[Xenonman]

I learned that HCl_ac + NH3 _ac  NH₄Cl_ac.
Both of those are gases at STP, which means they could coexist in the same container.

Would this happen in atmosphere?
HCl_(g) + NH3 _(g)  NH₄Cl_(s).
If possible, this would be a precipitation from plain air, which is awesome.

I think this would be somewhat more viable in a confined space. The salt would crystallize in a much more noticeable way, should that happen in the first place. 
HCl_(g) + NH3 _(g)  NH₄Cl_(s).
Since it is a closed space, even this
HCl_(g) + NH3 _(g)    NH₄Cl_(s).
looks possible in my head. This one is awesome in an additional level.

So that's it. Please tell me if this would happen.

I am not sure if this thread is in the proper section. I think the answer to possibility lies in physical chemistry magnitudes related in some way I don't know. The ΔG=-RT lnK jumps to mind, but that one requires the equilibrium constant, which only appears if the last equation is actually valid.

[Borek]

Would this happen in atmosphere?
HCl_(g) + NH3 _(g)  NH₄Cl_(s).
If possible, this would be a precipitation from plain air, which is awesome.

That's exactly the way it happens. It is enough to put open bottles of concentrated ammonia and hydrochloric acid close to each other to observe solid forming in the air.

But that's nothing. In the past I worked for a few weeks in a lamp factory, in the luminophore division. Our job was to mix chemicals, homogenize them in the ball mill, and then to roast the mixture. To avoid oxidation, we put some solid NH₄Cl on the surface in each crucible. After the crucibles were put in the kilns (or should I call them furnaces?), NH₄Cl starts to sublimate, and there were literally heavy clouds of solid condensing in the air*. The same effect, just on the industrial scale.

Note: state symbols should be not written as subscripts, so the reaction is perfectly valid as

HCl(g) + NH3(g) NH₄Cl(s)

*I am more than sure tenses are wrong. Sorry, I don't deal with a black magic.

[Corribus]

It's actually a somewhat unusual example of two gasses combining spontaneously to form a solid (at ambient temperature). It is unusual because the decrease in entropy from forming a solid from TWO gasses is enormous. In this case, based on standard entropy values for the reactants and product, the standard entropy change for formation of 1 mole of ammonium chloride is -285.1 J K^-1 mol^-1. The enthalpy change actually way more than compensates for this: ΔH° = -176.2 kJ/mol! Bond formation and breaking alone doesn't account for all of this energy change (if I add up the differences in bond enthalpies between reactants and products, I only get an enthalpy change of approximately -159 kJ/mol).  The different is likely related to crystal lattice energy effects - that is, you're forming an ionic solid with very strong intermolecular forces.

Because the entropy change is negative, this means that heating the reaction up actually makes it less favorable. (Equilibrium shifts to the left.) But the reaction is so exothermic that you'd have to heat it to approximately 618 K (based on the assumption that entropy and enthalpy changes are independent of temperature) to see this shift.