I saw this quite thick white smoke like thing in the cabin in my last flight, before takeoff that is, while it was taxiing and loading passengers:

A picture from the flight

Video Please watch in HD to best notice the phenomenon.

It seems to be coming from the AC vents of aircraft. I think it's a quite common phenomenon as the cabin crew didn't even pay any attention to this. But I saw it for the first time. What is this?

Aircraft Details : VT-IDE (A 3.5 year old A320)

The departure location was Bhubaneshwar and the approximate weather was:
Temperature : 28 °C
Humidity ~ 85%
Clear Sky, good visibility


Direct answer

This phenomenon is the condensation of air humidity into water droplets due to a large drop in temperature caused by air conditioning. It is similar to fog which appears when humid air is close to a cooler ground. It is also similar to dew which is fog directly on the ground.

Fog and dew appear when air temperature is lowered below its dew point, this is the whole point.

Quick explanation

The cabin was filled with unconditioned air from the outside, warm and nearly saturated in water vapor, and the dew point (see below) was close to the ambient temperature.

The injection of colder conditioned air in the cabin decreased the temperature around the AC vents below the dew point causing invisible water vapor (gas) condensation in visible water droplets (liquid). Because the temperature was decreased only around the AC vents, the water droplets are nearly immediately re-evaporated in warmer air into water vapor.

Cabin air with its high level of water vapor will be circulated into the air conditioning system, and the water separators will remove most of the water from air before it is returning to the cabin. As pointed out by @JanHudec the humidity level will be kept low during the rest of the flight, removing the possibility of condensation.

Another case of condensation:

enter image description here

The branch of physics which studies how liquids and solids boil or condense is named psychrometrics, it revolves around the notion of equilibrium vapor pressure, but for us mere mortals, the notion of dew point is more practical.

Detailed explanation

Why does water vapor condense?

The quantity of water vapor in air is named water saturation ratio (or relative humidity) and is expressed as a percentage. 0% means air is dry, 100% means air is fully saturated and cannot contain more water vapor. If vapor is added then it immediately condenses into water droplets. Contrary to water vapor, water droplets are visible.

Saturation occurs at certain combinations of temperature and pressure. For a given pressure, the higher the temperature, the larger the quantity of vapor than can be present without condensation. So condensation can occur when the temperature decreases.

Dew point

The temperature at which it occurs is called the dew point. When air temperature has not yet reached the dew point, but is close, and the temperature is for some reason decreased below the dew point, then water vapor condensation occurs. The quantity of water vapor at the dew point is called water vapor capacity of air, it increases exponentially with air temperature:

enter image description here

For sake of accuracy: When water vapor is present in air, it is continuously condensed and evaporated. The dew point is the point where condensation occurs at a greater rate than evaporation and water droplets start accumulating.

The dew point may be determined with a psychrometer (a system with dry and wet thermometers):

enter image description here

Pressure, dry and wet temperatures, dew point and relative humidity are linked by the mean of vapor pressure. By knowing three elements, the two other can be determined. So knowing pressure, dry temperature and wet temperature allows to determine the dew point. It exists precalculated charts (Mollier diagrams) to read dew point directly. There are also online calculators.

Dew point and fog/mist

The dew point is important in aviation because if the temperature is close to the dew point over the ground, then you may find fog at this location. When the dew point is very lesser than the ambient temperature, then fog is unlikely to happen. See KLAX ASOS reports, and compare relative humidity, visibility and temperatures:

enter image description here

As indicated when the dew point is close to the temperature, the humidity percentage is high, and the visibility low, with fog occurring.

Air conditioning on commercial aircraft

AC is provided by a system that usually takes high pressure air from the engines, mixes it with air taken from the cabin, and control the temperature and moisture (and pressure as part of the pressurization function) before injecting it in the cabin. Moisture is controlled by condensing water vapor and removing it by centrifugal force. For a full description see ECS for Boeing and ACS for Airbus. Relative humidity value in cabin is kept about 20%.

You case at Bhubaneshwar

The airport weather is 28°C and 85% humidity. Cabin air is assumed to be in these conditions before the AC is switched on and starts conditioning air at 20°C and 20% humidity.

This warm air is clearly in excess of water:

  • Relative humidity value must be decreased from 85% to 20%.
  • Air must be cooled from 28°C to 20°C, cooling decreases the water vapor capacity. From the curve above, saturated air contains 27g of water vapor per kg of air at 28°C, and 17g at 20°C.

