What was the great physical event that took place at the end of the Mesozoic era in North America?
Rocky Mountains formed.
First oceans formed
O Mount Saint Helens erupted.
Grand Canyon formed.

Mount Saint Helens erupted

if the temperature drops and becomes cool

then the atmosphere can be freezing as the water vapor in the air is at maximum (100%) and can be severely cold

also can be snowing lightly as the moisture in the air will start to solidify

and btw this is not chemistry but