Employing the principle of backscatter, a properly calibrated fog detector can sense when visibility lowers to a pre-set threshold value and will turn on the fog horn. Inside the fog detector is a strobe that flashes at the rate of three Hz (three flashes a second). If the air is clear, the vast majority of the light from this strobe is projected outward. If fog particles are present, some percentage of the strobe light will be bounced back to the collecting lens. Behind the collecting lens is a photodiode that converts the reflected light pulses to electrical pulses. These pulses are measured and at a predetermined value the fog signal is activated.