# How high before the sky turns black? **URL:** \ **Category:** Factual Questions\ **Created:** [March 23, 2008, 2:46am UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406 "2008-03-23T02:46:19Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Engywook](https://avatars.discourse-cdn.com/v4/letter/e/bc8723/32.png) [@Engywook](https://boards.straightdope.com/u/Engywook)\ **Post date:** [March 23, 2008, 2:46am UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406/1 "2008-03-23T02:46:19Z")
This has probably been done before… but not since search was available. My apologies if it’s a chestnut. A number of questions about what one would actually see as one approaches space: - How high above sea level does the sky turn black during the daytime? - Can any commonly used aircraft fly comfortably (normal engine power, enough lift from wings to stay there) at that altitude? - What happens while approaching that point? I’d guess that a circle of black gradually enlarges from the zenith until, at a certain altitude, it appears to reach the ground. All stuff you’d think you could just find without asking anybody, but so far no luck. ---
**Author:** ![ignis\_glaciesque](https://avatars.discourse-cdn.com/v4/letter/i/5f8ce5/32.png) [@ignis\_glaciesque](https://boards.straightdope.com/u/ignis_glaciesque)\ **Post date:** [March 23, 2008, 3:29am UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406/2 "2008-03-23T03:29:15Z")
I’m not sure what altitude you’d have to reach before the sky turns black, but I do know, from geeking out on a bunch of photographs, that the sky at around 70,000 feet is really deep purple, and at 100,000 feet is almost entirely black. Now, there’re very few aircraft that can reach that altitude without difficultuy, and none are in particularly common use. Most of them are reconnaissance planes and high-performance fighter jets. Not your typical Southwest Airlines 737. ---
**Author:** ![Byzz](https://avatars.discourse-cdn.com/v4/letter/b/90ced4/32.png) [@Byzz](https://boards.straightdope.com/u/Byzz)\ **Post date:** [March 23, 2008, 5:13am UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406/3 "2008-03-23T05:13:21Z")
A google image search reveals photos of black sky taken at 50k, but there are also photos of blue sky at that altitude as well (examples of black sky at 50k and 60k: ![Curvature of earth from 50,000 feet by ghipps](https://a4.pbase.com/v3/27/280227/3/51671740.100_2042.jpg) [http://www.nasaspaceflight.com/content/?cid=4993](http://www.nasaspaceflight.com/content/?cid=4993)). I suspect it depends on the conditions/time of day. ---
**Author:** ![Ponch8](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ponch8/32/9883_2.png) [@Ponch8](https://boards.straightdope.com/u/Ponch8)\ **Post date:** [March 24, 2008, 5:57pm UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406/4 "2008-03-24T17:57:00Z")
Even in an everyday commercial flight at 35 or 40,000 feet, if you look up through the window, you can see that the sky isn’t as bright blue as when you’re on the ground. I don’t know when exactly it’s pitch black, though. ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [March 24, 2008, 6:06pm UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406/5 "2008-03-24T18:06:05Z")
IIRC, the early high performance fighter jocks reported that around 80,000 feet, the sky was mostly black. ---
**Author:** ![cainxinth](https://avatars.discourse-cdn.com/v4/letter/c/7bcc69/32.png) [@cainxinth](https://boards.straightdope.com/u/cainxinth)\ **Post date:** [March 24, 2008, 7:56pm UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406/6 "2008-03-24T19:56:59Z")
My parents flew the Concorde several times. It flew at 60,000 feet, and they said that if you looked up through the window the sky was very black. ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [March 24, 2008, 8:04pm UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406/7 "2008-03-24T20:04:45Z")
[This picture](http://www.flickr.com/photos/75905404@N00/2179374647/) was taken at about 3 pm on a bright sun-lit day, near the middle of summer, less than 10 metres above sea-level. I suspect that the colour of the sky can be deceptive in photographs, since the sky did not look nearly black to me at the time. ---
**Author:** ![Tahriq](https://avatars.discourse-cdn.com/v4/letter/t/c6cbf5/32.png) [@Tahriq](https://boards.straightdope.com/u/Tahriq)\ **Post date:** [August 7, 2021, 4:06am UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406/8 "2021-08-07T04:06:00Z")
I’m assuming about 90000 ft, ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [August 8, 2021, 4:13pm UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406/9 "2021-08-08T16:13:33Z")
