a summer grinding through high humidity, your body builds a larger tank. When autumn finally arrives and the dew point drops, your expanded blood plasma volume means your heart can deliver massive amounts of oxygen to your muscles with ease. You will feel like you're flying.

The Golden Rule: Throw Away Your Pace Goals

If you try to run your standard, cool-weather paces when the humidity is pushing past 70%, you are setting yourself up for an incredibly miserable workout—or worse, heat illness.

As a general rule of thumb, you can expect your pace to slow down by 20 to 50 seconds per mile (or 12 to 30 seconds per kilometer) depending on how high the combined temperature and humidity are.

Instead of watching your GPS watch, switch your metric to Rate of Perceived Exertion (RPE) or heart rate zones. If your easy runs are usually an 8:30/mile pace at a 140 bpm heart rate, and suddenly you are running a 9:20/mile pace at that same 140 bpm, let it happen. Your body doesn't know what the watch says; it only knows how hard it is working.

Safety First: Knowing Your Limits

While pushing through a humid run builds mental toughness and physical endurance, there is a fine line between a hard workout and a medical emergency.

Before heading out, look up the Heat Index—which factors both air temperature and relative humidity together—rather than just the raw temperature.

Keep an eye out for the warning signs of heat exhaustion:

• Feeling dizzy, lightheaded, or suddenly nauseous.

• Developing a throbbing headache.

• Chills or goosebumps on your skin, despite it being blazing hot outside.

• Sudden confusion or a total inability to focus.

  
  

If you hit any of these markers, stop running immediately, find shade, sip water, and do not hesitate to call for a ride home.

Survival Tips for the Humid Months

1. Hydrate way ahead of time: Chugging a bottle of water 10 minutes before you walk out the door will just slosh around in your stomach. Start aggressively hydrating with electrolytes the day before a long, humid run.

2. Seek out shade: Concrete and asphalt absorb and radiate heat. If you can, take your runs to tree-covered dirt trails or greenways, which can feel a few degrees cooler.

3. Run early or late: The sun's radiation makes a humid day feel twice as oppressive. Aim for dawn or dusk to eliminate the direct solar beating.