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Haze is not considered to be a cloud or mist, and it also is not fog.

What is the ultimate reason that haze and general poor visibility ranges prevail in stable atmospheric conditions?

Is the cause concentration of pollutants, some other effect caused by the presence of moisture/humidity, a combination of both, or something else completely?

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  • $\begingroup$ I'd say concentration of pollutants is indeed the fundamental cause, with only small direct connection to moisture. But as you note stability is the overarching cause of the concentration... stable air means rising motion is suppressed, greatly limiting mixing into the higher atmosphere->dispersal by the stronger winds in mid/upper troposphere. Stable also tends to mean weaker horizontal winds, as high pressure is often more quiescent. But the same forced pooling is the reason moisture often builds to form fog/mist on stable days... so fog/mist often accompany haze if in a moist enough region. $\endgroup$ – JeopardyTempest Feb 6 at 17:03
  • $\begingroup$ Though in reconsideration, that is just one formation process of haze... it can also be caused by too much low level wind causing increased particulates from the ground. This can be why the day ahead of a active cold front can be quite hazy despite more limited stability (as it is today in Florida!) [note: again this often happens to match fog/mist, though this time because moisture levels are maxed due to wind advection] And of course things like fires can also add haze, sometimes aloft, by increasing particulate concentration, even when there's adequate mixing+weak horizontal winds. $\endgroup$ – JeopardyTempest Feb 6 at 17:37
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Traditionally, haze has been regarded as,

an atmospheric phenomenon in which dust, smoke, and other dry particulates obscure the clarity of the sky...

Sources for haze particles include farming (ploughing in dry weather), traffic, industry, and wildfires...

Whereas haze often is thought of as a phenomenon of dry air, mist formation is a phenomenon of humid air. However, haze particles may act as condensation nuclei for the subsequent formation of mist droplets; such forms of haze are known as "wet haze" ...

In meteorological literature, the word haze is generally used to denote visibility-reducing aerosols of the wet type. Such aerosols commonly arise from complex chemical reactions that occur as sulfur dioxide gases emitted during combustion are converted into small droplets of sulfuric acid. The reactions are enhanced in the presence of sunlight, high relative humidity, and stagnant air flow. A small component of wet-haze aerosols appear to be derived from compounds released by trees, such as terpenes. For all these reasons, wet haze tends to be primarily a warm-season phenomenon.

A haze can also be considered an atmospheric colloid, where,

one substance of microscopically dispersed insoluble or soluble particles is suspended throughout another substance.

Examples of atmospheric colloids include: fog, mist, clouds, condensation, smoke and atmospheric particulate matter.

The forces behind the suspension of haze particles includes: electrostatic interactions, Brownian motion, van der Waal forces, and entropic forces. The effect of these forces on the light (low mass) particles within a haze counter gravity to keep the particles suspended in air.

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Haze consists mostly of aerosols. To which extent varies on the location and conditions.

Aerosols are for example dust, pollen, particulate matter, ash (or other pollutants due to combustion) and... a lot of other stuff. Basically, anything which is ligthly enough to float in the air and small enough to mostly not be seen by the naked eye (dust/sand are an exception).

Aerosols function as condensation cores for water vapor - so if there is rather dry air and no rain, no aerosols are "washed" from the atmosphere and haze builds up. In special weather conditions (inversions aka next to no wind and no convection) the haze becomes very thick and a health hazard (e.g. Victorian London, today Delhi or Beijing or Stuttgart).

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Haze can have a number of causes. Sometimes it is due to moisture in the atmosphere, but not sufficient to form clouds, and sometimes it is due to pollution. In Britain it can on rare occasions be caused by dust picked up from the Sahara desert and carried northwards. As you suggest, it can sometimes be due to a combination of causes. In heavily polluted London, many years ago, the lunchtime winter sun was so reddened with pollution haze that I was easily able to see sunspots with the naked eye. These were special circumstances, and I wouldn't advise anyone to try it nowadays.

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