PURPOSE:To make raining/snowing and rain/snow decision, etc., with one set of apparatus by comparing the intensity of vertically polarized light with that of horizontally polarized light. CONSTITUTION:When polarized light 7 are projected into the air from an optical system 3, 4, 6, and 8, the polarized light are reflected, with the polarized states being kept, in case of raining. At the time of snowing, on the other hand, the polarized light are scattered by the snow and become almost equal to natural light. In the former case, moreover, the intensity of vertically polarized light 13 is stronger than that of horizontally polarized light 13 and the photoelectrically converted output (e) of photoelectric converters 17 and 19 is larger than the photoelectrically converted output (f) of photoelectric converters 18 and 20. Therefore, when outputs (e) and (f) are compared with each other by means of comparators 24 and 25, the outputs (e) and (f) become ef and raining is discriminated. In the latter case, the intensity of the light 13 is equal to that of the light 14, since the reflected light are equal to natural light and the comparators 24 and 25 discriminate that the output (e) is almost same as the output (f). As a result, snowing is discriminated.
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