When light strikes the raindrops, only a portion passes through while the rest scatters. The rain therefore blocks some of the light reflected by objects and less reaches the driver's eye. The backscatter acts as a veil and reduces contrast of everything in the field of view. The rain then blocks light coming back from objects in the road while simultaneous lowering their contrast by the veiling effects of backscatter. Backscatter also creates glare, which is defined as light much brighter than the driver's level of dark adaptation.
Glare creates visual discomfort and lowers the ability to see contrast.
Glare can impair vision for all drivers, but is especially bad for older people. The poor visibility causes people to concentrate their attention directly ahead in order to see where they are going. This decreases the probability of seeing with the peripheral field, so that, for example, it would be harder to see a car or pedestrian approaching from the side. Rain also impairs vision in other ways. Rain affects ability to see through the car windshield.