[JDulin]

A combination of three things: 1) The flash is air in contact with the super-hot rail ionizing instantaneously from the heat, and becoming plasma. 2) Friction (From the projectile against both the rail itself, and air) producing vast amounts of heat, which then a) causes water vapor in the air to flash to steam b) produces smoke from burning metal

I am not sure how much of the smoke is from 2)b), because I'm assuming the surface materials are designed to minimize friction and therefore surface burning.

[1] https://www.quora.com/When-a-railgun-is-fired-where-does-the...

[JDulin]

Ah! Some good answers in the rest of this thread noting that this gun vaporizes its sabot - That's probably the biggest source of smoke

[semi-extrinsic]

Nitpick on 2 a): you can't turn water vapor into steam, water vapor is steam.

[RantyDave]

No, it's water vapour. Steam is a gas.

[semi-extrinsic]

Water vapour is steam, i.e. it is the gaseous phase of water mixed with other gases (N2, O2, etc). Just like liquid water is still water if you mix it with alcohol. There is no possibility for a phase transition.

[grenoire]

Is it really that humidity in the air is classified as steam?

[crimsonalucard]

No steam references entropy or distribution of the molecules in the air. Concentrated water molecules visible as myst produced from hot water is steam. The water molecules evenly dispersed in the air is humidity.

[velosol]

>Concentrated water molecules visible as myst produced from hot water is steam.

Steam/water vapor/humidity is gas phase H2O and is transparent. Water droplets in air is often referred to as 'steam' or 'wet steam' but is liquid phase H2O suspended in air. If it mattered in context I assume it would be clearly stated as gas rather than liquid mist.

I doubt anyone would call the component of air that is 'humidity' by the word 'steam' but it seems it would be correct [1]. I am used to the term 'water vapor' for that gas phase H2O though.

[1] https://www.wordnik.com/words/steam