# Viewing Distance at Night?

Sunday night I was a passenger on AAL1500 KPHX/KDFW. At about 2135 CDT, from an altitude of 35,000 feet, I could see the lights of a city off the right side of the plane. It didn’t seem to be all that far off the flight path – miles away, but not that many.

Today I posted the 2135 position on Google Earth and went looking for the nearest city. Roswell, NM. GE’s line measurement tool showed Roswell to be 67 miles south of the flight track. The rounding of positional data to the nearest minute lat/lon indicates a potential rounding error of only about 1 mile.

I am more than surprised to get that kind of viewing distance at night from an altitude of about 7 miles.

My question to pilots is whether this seems reasonable or am I making some kind of naive error or assumption here. On a cloudless night, what would the normal distance to the horizon be from around 35,000 feet?

The viewing distance at night is phenomenal. I think that clear nights with a high overcast that you’re below is the best viewing circumstance since you can better see city lights with clouds above them for well over a hundred miles – that’s the norm.

When I fly between Houston and Austin at night (~150mi), I regularly see my destination as soon as I lower the nose to level out from the climb although I can’t actually see the airport (beacon) until 10-20 miles out due to the weak power of the beacon in comparison to city lights.

Not at all! You can see “forever” on a clear night. 67 miles isn’t surprising at all.

I remember one night flight that surprised me. From 5500 feet over South Carolina I could easily see the Oak Island Lighthouse beacon 74 nm away. That’s 85 statue miles.

Mid-sized cities are pretty easy to spot at night 40 or 50 miles away while under 10,000 feet if it’s clear.

Distance to horizon (line of sight):

Height (in feet) divided by 0.5736 = “X”
Square root of “X” = Distance in statute miles to horizon

Regards,

James

From 35,000 feet:

35000/0.5736 = “X” = 61,018
SqRt (61018) = 247 = Distance in statute miles to horizon

247 miles to the horizon from 35,000 ft. 500 miles visible from far left to far right. Now that’s quite a spread!

Thanks, James.

You’re welcome. Brings to mind standing on the observation deck of the St Louis Arch and seeing all those miles and miles of… miles and miles!

Regards,

James

You are exaggerating! It’s only 494 miles! Gosh…

That’s a nice formula. Thanks JHEM.

It shows that you can see a pretty good distance. The limiting factor is visibility through the air, between normal atmostpheric conditions and all of the crap we pump into it, you often can’t see more than 5-10 miles or so. But at altitude the air gets clearer and at night you get a far better contrast from bright lights in a dark area.

Bottom line: You can see a long way flying at night! And it’s fun, too!

You’re very welcome, As a sailor, I used to use it all the time (pre GPS) to determine when I could anticipate being able to see various things (antennae, smokestacks, mountains, etc.) coming over the horizon, or when triangulating current position from known points.

It’s not uncommon on short (~50 miles) night flights to see your destination before you even reach your cruise altitude, whatever it may be, although it works better in some locales than others.

Here in the NE, with all the smutch we’ve got in the air, we’re lucky we can keep our departure airport in sight shortly after takeoff. When my son and DIL were still out in Orem at school they would often wax eloquent about the clear air and the vistas of mountain flying, with SLC being easily visible most days right after takeoff.

They especially enjoyed night flights down to Vegas, as they would begin to see the tops of the brightly lit casinos when they were still more than two hours out.

Regards,

James

What worried me, flying in and around Orem, UT at night, was when lights suddenly went dark. That usually meant there was a mountain blocking them. Note to self: When flying around VFR at night in mountainous terrain, it is not fun when there is no moonlight.
Flying the Pilatus at night in the mid-20’s I can usually count at least 10-15 different airport beacons at any one time.

You’re back online!

Get another power brick or a loaner? Or did you get that one you have working?

Regards,

Pop

JHEM,

Say what

Sorry, CFIJAMES is my son. His laptop’s power brick took a dump on him on this week’s trip and he’s been offline since his laptop’s battery ran down.

I figured he got access to the Forum on someone else’s computer and simply wanted an update, this might be the only way we have to communicate right now.

He’s flying N558AF.

Regards,

James

JHEM wrote:

CFIJAMES is my son. …I figured he got access to the Forum on someone else’s computer and simply wanted an update, this might be the only way we have to communicate right now.

Yet again FlightAware saves the day and reunites lost family members!!! Pretty neat James.

skyhigh

Thanks. But I do apologize for hijacking the thread.

(In case no one had noticed I’m proud as punch of my son and his equally talented CFI and ATP rated wife.)

Regards,

James