Why are Nacreous Clouds considered the most elusive colorful spectacle mentioned?

Answer

Their formation is strictly confined by geography and the requirement for extreme cold in the stratosphere.

Nacreous Clouds are the most elusive because they are limited by geography (polar/near-polar regions) and the rare requirement for sustained, extremely low stratospheric temperatures, unlike general iridescence which can occur globally.

Why are Nacreous Clouds considered the most elusive colorful spectacle mentioned?
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