sun

[zedsded]

why is the sun cold in winter

[boognish76]

Because it's further away.

[Casey]

It is to do with the tilt of the earths axis why we have seasons. In the winter, the suns energy hits the earth at oblique angles and is less concentrated. In the summer, the hemisphere is receiving more direct rays of the sun than the opposite hemisphere where it is winter (so in winter the axis is tilting away from the sun)

[wildwood]

Notwithstanding other influencing factors such as wind and surface area, the main cause is because of the angle to the sun the energy has further to travel through the atmosphere, in particular the ozone layer. Without an atmosphere all life on Earth would fry within a very short period.