Short answer
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:
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.
Details
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:
(Source)
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):
(Source)
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:
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:
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 :-)
