Did you know that the optimal solar weather isn’t a bright cloudless day? In fact, sunny days that are partially cloudy are the best days. This is because as the radiation from the sun travels through the atmosphere, it is reduced by absorption, reflection and scattering. But, reflected radiation from sunlight bouncing off clouds (called diffuse radiation) can increase the irradiance-the amount of electromagnetic energy on a surface per unit time per unit area-that eventually hits the surface of the earth.