More Light

Why is winter colder than summer?

Obviously the days in winter are shorter, thanks to the tilt of the earth.  But this is only part of it.  During the winter the sun is lower in the sky.  Partly, that weakens the sunlight because it has to pass through more atmosphere to get to the surface.  (Although query whether that would really make things colder, on average.  If the sunlight's energy is getting absorbed in the atmosphere, it's still making it warmer somewhere.)

 But part of it is simply that the sunlight is coming in at a low angle, so that each unit of sunlight is spread out over a larger area of land, reducing the energy falling onto each square meter of the earth's surface.

Now all of this might seem very obvious.  I point it out because, atmospheric absorption aside, the angle of the sunlight doesn't affect how much you get when you are walking around outside.  If anything, you probably get more sunlight when it is coming in horizontally than you do when it is coming down from nearly straight up, at least if you are standing upright.  I think the sunlight is meaningfully weaker because of atmospheric absorption (which is why, I assume, it is less likely to burn your skin), but two of the big factors that make it cold in winter (short days and less sunlight per square meter) are non-factors in terms of your own exposure to sunlight.