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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

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Direct answer

Air conditioning causes a large drop in cabin temperature, air humidity present in the air condensates into water droplets. It is similar to dew, and more generally to fog (which is dew over the ground) which appears when humid air is close to a cooler ground.

Dew and fog appear when air temperature is lowered below its dew point (see further down).

Quick explanation

In your case, before air conditioning is started the cabin is filled with air from the outside, which happens to be warm and nearly saturated in water vapor. The ambient temperature is close to the dew point.

The injection of colder conditioned air in the cabin decreases the temperature around the AC vents, the temperature quickly goes below the dew point causing invisible gas (water vapor) condensation into visible liquid water droplets (fog).

Due to the limited instantaneous capacity of the air conditioning system, the temperature is decreased only around the AC vents, not in the whole cabin. When the water droplets mix with the warmer air in the rest of the cabin, they immediately return to the gaseous form (vapor), and doing this they also make this air a bit colder (evaporative cooling).

As pointed out by @JanHudec the humidity level must be kept low during the flight to prevent structure oxidation. Humid air from the cabin returns through the decompression panels to the air conditioning system where humidity is removed. After a full cycle the cabin air is rather dry and condensation doesn't occur anymore.

Another case of condensation:

Water Condensation in Aicraft


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% for dry air, 100% for air fully saturated. Saturated air cannot contain more water vapor. If vapor is added then it immediately condenses into visible water droplets.

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.

Psychrometrics, the branch of physics studying how liquids and solids boil or condense, revolves around the notion of equilibrium vapor pressure, but for us mere mortals, the notion of dew point is more practical.

Dew point

The quantity of vapor air can contain before being saturated is called its water vapor capacity, it increases exponentially with temperature:

enter image description here
(Source)

Let's define two cases:

  • Full saturation: If the quantity of vapor is the maximum allowed for the current temperature (100% relative humidity), decreasing the temperature or adding vapor will cause some vapor to condensate into water.

  • Partial saturation: If air is not fully saturated, there is a (lower) temperature for which it becomes saturated. This temperature is referred to as dew point. So whatever the current temperature, fog can be created by decreasing the temperature until the dew point is reached.

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.

Whether fog will form depends on the the dew point spread, that is the difference between current temperature and dew point. When the spread is large, the change in humidity or in temperature must be large. In your case, the spread is small, because air is wet and temperature is high.

Bhubaneshwar case

Before air conditioning system is started, cabin air is the same than at the airport: 28°C and 85% humidity. AC is going to inject air at 20°:

  • Quantity of vapor for 85% RH at 28°C is 23g per kg (85% * 27).
  • Vapor capacity at 20°: 17g per kg.

When air is injected and temperature is decreased to 20° around the vents, air becomes fully saturated, and the 6g of vapor in excess per kg of air are condensed into fog.

This air returns to the AC system, packs remove the excess of vapor to reach the 20% RH targeted for the flight. 13.6g of water are extracted by the packs from each kg of air before air is returned to the cabin, now containing only 3.4g of vapor per kg.

For typical cabin of 150m3 containing about 175 kg of air, 3.5 kg of water are extracted. This is without counting vapor resulting from passengers perspiration and breathing.

While fog in the aircraft is a problem, fog outside the aircraft requires flying using Instrument Flying Rules (IFR) which is only allowed with a specific pilot rating and aviation certification. Knowing the dew point is important to plan a flight, in particular to know whether IFR will be required. Therefore the dew point is measured and published as aeronautical information.

Measuring the dew point

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

Psychrometers
Whirling (how to use it) and digital psychrometers

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 information

KLAX ASOS report:

enter image description here

As previously mentioned, when the dew point is close to the ambient temperature and the humidity percentage is high, the visibility is low and fog occurs.

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 :-)

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    $\begingroup$ Please see the update about weather.. $\endgroup$ – anshabhi Jun 7 '16 at 9:47
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    $\begingroup$ Super-short answer: it's a cloud forming inside your plane. $\endgroup$ – David Richerby Jun 7 '16 at 10:02
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    $\begingroup$ 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
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    $\begingroup$ @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
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    $\begingroup$ @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
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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.


EDIT

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.

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