Conclusions
In sunshine, water drops residing on smooth, hairless plant leaves are unlikely to damage the underlying leaf tissue, while water drops held above leaves by plant hairs can indeed cause sunburn, if their focal regions fall on to the leaf blade. The same phenomenon can occur when water drops are held above human skin by body hair. However, sustained exposure of a given patch of skin to intense focused sunlight would require that a sunbather’s position remained constant relative to the sun; otherwise, the water drops receive sunlight from a continuously changing direction, and therefore focus it on to different skin areas. Therefore, we treat claims of sunburn resulting from water droplets on the skin with a healthy dose of skepticism.
Lastly, a similar phenomenon might occur when water droplets accumulate on dry vegetation (e.g. straw, hay, fallen leaves, parched grass, brush-wood) after rain. If the focal region of drops falls exactly on the dry plant surface, the intensely focused sunlight could theoretically spark a fire. However, the likelihood of this is considerably reduced by the fact that after rain the originally dry vegetation becomes wet, and as it dries water drops also evaporate. Thus, claims of fires induced by sunlit water drops on vegetation should also be treated with a grain of salt.
