Here's why:

* Earth's Tilt: The Earth is tilted on its axis at approximately 23.5 degrees. This tilt causes different parts of the Earth to receive varying amounts of direct sunlight throughout the year.

* Summer Solstice: December 21-22 marks the summer solstice in the Southern Hemisphere. During this time, the South Pole is tilted towards the sun, receiving the most direct sunlight and therefore the most intense solar radiation.

While the Northern Hemisphere experiences winter during this period, the Southern Hemisphere enjoys its summer season.