"Alpenglow"is the name for the refelcted sunlight that you see (reflected from) mountans,just as the sun sets. Because of the rosy-red hue of the setting sun, the mountains appear to be red.
Anyway, last night was clear, and about a half hour after sunset, I looked up and saw alone cloud glowing bright orange, from the rays of the sun. How long can this go on? Assuming the highest clouds are about 40,000 feet, how long after sunset can such a cloud be illuminated?
At what latitude, at what time of year? I know you were not looking for an “it depends” answer, but stop and think about it. On June 21, the sun will never set at the North Pole (and it will never rise on December 21) and as you move South from there, the times that the sun will actually set will be affected by the latitude and the time of year. Since the sun “goes down” at different rates, depending on whether it is setting directly in the West or if it is sliding down to the North or South of West, your question, expressed as a unit of time has too many variables.
Arbitrarily using the Equator on the evening of the Equinox, and using the “accepted” diameter of 12,756 kilometers for the Earth, your 40,000 feet gives an additional 12 kilometers to the distance of the cloud from the Earth’s center which means it should be visible to a ray from the sun glancing across the Earth at a tangent from 553.43 kilometers away. Using the convenient (if not wholly accurate) “speed” of the sun at the Equator of 1669.8 kph, we find that the sun’s rays touching your toes at sunset will last touch that cloud in just under 1/3 hours or a tiny bit less than twenty minutes.
But that is at the Equator and you asked about the glow. When does the “glow” begin after sunset? And How far North are you, increasing the time the sun takes to “angle” across the Earth after setting? It depends.
I get a different result.
Assuming a spherical earth with a radius of 12756km, where the sun rays touch tangentially at a faraway point, and then strike a cloud at 12km. the angle between the local surface and the sun rays is then acos(12756/(12756+12)) ~=2.5[sup]o[/sup]. (Using tomndebbs figures.)
Knowing that the sun moves at 15 degrees/hour (or fifteen minutes per minute, if you so like), this would correspond to about 10 minutes. If you’re at the equator.
There are also other factors, such as refraction in the athmosphere which supposedly can make the sun visible even up to a degree under the horizon. (But this would probably apply similarly to the rays that touch you and the rays that iluminate the cloud.)
Assuming that we’re close to the equinoxes, the actual perceived angle between the horizon and the path of the sun is close to your latitude. If you’re at 60[sup]o[/sup] it would take the sun twice as long to set, so it’s not altogether impossible that it was a very high cloud iluminated by the dying rays of the sun.
(It is of course entirely possible that I’ve screwed up, I only jotted at a napkin and threw the numbers at my trusty old HP48. If I’m wrong, please correct me!)
Thanks for he info! How high are the highest clouds? I understnad that sometimes clouds form at very high altitudes (>80,000 feet). Is it possible?