Weather Why: Measuring Visibility

Weather Blog

Bob asks, “When it is foggy,  you say visibility is a quarter mile, half mile and so forth. Who measures it and how come it’s never 7/16th of a mile?”

Starting with the last question first, we keep the description of length of visibility simple by stating the distance in simple lengths at quarter mile, half mile, mile, etc. Breaking it down to 7/16th or any other similar exact detail would get too complicated and isn’t necessary.

Visibility is measured by a visibility instrument located at weather stations set up around the country at airports, National Weather Service offices and other recording sites. This tool is called a Forward Scatter Sensor. This sensor emits a blue light and the visibility is determined by the intensity of how much light is scattered. A receiver watches for the scattered light. The more light that is scattered, the more fog, dust or other particles there is in the air leading to a lower visibility. The average result for each minute during a ten minute period gives us the reported visibility.

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