From a Lear jet at 55,000 feet it’s quite dark, but I wouldn’t say black. Contrails of commercial jets flying about 25,000 lower are dark gray, not white as viewed from the ground. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [August 9, 2021, 11:17am UTC](https://boards.straightdope.com/t/how-high-before-the-sky-turns-black/442406/10 "2021-08-09T11:17:53Z")
> [@Engywook](#): > > A number of questions about what one would actually see as one approaches space: > > - How high above sea level does the sky turn black during the daytime? > - Can any commonly used aircraft fly comfortably (normal engine power, enough lift from wings to stay there) at that altitude? > - What happens while approaching that point? I’d guess that a circle of black gradually enlarges from the zenith until, at a certain altitude, it appears to reach the ground. The OP is about 13 years old now; hopefully they’ve found their answers somewhere, but just in case… 1. It’s hard to define an altitude where the sky “turns” black. It’s not like you flip a switch, it’s just a gradual transition. Some years ago James May hopped a ride on a U-2, and his video shows that the sky is pretty dark, almost-black, at around 70,000 feet (see at 6:50): [![](https://i.ytimg.com/vi/1PmYItnlY5M/hqdefault.jpg?sqp=-oaymwEmCOADEOgC8quKqQMa8AEB-AHUBoAC4AOKAgwIABABGFIgYShlMA8=&rs=AOn4CLCv1CPGGhytttO3ZIy7BONfEy-NMA "Ride on a U2 spy plane") ](https://www.youtube.com/watch?v=1PmYItnlY5M) As for “commonly used” aircraft, that also depends on your definition. most fighter jets have a max service ceiling of around 60,000 feet (as did the Concorde). In the early days of the space age, some military fighter jets could “zoom climb” to altitudes where they could not sustain level flight: > **[Zoom climb](https://en.wikipedia.org/wiki/Zoom_climb)** > > A zoom climb or an unrestricted climb is a maneuver in which the rate of climb is greater than the maximum climb rate using only the thrust of the aircraft's engines. The additional climb rate is attained by reduction of horizontal speed. Before a zoom climb, the aircraft accelerates to a high air speed at an altitude at which it can operate in sustained level flight. The pilot then pulls steeply upward, trading the kinetic energy of forward motion for altitude (potential energy). This is diff The only planes I’m aware of that can sustain level flight at black-sky altitudes are the U-2 (80,000 feet) and the SR-71 (85,000 feet): > **[Lockheed U-2](https://en.wikipedia.org/wiki/Lockheed_U-2)** > > The Lockheed U-2, nicknamed "Dragon Lady", is an American single-engine, high altitude reconnaissance aircraft operated from the 1950s by the United States Air Force (USAF) and the Central Intelligence Agency (CIA). It provides day and night, high-altitude (70,000 feet, 21,300 meters), all-weather intelligence gathering. > Lockheed Corporation originally proposed it in 1953, it was approved in 1954, and its first test flight was in 1955. It was flown during the Cold War over the Soviet Union, Ch... > **[Lockheed SR-71 Blackbird](https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird)** > > The Lockheed SR-71 "Blackbird" is a retired long-range, high-altitude, Mach 3+ strategic reconnaissance aircraft developed and manufactured by the American aerospace company Lockheed Corporation.\[N 1\] The SR-71 has several nicknames, including "Blackbird" and "Habu". > The SR-71 was developed as a black project from the Lockheed A-12 reconnaissance aircraft during the 1960s by Lockheed's Skunk Works division. American aerospace engineer Clarence "Kelly" Johnson was responsible for many of the a... The blackbirds are no longer flying, but the dragon ladies are still in service. As far as what you see during your ascent to black-sky altitude, James May’s U-2 flight seems to show something like what you describe: the ambient blue of the atmosphere recedes to form a tall, fuzzy wall at the horizon. The apparent height of that wall decreases as you climb, and the visual fuzziness of it gets reduced. By the time you get to low earth orbit (far higher than the U-2 flight), that blue wall is just a thin , bright blue tinge on the rim of the earth: [![](https://img.youtube.com/vi/DDU-rZs-Ic4/maxresdefault.jpg "Earth Views: Seen From The International Space Station") ](https://www.youtube.com/watch?v=DDU-rZs-Ic4)