by Chris Peterson » Sun Jan 02, 2022 7:32 pm
JohnD wrote: ↑Sun Jan 02, 2022 6:50 pm
Chris,
I beg to differ, from personal experience and with the agreement of NASA!
See:
https://www.nasa.gov/vision/universe/wa ... light.html
They cite Said & Patternaik, J. Vision
http://citeseerx.ist.psu.edu/viewdoc/do ... 1&type=pdf "The rod cells are highly sensitive to light. Rods work best in dim light less than 10-1cd/m2. They provide achromatic vision in dark conditions known as scotopic vision. The cone cells on the other hand are less sensitive than the rod cells. They are active in dim to bright light (10-1 to 106 cd/m2)."
Trying to match that with known light intensities, I read that bright moonlight provides less than1 lux of light intensity (compare 120,000 lux in bright sunlight). Converting that to the "candelas per square meter" used by Said & Patternaik escapes me!
Hoever, haloes are produced by internal reflection in the ice crystals, thus producing the typical 22 degree ring. Refraction will also occur, at the air/ice boundary, seperating the colours in sunlight, as in rain drops. So with respect, Chris, to say that "colour isn't there" in moonlight is wrong, and we need an explanation for seeing colourless moon haloes. This site ascribes a diferent mechanism to the appearance of 'coronas', and the Met Office should know!
https://www.metoffice.gov.uk/weather/le ... nd-coronas
I also note that the index of refraction for water ice (1.31) is slightly less than for liquid water ( 1.33) I am quite unable to put that into maths, but presume that ice will refract sunlight slightly less, so producing a lesser seperation of colours.
John
Do not confuse the ability to see color by moonlight with the ability to see color in ice structures around the Moon. They are very different things. We easily see color in coronas and various ice halos, and even in moonlit rainbows.
My point was that neither the 22° or 46° halos have much color. They are pale white, both in moonlight and in sunlight. So the color is not there. It is there, however, in the infralateral arc... which is how we know that's what we're seeing here.
Not sure about your comparison between water and ice. Whether illuminated by the Sun or the Moon, all of these features are caused by ice crystals, not water droplets. (Except for coronas, which are caused by water, day or night.)
Nothing in the Met Office information contradicts what I said.
[quote=JohnD post_id=319559 time=1641149448 user_id=100329]
Chris,
I beg to differ, from personal experience and with the agreement of NASA!
See: https://www.nasa.gov/vision/universe/watchtheskies/28sep_strangemoonlight.html
They cite Said & Patternaik, J. Vision http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.91.7935&rep=rep1&type=pdf "The rod cells are highly sensitive to light. Rods work best in dim light less than 10-1cd/m2. They provide achromatic vision in dark conditions known as scotopic vision. The cone cells on the other hand are less sensitive than the rod cells. They are active in dim to bright light (10-1 to 106 cd/m2)."
Trying to match that with known light intensities, I read that bright moonlight provides less than1 lux of light intensity (compare 120,000 lux in bright sunlight). Converting that to the "candelas per square meter" used by Said & Patternaik escapes me!
Hoever, haloes are produced by internal reflection in the ice crystals, thus producing the typical 22 degree ring. Refraction will also occur, at the air/ice boundary, seperating the colours in sunlight, as in rain drops. So with respect, Chris, to say that "colour isn't there" in moonlight is wrong, and we need an explanation for seeing colourless moon haloes. This site ascribes a diferent mechanism to the appearance of 'coronas', and the Met Office should know! https://www.metoffice.gov.uk/weather/learn-about/weather/optical-effects/haloes-and-coronas
I also note that the index of refraction for water ice (1.31) is slightly less than for liquid water ( 1.33) I am quite unable to put that into maths, but presume that ice will refract sunlight slightly less, so producing a lesser seperation of colours.
John
[/quote]
Do not confuse the ability to see color by moonlight with the ability to see color in ice structures around the Moon. They are very different things. We easily see color in coronas and various ice halos, and even in moonlit rainbows.
My point was that neither the 22° or 46° halos have much color. They are pale white, both in moonlight and in sunlight. So the color is not there. It is there, however, in the infralateral arc... which is how we know that's what we're seeing here.
Not sure about your comparison between water and ice. Whether illuminated by the Sun or the Moon, all of these features are caused by ice crystals, not water droplets. (Except for coronas, which are caused by water, day or night.)
Nothing in the Met Office information contradicts what I said.