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Space Forum / Astronomy / November 2004Why is the sky red?
Hi! Some days when the sky goes up or down the sky is very very red so the air around us looks red to. Why is this happening? Guess there must be something special with the air those days that changes the frequency of sunlight that we observe. Thanks > Some days when the sky goes up or down the sky is very very red so the > air around us looks red to. Why is this happening? See the blue colour of the bulk of the sky? That's light that's been scattered from the light passing through the atmosphere. It leaves behind light that is sensibly redder than the original colour balance. In the morning and evening you're going to be seeing light in the sky which has passed through a relatively thick section of atmosphere. So it will have had a lot of it's blue light scattered and be sensibly red. Since there's no other significant light source, you see the sky as being red.  SignatureAidan Karley, Aberdeen, Scotland, Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233 Thanks. Yes i understand that but once in a while the sky is much more red than usual. That can´t be the angle changing. It varies with temperature or pollution? Thanks > See the blue colour of the bulk of the sky? That's light that's > been scattered from the light passing through the atmosphere. It leaves [quoted text clipped - 9 lines] > Aberdeen, Scotland, > Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233 > That can´t be the angle changing. It varies with temperature or > pollution? Not sure about temperature (per se). Having had to clear up chemical spills which made it impossible to see across a 2m wide room due to the liberation of large quantities of N2O4 ("nitrogen dioxide", commonly "NOx" in pollution literature, for reasons too boring to go into here) ... sure the amount of pollution between yo and the sun would be able to affect the colour of the sky. There is a lot of feedback between air temperature (at different heights) and the composition of pollutants and their interactions. Dust from sandstorms in the Sahara is also a frequently cited reason for changes in sky colour on this side of the puddle. SignatureAidan Karley, Aberdeen, Scotland, Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233 > Thanks. Yes i understand that but once in a while the sky is much more red > than usual. That can´t be the angle changing. It varies with temperature or > pollution? It varies with particle density, and particle size, and possibly depends on whether the particles are sand, volcanic dust, smog, water droplets or pure ice. SignatureI recommend Macs to my friends, and Intel machines to those whom I don't mind billing by the hour > It varies with particle density, and particle size, > and possibly depends on whether the particles are > sand, volcanic dust, smog, water droplets > or pure ice. Aerosols in your vicinity can give the red-filtered sunlight something to shine on. However the primary effect (Rayleigh scattering) is purely a molecular phenomenon. The kind of molecule doesn't matter. Ben > > It varies with particle density, and particle size, > > and possibly depends on whether the particles are [quoted text clipped - 6 lines] >phenomenon. The kind of molecule doesn't >matter. Rayleigh Scattering is also a rock and roll band from San Diego. Just thought you'd like to know :-) > Hi! Some days when the sky goes up or down the sky is very very red so the > air around us looks red to. Why is this happening? Scaring thing... maybe the sun is escaping from us at big speed, so we see red light (the well known Doppler effect). Maybe for this reason it's really cold these days in Italy... :-) Luigi Caselli Jimmy (or somebody else of the same name) wrote thusly in message <0HFld.121844$dP1.419958@newsc.telia.net>: > Hi! Some days when the sky goes up or down the sky is very very red so the > air around us looks red to. Why is this happening? [quoted text clipped - 3 lines] > > Thanks The blue of the sky is due to reflected (scattered) light being mostly blue while the red of a rising or setting sun is due to transmitted light which is correspondingly richer in red. For the same reason blue-eyed people (myself included) have their eyes appearing red in flash photographs. There is no blue or red pigment involved. The "normal" blue arises from scattering of light by the cloudy but *unpigmented* iris. The red-eye effect arises from the flashlight passing through the iris (so losing its blue by scattering) and being reflected back by structures in the eye behind it. SignaturePaul Townsend Pair them off into threes Interchange the alphabetic letter groups to reply > Jimmy (or somebody else of the same name) wrote thusly in message > <0HFld.121844$dP1.419958@newsc.telia.net>: [quoted text clipped - 20 lines] > passing through the iris (so losing its blue by scattering) and being > reflected back by structures in the eye behind it. But it doesn't just happen in blue-eyed people..... cheers Bill > Jimmy (or somebody else of the same name) wrote thusly in message > <0HFld.121844$dP1.419958@newsc.telia.net>: [quoted text clipped - 20 lines] > passing through the iris (so losing its blue by scattering) and being > reflected back by structures in the eye behind it. Sorry but the red eye effect has nothing to do with the colour of the iris. Normally the pupil looks black because almost all of the light that enters the eye is internally reflected. If the source of light is nearly on the same axis as your eyes then the light is reflected back by the retina and you see a red colour from the blood in the retina. This is how an ophthalmoscope allows you to see the retina. A mostly parallel light source is beamed onto a mirror just below the optical axis and reflected to be parallel with the vision of the observer. i.e. you look just over the top of the source of light. This light is aimed into the pupil and is reflected back by the retina. The image is magnified and focused by a lens. If the pupil is small due to bright light it is harder to visualise the retina. If it is dilated due to darkness (or eye drops) then it is easier. The same principle happens with flash photos. The pupil is dilated due to the dim light (hence the need for flash) allowing the flash to reflect back to the camera lens. The effect is prevented by either using a preflash to make the pupil constrict or by moving the flash further away from the optical axis of the camera- a difficult problem with todays compact cameras but easy with a separate flash on a SLR. Clear skies Terry B Moree Australia Jimmy (or somebody else of the same name) wrote thusly in message <0HFld.121844$dP1.419958@newsc.telia.net>: > Hi! Some days when the sky goes up or down the sky is very very red so the > air around us looks red to. Why is this happening? [quoted text clipped - 3 lines] > > Thanks Air has a greater refractive index for red light than for blue, the red travels slower through the air so is the last to reach the ground. :) SignaturePaul Townsend Pair them off into threes Interchange the alphabetic letter groups to reply |
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