Why is it cold in the frigid regions? Tropical fever? Is it caused by the way the sun shines?

Yes, the main reason why the sun's oblique rays cause the climate temperature to drop is that the atmosphere penetrated by sunlight is thick, and the atmosphere blocks some sunlight.

For example, in spring and autumn, the sun shines directly on the equator, and we are in a semi-direct state with a suitable climate.

In summer, the sun shines directly on the Tropic of Cancer, and our weather is very hot.

In winter, the sun shines directly on the tropic of Capricorn, and our weather is cold.

In the north and south poles, there is no direct sun all year round, and even it is extremely dark for several months every year, and there is no sun at all.

The sun is oblique, which is the main reason why the atmosphere it wants to penetrate is thick.