Brrrr! Why is it always so cold on sunny days in winter?
You may have heard that clouds trap heat in the Earth's atmosphere on cloudy days in winter, insulating the Earth. You may have also heard that it is cold in winter because the Earth's axis is tilted away from the Sun, therefore the Sun is further away from the Earth, providing less heat.
Both of these reasons for winter's chilly sun are great hypotheses, but they are not entirely accurate.
In summertime in the Northern hemisphere, the Earth is tilted towards the Sun, which means that the Sun spends more time high in the sky. Since the Sun is high in the sky, the sunlight received by the Earth is more direct.
In contrast, during winter in the Northern hemisphere, the Earth is tilted away from the Sun. In winter, the Sun is generally lower in the sky, this means the Earth receives less direct sunlight because the sunlight is coming in at a low angle. This low angle means the Sun's energy is radiating across a greater area when it hits the surface, and, because the sun is shining over a greater area, winter sunlight has less power, and therefore heats the Earth less. Less heat energy means cold winters.