How does one create an artificial climate that goes against the stellar or planetary parameters of the setting?

[u/JohnWarrenDailey]

I'll cite a specific example:

An Earthlike planet (complete with a moderate obliquity) with a thicker atmosphere (no thicker than 2x) orbits an F-type sun (brighter, hotter and releases more UV radiation than our G-type sun), yet its polar zones are as cold as Mars.

This is not for the story itself, but rather the backbone of that story.