The term "nacreous clouds" is often treated as synonymous with "polar stratospheric clouds", but they can form at non-polar latitudes. Besides volcanic eruptions, they can emerge from upper-air flows over mountains:

[Nacreous clouds] are most common in two situations. One situation is when strong winds (and winds increasing with height) cross a long mountain ridge, such as the Rocky Mountains in North America. The resulting high-amplitude gravity waves may propagate into the stratosphere. Nacreous clouds there are the stratospheric equivalent to lee wave clouds (Ac lenticularis) in the troposphere.

And in thunderstorms:

Large thunderstorm systems (mesoscale convective systems, MCSs) may also trigger nacreous clouds. The residual kinetic energy of convective updrafts within MCSs repeatedly poke through the tropopause. The combined effect of these overshooting tops is to bulge the tropopause upwards, and also to diffuse some water vapour in the lower stratosphere. The combination of water vapour transfer and uplift may result in a nacreous cloud above the remnants of an MCS.

A photograph of nacreous clouds taken by Hans Nilsson of Sweden: