What Mountain Is That?

Question: What mountain is that? It’s really low on the horizon, and it’s not always visible depending on how clear the air is. It’s like a “ghost ship” … appearing infrequently and without warning. — DH, New River, AZ

Answer: Yes, I asked this question of myself. I’ve lived here for years but only see that distant peak (Mt.?) occasionally. Unlike Table Mesa, which is almost always visible, Mt.? rarely grabs my attention. After some recent windy weather the air cleared and it popped into view again. It’s fairly low contrast. Just look right under that question mark. That’s the highest visible point, but you can see the mountain extends to the right, passing behind Unnamed Peak and continuing on from there. N marks the direction of north.

This time, with the help of that photo, I was determined to identify Mt.?. The methods I’m about to describe will work for anyone in any location. If you’re privileged to live in the mountains, or the foothills thereof in my case, I’m sure there’s some distant peaks you’ve always wondered about.

I’d always suspected that mountain was Sunset Point, a protrusion of the Colorado Plateau that extends throughout most of northern Arizona. Interstate 17 climbs that escarpment along winding switchbacks on the way to Flagstaff. The elevation change is around 1300 ft. I’ve watched with powerful binoculars for the lights of cars and never seen even a flash. But then, Sunset Point is 18 miles from here.

So I started my investigation with Google Earth Pro. I created a path from my location to Sunset Point (below). This path is my hypothesized sightline. I knew the sightline direction was a few degrees west of north, and that it had to run midway between Table Mesa and Unnamed Peak. Interstate 17 is the yellow line. Click to enlarge.

Next step was to create an elevation profile for that sightline. The result (below) shows there are no intervening mountains between me and Sunset Point. Note that Google Earth exaggerates the vertical scale by a factor of 11.3X (in this case) compared to the horizontal scale. The calculated slope of that sightline (0.63°) looks about right compared to the mountain’s visual elevation above the horizon (top photo). Click to enlarge:

Google Earth has another feature I decided to try out, as long as I had the sightline path loaded. It’s called a fly-through and allows you to “fly” along your path through Google’s virtual world. Oftentimes, when staring at that distant mountain, I imagined being able to fly from here to there and seeing Mt.? up close. So I created a fly-through. By default, it starts at altitude then swoops down to the path:

You can see how the “flight path” passes between Table Mesa and Unnamed Peak shortly after “takeoff”. The flight covered 18.1 miles in 106 seconds = 614 mph (946 kph) — about the speed of a jetliner. At the end, when it pauses over Sunset Point, I’m sure you noticed those even higher mountains just ahead. Could I possible see them too, on a very clear day? To answer this question, I extended my path and ran a new elevation profile. Click to enlarge:

Sure enough, it looks like those more-distant mountains should easily intersect my line of sight. In fact, there should be multiple overlapping mountains visible. Unfortunately, two factors prevent that from happening: Earth curvature and atmospheric extinction.

Atmospheric extinction limits how far you can see through air, even on a clear day. This is what meteorologists report as the visibility, which can range from zero on a foggy day to 120 miles on a clear day (in wilderness regions). The maximum distance really depends on what you’re trying to see, and factors like size, brightness, and contrast make a big difference. Sure, Sunset Point is large, but it’s 18 miles away. And here in the dusty Arizona desert, the average visibility at my location is 10–12 miles.

But even with perfectly transparent air, Earth curvature geometrically limits how far you can see. Earth’s surface curves downward by 2017 ft over a distance of 55 miles, so that 6375 ft peak in the elevation profile is actually 829 ft below the sightline. I “bent” that elevation profile to conform to the curvature. There’s some distortion in the process, but it’s close enough for my purpose.

There is one tantalizing peak (?) that could possibly be seen, but it’s 42 miles away and would require an extremely clear day — plus a telescope.

It’s worth reiterating that Google Earth does not account for Earth curvature in its elevation profiles. You can easily see this by creating a profile between continents. This is helpful to know if you’re doing these kinds of investigations. Are there any “mystery peaks” you can see that you’ve always wondered about?

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