# What is this white smoke like thing, seen inside the cabin before takeoff?

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:

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

• See this video for an explanation of that vapor: youtube.com/watch?v=om8Xwa3MhaU Commented Jun 7, 2016 at 12:39
• Oh no! The ground crew accidentally connected chemtrail feed line to the ac connector 🫠 Commented Jul 31 at 10:51

Short explanation

You're basically seeing this:

In this picture fog appears because air is humid and the ground is colder than air. In your case humidity from the air has condensed into water droplets because of a temperature drop in the cabin, near the AC vents.

Air is a mix of invisible gases, of which water vapor. Water is air-soluble, but only to a certain extent for a given temperature. Humidity is measured by the relative humidity index, the ratio between the mass of water vapor dissolved and a mass of dry air: 0% for dry air, 100% for saturated air, i.e. containing the largest quantity possible given the temperature.

If drying time is considerably reduced in summer, this is because hot air can absorb much more vapor than cold air, as depicted on this psychrometric chart calculated for sea level and 100% RH:

(Source)

Read this curve as: For temperatures -18°C, 20°C and 28°C, one kg of dry air can be saturated (100% RH) with respectively 2g, 17g and 27g of water vapor. When the mass of water vapor exceeds this ratio, water vapor in excess condensates into liquid droplets which separate from gaseous air and form with it an aerosol.

This condensation can occur after two events:

• A large quantity of water vapor is introduced in air, e.g. by boiling water or by evaporating water.
• Air temperature is lowered, mechanically increasing the RH, this is a way to get water in dry deserts.

From left to right: Boiling water, evaporation tower, liquid nitrogen. All cause water in air to condense after reaching 100% RH. This same mechanism is able to create mist, fog, clouds and dew.

The other parameter that can decrease water capacity and induce condensation is a significant change in pressure. In aviation this can be seen in case of explosive decompression, fog fills the whole aircraft in 1/10s, and more casually in the low pressure areas of the wing:

(Source: JetPhotos, by Seth Jaworski)

• Ambient air temperature is 28°C, it means 100% RH corresponds to 27g of water vapor in one kg of air, and 23g/kg for a RH of 85%.
• The temperature for which 23g/kg represents 100% RH is 27°C (the dew point).
• The spread between current temperature and dew point is only 1°C, condensation is going to happen easily.
• Around the AC vents, air is cooled at 20°C, this is far below the dew point. For this temperature there are 6g/kg of gaseous water vapor in excess, they are condensed into 6g of visible water droplets. This happens only around the AC vents, remote from the vents the temperature is warm again, water droplets are evaporated accordingly.
• Water (either vapor or liquid) is progressively eliminated from the cabin by the AC system, RH is maintained around 20%, condensation cannot occur anymore.

Condensation appears near the AC inlets:

Air circulation in large pressurized aircraft, from this answer

Dew point

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.

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

Measuring the dew point

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

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

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.

When water vapor changes into ice

When the dew point is below 0°C, water vapor is not condensing into water droplets, it is converted directly into ice crystals (frost).

This effect is particularly dangerous and can lead to icing in the carburetor of a piston engine. This condition can happen in areas of low pressure and low temperature, commonly created by the sudden pressure drop in the throttle venturi, due to venturi effect and the temperature drop due to fuel vaporization.

For a typical piston engine, this effect is centered on a external temperature around 10°C but extends to a large range of temperature when the engine is slowed down (descent):

Induction icing conditions

The remedy to carburetor induction icing is to heat the carburetor and raise the due point to a safe area, especially when descending.

For jet engines, induction icing (not to be confused with flying in atmosphere with ice crystals or supercooled water droplets) can cause ice to form at the engine inlet. Blocks of ice can then break, causing damage to blades, compressor stall and/or combustor flameout. It can also cause EPR probe (PT2) reading errors and subsequent FADEC fatal behavior.

• Please see the update about weather.. Commented Jun 7, 2016 at 9:47
• Super-short answer: it's a cloud forming inside your plane. Commented Jun 7, 2016 at 10:02
• 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. Commented Jun 7, 2016 at 14:30
• @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. Commented Jun 7, 2016 at 18:44
• @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. Commented Jun 8, 2016 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.

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.

The above explanation is only a technical explanation of how mist forms.

The reason you see it on the A320 in those conditions is this.

Every AC system (home, building, car) uses a plant to generate a cold refrigerant/water, which in turn cools a heat-exchanger. The supply air is passed over this heat exchanger, and the humidity condenses on it and drops off. You never see the mist becasue what is discharged is dry, cold air.

Aircraft AC is different. A plant is too heavy, and AC is rarely required on an aircraft anyway since they spend most of their time in an environment which is -60C. What they do require is pressurisation.

Therefore, the pressurisation and AC are a combined system - and it works on the supply (ambient/outside) air. There is no refrigerant. The air is compressed in the engines or APU, that air is extraordinarily hot. It is cooled (it only needs to be cooled a bit - maybe 50C or so) and then brought back to ambient pressure. When brought back to ambient pressure - even ambient air of 50C is now below freezing. This air is discharged into the cabin. This can't hold the same moisture as outside air, so mist forms. You'll notice that nowhere in the cycle is the moisture able to condense onto a heat exchanger. Some aircraft use dessicant wheels, but these are not as effective as condensing moisture onto a heat exchanger.

After the aircraft takes off, the humidity is lower, the temp is colder and the phenomena is no longer visible.

• The air is dry in every case, home AC, car, airplane, etc, because it was cooled. The method of cooling makes no difference. Your explanation is good, right up until "so mist forms," at which point you've lost me. Mist forms because the air from the pack is cold, not because it's any different from any other parcel of air otherwise. Cold air + humid air = mist. Refrigerant or not plays no role in the formation of mist.
– Ralph J
Commented Dec 10, 2020 at 1:38
• "When brought back to ambient pressure - even ambient air of 50C is now below freezing." What are you trying to say here? Downvoted for this confusing statement, and for "me-too" answer to a 4 year old question. Commented Dec 10, 2020 at 16:58