Besides high altitude, what extremely specific condition is required for the formation of Nacreous Clouds?

Answer

Extremely low temperatures in the stratosphere, usually below -78°C (-108°F).

Nacreous Clouds require the deep cold of polar winter conditions in the stratosphere, typically needing temperatures below -78°C (-108°F) for formation.

Besides high altitude, what extremely specific condition is required for the formation of Nacreous Clouds?
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