Every once in a while, especially on humid days, you’ll sit on an airplane waiting for takeoff and see fog pouring from the ceiling vents like a low-budget concert. Someone always panics and asks if something’s on fire. It’s not smoke—it’s condensation. You’re looking at thermodynamics and psychrometrics in action.

So, why does it happen, and why only sometimes?

To explain that, we’ve got to talk about how airplane air conditioning works—and it’s not what you think.

Airplane A/C Isn’t Like Building A/C

Airliners don’t have a compressor, condenser, TXV, or evaporator coil like you’d find in a rooftop unit or mini-split. They use air-cycle refrigeration: cooling air with air.

The air starts as bleed air, which is high-pressure, high-temperature air pulled from the jet engine’s compressor section or the APU when the engines aren’t running. That air can be over 400°F. The system cools it through two heat exchangers, compresses it again, and then expands it through a turbine, dropping both its temperature and pressure.

That cold, dry air is mixed with some warmer return air to reach the target cabin temperature. The whole setup is called a PACK (Pneumatic Air Conditioning Kit), and it’s part of what’s known as the environmental control system on a jet.

There’s no refrigerant. The “working fluid” is just air. When you expand air quickly, it cools—the same principle John Gorrie used in the 1840s when he built the world’s first mechanical cooling system in Florida.

He compressed air, cooled it with water, and then expanded it through a valve to produce cold air (and sometimes even ice). He was proving that air could be its own refrigerant. Airplanes still use that principle today; they just use turbines instead of pistons.

Why It Fogs

Now that you know where the cold air comes from, the “fog show” makes sense.

Here’s what’s happening step by step:

The Setup

Before takeoff, the cabin doors have been open. Warm, humid air from outside fills the cabin. Let’s say it’s 85°F and 70% RH outside—a dew point around 74°F.

The Cold Air Arrives

The PACKs start feeding in conditioned air at roughly 40°F and very low humidity (maybe 10% RH).

The Mix Zone

The cold, dry supply air mixes with the warm, humid cabin air. When those air masses mix, the resulting dry-bulb temperature can easily fall below the cabin air’s dew point.

Condensation Happens

When the local air temperature falls below the dew point, water vapor condenses into visible droplets: fog.

It’s the same thing you see when you open a walk-in cooler door on a muggy day or watch your breath in winter. The droplets hang in the air long enough for light to scatter, and your eyes see a cloud.

Once the cabin air dries out, the dew point drops, the fog disappears, and everyone relaxes.

Dew Point, Dry-Bulb, and RH% in Play

Let’s tie this to psychrometrics—because this is exactly what the chart is for.

• Dry-bulb temperature (DB) – the actual air temperature, what you’d read on a thermometer.
  
• Relative humidity (RH) – how “full” the air is with water vapor, expressed as a percent.
  
• Dew point (DP) – the temperature at which the air can no longer hold its water vapor, where condensation begins.

In our airplane example:

• Cabin air (boarding): 85°F DB, 70% RH → DP ≈ 74°F
  
• Conditioned supply air: 40°F DB, 10% RH → DP ≈ 10°F

When you mix those two air masses, the resulting temperature in the supply plume might be around 60°F. That’s well below 74°F (the dew point of the humid cabin air), so the vapor condenses.

You could plot it straight on a psych chart and watch the process curve cross the saturation line; that’s the same math we use for dehumidification, but it’s happening in midair instead of on a coil.

What “Ram Air” Means

A quick side note: you’ll often hear the term ram air when talking about aircraft A/C. It just means the outside airflow that passes through the heat exchangers to cool the bleed air before it enters the turbine.

At cruising speed, the aircraft’s motion pushes air through those exchangers at high velocity—no fan needed. On the ground, where there’s no forward motion, fans pull air through instead.

Think of ram air as the airplane’s version of condenser airflow, just powered by flight instead of a motor.

The Cousins: Other “Air-Only” Cooling Systems

This “cool by expansion” method isn’t unique to airplanes. There are a few other systems that prove you can move heat without refrigerant.

1. John Gorrie’s Machine (1840s)

Gorrie’s design compressed air, cooled it, then expanded it. The expansion caused a pressure and temperature drop, making cold air for cooling hospital rooms. He used pure physics—no phase change, no chemical refrigerant.

2. Vortex Tubes (1930s)

A vortex tube takes compressed air and spins it at high speed inside a chamber. It naturally separates into two streams, one hot and one cold, because of angular momentum. The cold stream can reach -40°F or lower.

It’s inefficient but simple. You’ll find them cooling machining tools or control boxes in factories. No moving parts, just compressed air.

3. Air-Cycle Machines (Modern Aviation)

The air-cycle machine takes Gorrie’s physics and adds turbines, bearings, and precise control. It uses air compression and expansion, not angular separation like a vortex tube, but both are “air-only” cooling systems.

4. Vapor-Compression Systems (Everything Else)

Our normal HVAC systems rely on refrigerant phase change—liquid to vapor and back—to move large amounts of heat efficiently. The latent heat of vaporization lets us transfer thousands of BTUs with only small temperature differences.

Phase change makes modern systems efficient, but it isn’t required to make cooling happen. The airplane system proves that.

Efficiency vs. Simplicity

An air-cycle machine isn’t very efficient by HVAC standards. Air has low density and specific heat capacity, so it takes a lot of compression work to get a meaningful temperature drop.

But airplanes already have compressed air available from the engines, and they can’t afford the weight, flammability, or leak risks of refrigerants. So air-cycle systems win on simplicity and safety, even though they lose on efficiency.

Same trade-off John Gorrie faced 180 years ago: efficiency versus practicality.

Why It Matters for HVAC Techs

The same physics that fog a jetliner cabin are what we deal with every day in buildings and equipment rooms.

Mix warm humid air with cold dry air, and if the mixed temperature crosses the dew point, condensation shows up in ducts, on grilles, or on your beer can.

In a building, we prevent that with insulation and vapor barriers. On a plane, it’s harmless and temporary—a psychrometric flash show before the cabin stabilizes.

It’s a great teaching tool for understanding dew point control, latent load, and air mixing, all things that make or break humidity control in HVAC systems.

The Big Picture

Every cooling system, whether it’s a jetliner, a mini-split, or a walk-in cooler, relies on the same basic sequence:

1. Compress a vapor (add energy and pressure).
   
2. Reject heat to the surroundings.
   
3. Expand it (drop pressure and temperature).
   
4. Absorb heat from the space you want to cool.
   

Airplanes just skip the refrigerant and do it all with air. The fog you see before takeoff is that last step: the cold, expanded air absorbing heat and crossing the dew point of humid cabin air.

It’s pure thermodynamics, happening right in front of you.

Final Thoughts

The next time you’re on a plane and see mist curling through the cabin, remember:

• It’s not smoke, it’s water vapor condensing because the cold supply air cooled below the dew point.
  
• The aircraft’s “A/C” is actually an air-cycle refrigeration system, a direct descendant of John Gorrie’s first air machine.
  
• The system uses ram air for heat rejection and turbine expansion for cooling, no refrigerant required.
  
• The fog is a temporary psychrometric event, not a mechanical failure.
  

Every HVAC tech can appreciate that what looks like “smoke” to a passenger is really just dew point, dry-bulb, and relative humidity doing their thing—the same rules we live by every day.

The airplane just happens to be the most expensive psychrometric lab in the sky.

—Bryan