Why is it suddenly so cold?

The direct cause of the cold weather is the decrease of sunshine. From summer solstice, the sun moves to the southern hemisphere, and in winter solstice, the sun is located on the tropic of Capricorn in the southern hemisphere. For the northern hemisphere, the sun is oblique, and the solar energy obtained by the northern hemisphere is greatly reduced, so it will get colder and colder. For our own living area, it will be very cold because we get less heat from the sun and can't resist the cold air from the North Pole.