We can see smoke at some regions before the main ignition takes place. Why is that?
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$\begingroup$ Can you be more specific i.e. at what video time index do you see smoke before ignition? As I see it, there are fog clouds which I assume are being formed by vented hydrogen and oxygen, and then there is the water deluge, and finally the exhaust plume as the engines are started a few seconds before lift-off. The only odd thing I see is a rather large bank of fog in the foreground, which I assume is from vented cryogenic propellant (hydrogen or oxygen) somewhere off screen. $\endgroup$– Anthony XFeb 18, 2018 at 2:18
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The white smoke coming from the second stage, and three boosters is LOX venting to relieve the pressure of the LOX turning to gas.
The Liquid Oxygen is sub cooled, so it is even cooler than usual LOX (increases density, more can be stored in tank, and engines produces more thrust) and in the warm Florida sun the tanks start to warm up as soon as the LOX is pumped in.
Normally a booster would just refill the LOX that vents out (And I guess SpaceX does that as well) but because this is sub cooled, just refilling it does not entirely help. This is why SpaceX cannot recycle a launch very fast. Once it warms up too much, they have to pump out all the LOX and refill it.
So you are seeing literally air. Liquid Oxygen, warmed up in the tank to return to gaseous form, and released to relieve pressure in the tanks.
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3$\begingroup$ "So you are seeing literally air." No, you are literally seeing water. $\endgroup$– uhohFeb 18, 2018 at 3:29
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$\begingroup$ You may have missed the comment; is the thing which is visible "air", or "liquid oxygen", or is it more likely to be water vapor present in the Florida air which has condensed into an aerosol of water droplets, not unlike what's seen opening the door to a freezer to retrieve ice cubes for beverages suitable for watching such launches? $\endgroup$– uhohFeb 20, 2018 at 0:08