Why the polar regions are colder: Effect of the Earth's shape and atmosphere on incoming solar radiation.
Compared to equatorial regions (b), incoming solar radiation of the polar regions (a) is less intense for two reasons:
Tropical areas (i.e. lower latitudes, nearer the equator) receive solar radiation which is closer to vertical.
The angle of incidence of the rays, combined with the albedo of the surface has also a strong influence on the amount of energy being absorbed (or reflected) at the surface. In the ice-covered polar zones, almost all direct energy from the sun is reflected because it is white and the angle is small. In short, the angle of incidence affects the heating of the surface in 3 different ways: length of atmospheric track, variable flux and variable reflection
For simplicity, the diagram ignores the axial tilt of the Earth, which causes each pole to slip into darkness for around 6 months of the year, and means the equator's ground is generally not perpendicular with the sun's light.
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