Visibility: more than meets the eye

Visibility, unlike so many other weather terms in aviation, seems so simple. It’s a measure of how far you can see, right? What nuances can there possibly be?

Turns out, a lot.

Visibility may be the single most important weather metric for VFR pilots, and it’s right up there for IFR pilots too. While weather conditions are usually reported in terms of ceiling and visibility, it’s the latter that usually matters more. An 8000 ft. overcast may sound like good VFR, but if it’s 3 miles underneath that cloud deck, things will look very murky. I’ll take good visibility over high ceilings every time.

That’s especially true if it’s raining. Rain can have a dramatic effect on visibility, and if it’s showery (with bands of rain moving through the area) the visibility could swing from good VFR to low IFR within the span of a few minutes. So don’t stop at the visibility number when reading a METAR – make sure you consider any precipitation too. As a rough guide, you can almost cut the observed visibility in half if it’s raining.

But visibility is affected by more than just the wet stuff – haze is actually the VFR pilot’s worst enemy. The John F. Kennedy, Jr. crash in 1999 is a classic example. While weather conditions were legal VFR, hazy conditions made the horizon nearly impossible to see (the open water environment made this even worse). The VFR-only pilot lost control over the ocean and crashed. Legal to fly? Absolutely. Safe? Probably not. At least not for this pilot in this airplane.

I thought of this accident on a recent flight from Chicago to Cincinnati, when I was reminded just how disorienting it can be to fly in thick haze. On this day, the combination of hot summer haze and smoke from some large Canadian wildfires had brought visibility down to about 4 miles (see photo at right). The whole world took on a fuzzy, gray appearance, and I was glad to have an instrument rating.

As we flew on at 9000 ft., we were comforted by the fact that we could still see the ground. How bad can it be, right?

But this was a trap. After all, at 9,000 ft., seeing the ground doesn’t mean much: visibility could be as low as 2 miles. Looking out the front windshield, which is what really matters, visibility was effectively zero. In fact, we nearly stumbled into a cloud because we couldn’t see it. Not to mention the traffic issue: while we saw multiple traffic targets on our G1000 screen, we never saw one with our eyes.

The lesson is clear: what you see is what you get. No matter what the METAR says, if you’re uncomfortable with the situation, it’s time to file IFR (if you’re instrument rated) or land (if not).

Another trap, although less dangerous, is the catch-all report of “visibility 10 miles.” Most airport weather sensors only go to 10 miles, so even on days when visibility is 35 miles you’ll hear 10 on the AWOS or ATIS. This can cause your brain to subconsciously calibrate a report of “10 miles” with nearly unlimited visibility. But the first day you fly when the visibility really is 10 miles, you’re likely to be surprised. It’s not unsafe, but it’s a little unsettling. 

A good way to counteract this is to always have the big picture in mind. Is there a big high pressure system that’s packing the haze in or has a strong cold front come through recently to clear out the atmosphere? Both situations could result in a report of “10 miles,” but the view out the front window will be different. Go beyond the METAR for the true story.

A final consideration is the sensor on the airport that determines visibility. These work well most of the time, but they are subject to errors. Even when they’re not, they can only report conditions at a single point on the airport. That’s helpful, but what really counts is what you see as the pilot on short final. Don’t become a slave to the electronic eye.

Even the FAA trusts pilots more than electronics: instrument pilots must have the “required flight visibility” in order to complete an approach. Note that the term is flight visibility, as observed from the cockpit, not what the airport weather station senses. That is recognition that, once again, “what you see is what you get.”