Other humidity values:

enter image description here

When air is cooled at 20°C, 6g/kg are condensed into water droplets. When the relative humidity is lowered to 20%, the water vapor capacity is decreased to 3.4 g/kg and 13.6 additional grams are condensed.

Each kg of conditioned air creates 19.6 g of water droplets. A typical cabin of 150m3 containing about 175 kg of air, delivers 3.5 kg of condensed water (that's only for a cabin without passengers, as perspiration and breathing will add more water, continuously):

  • Most of the condensation happens in the AC system and the water is collected by the water separator.
  • Before the cabin is initially cooled, air from the AC system meets the warm and nearly saturated air of the cabin. This triggers condensation in the cabin around the AC outlets. But most of the water evaporates again at some distance and vapor will eventually be collected by the water separators.

Common examples of condensation

  • Frost around the freezer pipes: Air with water vapor enter the fridge when the door is open. When air starts to be cooled below the dew point by the pipes, condensation occurs and ice forms around them.

  • Air exhaled when the weather is cold. Air in our lungs is not saturated due to the high body temperature, but becomes saturated when it starts cooling.

  • Air saturated by water vapor coming from a pressure cooker.

  • As pointed out by @RyanMortensen, saturated air on car windows (when the temperature decreases during the night, or when air is saturated without temperature change, because there is more water/rain in the air that can be evaporated naturally -- Increasing car temperature above the dew point is the solution).

  • Fog and clouds formation: Fog because the ground is colder than the surrounding atmosphere (early in the morning), cloud because air is cooled below the dew point while it climbs (pressure and suddenness play also a role).

  • Dew :-)

  • 1
    $\begingroup$ Please see the update about weather.. $\endgroup$ – anshabhi Jun 7 '16 at 9:47
  • 23
    $\begingroup$ Super-short answer: it's a cloud forming inside your plane. $\endgroup$ – David Richerby Jun 7 '16 at 10:02
  • 1
    $\begingroup$ I was once informed by the Weather Channel that I could expect 103% humidity on an absolutely miserable June day in New York City. I was never sure if that was some kind of error in their prediction algorithms, or if it was possible to achieve super-saturation in a whole city like that. $\endgroup$ – KRyan Jun 7 '16 at 14:30
  • 1
    $\begingroup$ @mins you're right that makes sense as well. The colder air coming out cools the warm moist air around it and causes condensation. Am I correct that transport aircraft don't use systems similar to those found in cars? Ie. no freon with condenser and evaporator heat exchangers? I'm curious because the company I work for manufactures heat exchangers for Nissan. $\endgroup$ – Ryan Mortensen Jun 7 '16 at 18:44
  • 1
    $\begingroup$ @RyanMortensen, the air is only hot in the bleed air ducts and there is nowhere to pick moisture from there. Quite the opposite; the aircraft systems specifically remove moisture from the air. The only case where this happens is when the cabin is filled with moist ambient air and the packs just start feeding cooler, though also drier, air. $\endgroup$ – Jan Hudec Jun 8 '16 at 9:14

It's condensed water droplets. Outside air with high humidity is reduced in temperature from 28 - 20 ºC (after going through a compression - cooling - expansion cycle), and some of the water vapour condenses since colder air cannot contain as much water vapour as warmer air. The condenser is located before the expansion turbine. Indeed like @David Richerby says in a comment: it's a cloud forming inside your plane.

A home air conditioner has a compression - cooling - expansion cycle as well, yet this white cloud never appears at home. Home air conditioners cool the air to a temperature lower than final temperature, condense the water out, then heat to the final temperature so that outflow relative humidity < 100%. Aircraft A/C systems are not dimensioned for this, since the phenomenon only occurs in some airports, during ground handling and taxi. As soon as the outside air temperature is less than 20ºC the clouds cannot appear within the aircraft.


For a brief moment an answer that was a question and should have been a comment appeared: why do the droplets not condense, and why don't they feel wet when you put your hand in. Because there is not enough time for all of that: the water is blown into air that is warmer and has a lower humidity than 100%, and dissolves again. Water condenses on a surface when the dissolved water vapour meets a surface that is colder, cools the air locally, and provides opportunity for the water molecules to find each other and congregate into condensation drops.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.