Why do seasonal variations of solar radiation occur?

Seasonal variations of solar radiation occur primarily because the Earth's axis is tilted. This tilt is approximately 23.5 degrees from the vertical. As the Earth orbits around the sun, this tilt causes different parts of the Earth to receive varying amounts of sunlight throughout the year, leading to seasonal changes. During summer in one hemisphere, that region is tilted toward the sun, resulting in longer days and more direct sunlight. Conversely, during winter, that hemisphere is tilted away from the sun, leading to shorter days and less direct sunlight.

The other factors listed do not significantly affect the seasonal variations of solar radiation. Changes in solar wind primarily relate to solar activity and do not influence the distribution of sunlight on Earth's surface across the seasons. The Earth's rotation speed affects daily sunlight distribution but does not play a role in the seasonal changes as it is the axis tilt and the orbit that are responsible for these variations. Volcanic activity can influence climate over shorter periods but does not establish the regular seasonal patterns associated with the Earth's tilt and orbit.