You need to set it up right and be prepared for significantly longer render times.

The atmosphere shader setting shown, from a small collection I've developed based (of course) on something BagginsBill posted years ago -  is one I've called 'mist'; you might want to darken the 'sky' colour of the clouds node to give some differentiation. Note that you can ignore the DepthCue start and end distances because we're not using that, but all my units are in Metres.

The atmosphere strength settings on the light (a spot here) are easy to overdo: 0.025 will give you quite a thick effect, you can drop this to 0.01 or less for a more subtle effect.

NB: do not use the volumetric atmosphere with Infinite Lights! If you need atmosphere for an outside scene, convert an infinite light to a spot, constant attenuation, use angle start / end, distance start / end and intensity to simulate daylight or moonlight. 

Set test renders very small because volumetric atmosphere really slows FireFly down...

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Windows 10 x64 Pro - Intel Xeon E5450 @ 3.00GHz (x2)

PoserPro 11 - Units: Metres

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