When the sun is overhead, light rays pass through less atmosphere, so less UV radiation is screened out. A solar cooker works like a one-way lobster trap. It lets UV light rays in and then converts them to longer infrared light rays that can't escape.
A sort of solar thermal collector is a solar cooker. It "collects" and retains the thermal (heat) energy of the Sun. When high frequency light (visible and ultraviolet) is transformed into low frequency infrared radiation, heat is created.
