Question

What factors affect the amount of solar energy that reaches the Earth’s surface?

Answer 1

Weather, Location and Time of year.

Answer 2

One of the factor is the Sun's elevation where the UV reaches us at a much lower frequency when the sun is more distant. Next is our ozone, with our ozone depleting as time goes by, the amount of solar energy it can diffuse is much lesser thus the heat is much more than the previous decades.