The Sun is the Earth's only source of radiative energy, heating the surface by daytime. Greatest heating occurs under cloudless skies, but even when the sky is overcast, heating is usually sufficient to raise the surface temperature above the night-time minimum.
Many weather stations record the amount of bright sunshine during daylight hours. A typical measuring instrument is the Campbell-Stokes sunshine recorder. This consists of a glass ball which focuses the Sun’s rays, burning a hole in an index card. As the Sun moves around the sky, so the burnt hole extends along the index, which is marked in hours to record the amount of direct sunshine.