Sunspots over Santorini, Greece. Zoom-in view from a wide-angle sunset image of Santorini, Greece, to large sunspots on the solar disc. The dark colour of sunspots is due to their temperature being lower than the surrounding surface of the Sun. It is thought that intense localised magnetic fields inhibit the convection process that brings hot material to the surface. Sunspots are normally associated with intense solar activity. The number of sunspots varies with time according to a cycle which lasts about 11 years. Sunspot maxima are often linked to an increase in the strength of the solar wind - causing an increase in aurorae and interference to radio transmissions on Earth. Sunspots are massive structures, ranging in size from hundreds to many thousands of kilometres across.