Chemistry gas pressure help?

If a pressurized gas is releases from a container, ice can be seen forming on the outside of the vessel. Why does this happen?

Answer:
Expanding gas gets cooler (due to less pressure). As the gass gets cooler the amount of water vapor that can exist in the air decreases (relative humidity increases).
Eventually the air is cool enough that it reaches its capacity to hold water vapor (becomes saturated) and further rising and consequent cooling leads to condensation into droplets. Condensation occurs on the surfaces of particles. If the expanding gass creates a large enough temperature gradient, then the condensed water will freeze leaving ice buildup, typically at the point of expansion. This process is basically how air conditioning and refrigeration works, though it is done in a closed system - a compressor pressurizes the gas, moves it down a tube to an expansion chamber where it rapidly expands getting very cold, then you run air around the coldness to cool it.

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