Uniform Scattering of Light to Nearly White – The Mie Scattering and Haze

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One process is particularly important for the formation of the sky colors: the scattering of the light at the different particles in the atmosphere. This process, which can act uniformly across all wavelengths or favor specific areas of the spectrum, directs a significant portion of the wavelengths in a light beam in a direction different from the original beam. The distribution of the light depends essentially on the ratio between the particle size and the wavelength of the incident light. Based on this, we can primarily distinguish between two types of scattering.

We refer to the light we perceive on earth as sunlight. However, daylight is a combination of sunlight, scattered skylight in the atmosphere, and reflected light from the surface.

Haze is a nasty enemy for us photographers. It robs the sky and landscape of almost all color. It darkens the low sun noticeably – in extreme cases you think our light source has already set an hour before its time. It blurs the contours so that foreground and background are no longer properly separated. And both our perception and our photographs lack depth. And last but not least, haze often dampens our mood in general.

But have you ever looked through the brochures of the well-known photo accessory manufacturers and discovered a filter that promised to free your photos from the frequently occurring milky-white afternoon haze? No? Not even with technical finesse. A look at the physical processes behind the all-suffocating haze quickly reveals why this isn’t possible.

Generally speaking, haze is a clouding of the earth’s atmosphere that reduces visibility to five to eight kilometers or less, but not below one kilometer, and is divided into „moist“ (caused by water vapor in the air) and „dry“ (caused by solid particles in the air such as soot particles or smoke, pollen, water droplets, salt crystals, dust from the earth’s surface, carbon, plant particles, and especially aerosols). When visibility is less than one kilometer, we speak of fog.

Aerosols are mixtures of at least two substances that do not or hardly dissolve in each other or combine. Typically, they consist of a mixture of a liquid and a solid that are finely dispersed in a gas, often air. Their size spectrum ranges from a few nanometers to a few millimeters in diameter. The atmosphere contains aerosols in the form of mixtures such as pollen, bacteria, spores, dust, smoke, sea salt or ash, and water droplets. Volcanic eruptions have the ability to release large quantities of natural aerosols, which can significantly impact the weather globally.

80% relative humidity is the limit in distinguishing the two types of haze. When this value surpasses 80%, the air contains a sufficient amount of invisible water vapor, a gaseous form of water in which molecules move quickly enough to stay apart, causing a reduction in visibility, a phenomenon known as humid haze. In our Central European latitudes, however, this is comparatively seldom the case, and that is why we are much more often dealing with dry haze, which is favored especially over the land masses by high-pressure weather conditions with little air exchange. Under such inversion weather conditions, an accumulation of suspended particles and aerosols can occur in the atmosphere over several days, leading to the development of extreme smog.

The most common weather conditions are characterized by warm air rising from the earth’s surface and cooling down in the process. In an inversion weather situation, this relation is reversed: cold air below and warm air above. However, cold air is heavier than warm air; it does not have the tendency to rise, and thus the wind necessary for the mixing of the air layers is missing. As a result, the pollutants and particles that cause haze accumulate in the cold layer. Inversions occur mainly in autumn and winter, when the sun no longer manages to warm the layer near the ground.

However, especially over the heavily industrialized areas of North America, Europe and Asia, sulfur sulfate released by coal-fired power plants also plays an important role in the formation of haze during the hot summer months. In the atmosphere, the sulfate particles mix with condensed water vapor to form haze-causing aerosols. From this list, we draw the conclusion that haze is due to both natural and, since the last century, increasingly man-made causes.

After determining which particles are needed for haze, we need to determine their light scattering contribution. German physicist Gustav Mie’s calculations from 1908 provided this information. According to the theory named after him, regularly shaped particles with a diameter larger than the wavelength range of visible light (400 to 700 nm) scatter the incoming radiation more and more forward as they get larger and more evenly across the entire spectrum. „Forward“ in this case means against the direction from which the light is falling, and „evenly“ means that no wavelength range is favored and all the colors complement each other to form a more or less distinct white. This is why, when we refer to the sky as hazy, we perceive it as a bright, milky white.

According to the two discoverers, Gustav Mie and Ludvig Lorenz, we should correctly speak of Lorenz-Mie scattering.

Because the scattering is so uniform, we don’t have many means of suppressing haze in a photograph. A UV filter helps to eliminate the UV radiation that causes a slight blur because the haze particles scatter the shorter wavelengths the most, and the ultraviolet range is therefore the most affected. The polarizing filter can improve image quality in light haze by removing the reasonably uniformly polarized scattered light. Finally, we can select the shooting location to direct the view away from the sun, which is the main scattering direction.

Fortunately, the atmosphere has some mechanisms in place to rid itself of the haze particles after a while. First, there is gravity, which forces the larger and heavier troublemakers to settle downward. The smaller and lighter substances are then either transported to higher air layers by turbulence-causing rising air masses or washed out in an all-cleaning thunderstorm. The latter, however, requires a considerable amount of rising and condensing water vapor.

With the cloudiness and the fog we find in the sky and on the earth, there are still further factors that promote the Mie scattering. Both differ physically from haze in that they consist of water droplets, i.e., condensed water vapor, or ice particles. With an average diameter of about 20 µm, both meet Mies‘ theoretical specifications and scatter the incident visible light to a stronger and more uniform white than the smaller water vapor molecules.

But physics is not always for practice, and in practice, haze and fog, but also haze and low-lying clouds, are not necessarily always easy to distinguish from each other. An aid to this may be the visibility values already given. Haze always allows us to see at least a little bit, while fog often encloses us and low clouds are somewhere on the horizon. Colors are a second guide. In clouds and fog, they range from a pure white to a washed-out gray, depending on the intensity of the sun’s rays. Haze, on the other hand, tends to be milky white or hazy.

But „uniform“ and „white“ are boring terms and not the ingredients we need for an exciting picture. Instead, contrasts and colors are the order of the day. With the sky blue, the iridescent sister of the Mie scattering provides us with at least one important building block.

Nanometer (nm), from Greek nãnos = dwarf, the billionth part of a meter, value 10 to the 9th power or 0.000.000.001

Micrometer (μm), from Greek mikrós = small, the millionth part of a meter, value 10 to the power of -6 or 0.000.001

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