Answer and Explanation
In the winter, because of the 23.5° tilt of the earth's axis, the sun's beams hit the earth's surface at an angle and subsequently cover a larger surface area making the heat less intense.
In the winter, the Earth is closer to the Sun. So the sun's rays spreads out to cover more area, making the heat per unit area, much reduced. In summer the Earth is further away from the sun, and focuses on less area, causing a much more intense heat flux (heat per unit area).