Question

The polar climate zone is the coldest of the three zones. Why is this?

Answer 1

This is because the sun's rays strike Earth at the most oblique angles in the polar regions. In winter, the Sun never rises above the horizon beyond the arctic and Antarctic circles. In summer at mid day, the Sun is still at a very low angle, even though it never sets above the arctic and Antarctic circles. The rays striking the Earth at these low angles spread the light over a very large area, limiting the amount of heating.