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Atmospheric light scattering: Difference between revisions
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== Atmospheric haze == | == Atmospheric haze == | ||
=== General considerations === | |||
What makes the problem complicated to solve in practice is that the only thing that can be calculated reliably is the density of air molecules in the atmosphere as a function of altitude, but there are only one ingredient in the light scattering problem. Dust or water vapour are at least equally important, but their distribution in the atmosphere cannot be cast into a simple form - it is in general a full 4-dim function of space and time, equal to the evolution of the weather itself. The information about the distribution of haze must then come from the weather system. | What makes the problem complicated to solve in practice is that the only thing that can be calculated reliably is the density of air molecules in the atmosphere as a function of altitude, but there are only one ingredient in the light scattering problem. Dust or water vapour are at least equally important, but their distribution in the atmosphere cannot be cast into a simple form - it is in general a full 4-dim function of space and time, equal to the evolution of the weather itself. The information about the distribution of haze must then come from the weather system. | ||
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* Compared to diffuse scattering, the effect of Rayleigh and Mie scattering is much less pronounced, as these apply to optically thin media with hardly any light attenuation. Thus, one can to a good approximation base the whole fog fading with distance on diffuse scattering. This neglects phenomena like the [http://en.wikipedia.org/wiki/Blue_moon#Visibly_blue_moon Blue Moon] which are caused by almost pure Rayleigh scattering in the absence of diffuse scattering. | * Compared to diffuse scattering, the effect of Rayleigh and Mie scattering is much less pronounced, as these apply to optically thin media with hardly any light attenuation. Thus, one can to a good approximation base the whole fog fading with distance on diffuse scattering. This neglects phenomena like the [http://en.wikipedia.org/wiki/Blue_moon#Visibly_blue_moon Blue Moon] which are caused by almost pure Rayleigh scattering in the absence of diffuse scattering. | ||
=== The color of the horizon === | |||
An interesting problem with a surprising solution is how the horizon should be coloured. Suppose it is noon, the sun is right above the scene, we stand on the seashore and look towards the horizon - what do we see? If there were no atmosphere, we'd see the darkness of space (as pictures from the surface of Moon demonstrate nicely). Since there is atmosphere, we see black, fogged by the atmosphere between us and the edge of the atmosphere. | |||
If we approximate the atmosphere by a 20 km thick layer with a constant density, the horizontal view ray exits the atmosphere after passing about 500 km of air. If the air is very clean and hence only Rayleigh scattering present, the horizon is a relatively dark blue. Even small amounts of fog (a visibility of 500 km is quite good!) change this to a light blue. | |||
If the visibility is somewhat less, i.e. between ~30 and 400 km, the horizon is a brilliant white. Every fog particle along the ray is fully illuminated from above, but we cannot see through the atmosphere, as the visibility is less than the distance we'd need to see. | |||
Once the visibility drops below about 30 km, it is however no longer true that every fog particle is fully illuminated - a lot of light now gets absorbed in the upper layers of the atmosphere before it can reach fog particles along the view ray, and hence the horizon becomes grey. Eventually, under an overcast sky and in thick haze, this turns into a dark blue-grey. | |||
Thus, as the visibility increases from zero to infinity, the horizon first gets lighter until it reaches a brilliant white, then it darkens again. | |||
== Basic atmosphere model elements == | == Basic atmosphere model elements == | ||