Visibility is pretty important to safe driving. Here in Teton Valley, visibility can be reduced by falling snow, blowing snow, dust, smoke, and fog. But just how do you measure and report how far you can see?
Measuring visibility isn’t as easy as it seems. For most of us, we look toward something a known distance away and if we can’t see it, that distance or less is the visibility. Makes sense. But then there is also this definition of visibility:
To define visibility of a perfectly black object being viewed against a perfectly white background, the visual contrast CV(x), at a distance x from the black object is defined on a scale of zero to one as the relative difference between the light intensity of the background and the object.
CV(x) = [FB(x) − F(x)] /FB(x)
where FB(x) and F(x) are the intensities of the background and the object, respectively.
Everybody got that?
The international definition of fog is when visibility drops below 3300 feet (.625 mile or 1 km). If visibility is twice that, 6600 feet (1.25 miles or 2 km), then technically it’s mist, not fog. When we say haze is present, visibility can be just over 3 miles (5 km). And since haze can be composed of smoke instead of water droplets, that can fool automated visibility sensors. Your eyeballs looking for objects at known distances are the best source of information on visibility unless you’re having a bad day. By the way, applications like the measuring tool on Google Earth work really well to find the distance to the towers, buildings, or peaks you wish to use.
Using distant objects to determine visibility can also be used to estimate how heavy the snowfall is. In fact, the official National Weather Service definition of a blizzard includes the requirement that visibility be under ¼ mile for at least 3 hours. And when your clear field of view falls below about 30 feet with no visible horizon or shadows, you are in whiteout conditions, and if traveling by foot or vehicle, you are likely to become disoriented and get lost if you keep traveling.
But the look and see method raises other questions. The object you are looking at to measure visibility must be clearly identifiable as a specific object against the background, not just barely seen. And the object must be at ground level, so being able to see 25 million miles to Venus doesn’t count. And when the distance to visible objects changes depending on which direction you look, you should report the lowest value you see.
Automated visibility sensors are used at airports and transportation department weather stations to measure visibility. They can give you a good indication of the visibility at the moment in the place you plan to drive or land your plane. It’s important to keep in mind, though, that visibility changes quickly from minute to minute and mile to mile. Again, your eyes will pick this up where automated sensors are very focused on one place and time.
In and around Teton Valley, you can leave home in perfectly clear weather and suddenly find yourself surrounded by thick fog or heavy, blowing snow. The best thing is not to get into these spots; stop or turn back when you notice them getting worse. Once you are in near zero visibility, motorists who just stop on the road are in danger of being rear ended by the blinded driver behind them. But if you keep going, you are in danger of becoming disoriented, driving off the road, and getting stuck. Trying to find a safe place to pull over and wait it out before it gets that bad is definitely a better plan. And once there and safely parked, why not report the situation on the Teton Valley Weather Facebook page so your neighbors can avoid it, too.
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