What is the physical phenomenon that occurs because of the heat ?

The physical phenomenon would be the sighting of mirages by the boys due to the sun and heat.

Strange things happened at midday. The glittering sea rose up, moved apart in planes of blatant impossibility; the coral reef and the few stunted palms that clung to the more elevated parts would *oat up into the sky, would quiver, be plucked apart, run like raindrops on a wire or be repeated as in an odd succession of mirrors. Sometimes land loomed where there was no land and *icked out like a bubble as the children watched. Piggy discounted all this learnedly as a "mirage"; and since no boy could reach even the reef over the stretch of water where the snapping sharks waited, they grew accustomed to these mysteries and ignored them, just as they ignored the miraculous, throbbing stars. At midday the illusions merged into the sky and there the sun gazed down like an angry eye. Then, at the end of the afternoon; the mirage subsided and the horizon became level and blue and clipped as the sun declined.