Winter has the coldest weather of the four seasons as the days become shorter and colder with snow dropping in the coldest areas. In the Northern Hemisphere winter comes in the December, January, and February months and in the Southern Hemisphere it comes in the June, July and August months. Winter comes about by the Earth decreasing its axis tilt relative to the sun till the Winter Solstice (December 20-23 in Northern Hemisphere) then the sun relative to the earth changes course and starts to move back towards the equator.
Winter Solstice
During the Winter solstice the sun shines directly on the Tropic of Capricorn in the Northern Hemisphere and the Tropic of Cancer in the Southern Hemisphere and the day is shortest with nights the longest. As the winter season progresses from its start around December 20th to 23rd (December Solstice - Northern Hemisphere) with the sun overhead the Tropic of Capricorn, it changes course and begins to move back toward the equator. In the Southern Hemisphere the sun moves on June 20th to 22nd (June Solstice) from its overhead position on the Tropic of Cancer, changing its course and moves back toward the equator.