Visibility and atmospheric aerosols
The visibility (or how far we can see), is determined by the mass and size distribution of aerosols in the air; these are either naturally produced or man-made and can be in the form of liquid water, crystals of ice or solid 'particulates' from various natural & human-activity sources. Most of this note deals with horizontal visibility, but vertical visibility is of great importance, especially to aviators & astronomers.
The distance at which any object can be seen (& recognised for what it is), depends upon the position and 'personal attributes' of the viewer (i.e. keen-ness of sight) and of course, the 'blocking factors' (or degree of obscurity) between the two. For an object to be clearly visible, the eye must receive enough light energy to be able to resolve the object sufficiently for the brain to recognise what it is: packets of light flowing from an object will be subject to absorption & scattering on its way from object to eye, scattering being by far the most important factor.
Fog, mist, cloud and precipitation are obvious reasons why you might not be able to see as far as possible, these being composed of either water droplets or ice particles of varying sizes: these scatter the light being emitted from an object, dependent upon the number, character and size of the particles: the greater the scattering (diffusion) the poorer the visibility(**).
In a marine environment, spray due to very strong winds will produce a large amount of minute water droplets which will also reduce the visibility, similar to the action of a thick mist or fog.
(**): Although precipitation will reduce the visibility, even if only slightly, under certain circumstances it can actually lead to an improvement on overall visibility as the rain etc., 'washes-out' impurities in the air.
Suspended solids (or "particulates") can be highly effective restrictors of atmospheric visibility, under the right circumstances & in the appropriate quantity.
Air contains a variable amount of impurities, such as dust, soot and salts. The category of "dust" includes the whole range of pollutants due to industrial processes & the exhaust from combustion-based transport activity and also the lifting & suspension of fine soil-based particles, where very dry, exposed fields are subject to a persistently strong wind.
The main source of natural dust is the arid regions, such as deserts and steppes. Course material, lifted by strong and turbulent surface winds, is never carried far from its source, but minute dust / soil particles are readily distributed throughout the lower troposphere and carried far from the source.
Industrial regions, forest (or heath / scrubland) fires & volcanoes provide the main source of soot (or particulates). Smoke particles of course can produce a dramatic, though relatively temporary drop in visibility.
Combustion at very high temperatures is usually an efficient process, producing only small amounts of unburned residue. However, many fuels (including most used in transport) are burnt at relatively low temperatures, and carbon residues are injected into the lower atmosphere and under the right conditions can be carried large distances.
Air also contains a considerable amount of salt. Through the action of the world's winds, spray is whirled up from the oceans, and when it evaporates the salt remains in the air in the form of minute particles. Indeed, of all the particles floating around in the atmosphere, by far the greatest quantity are in the form of sea-salt of varying sizes. This is hardly surprising given the greater proportion (relative to land areas) of exposed sea-water covering the surface of the earth.
All these particles (above) which constitute the impurities of the air are so small that they cannot be seen individually with the naked eye, but their effect on visibility & on the colouring of distant objects is easily observed as they interfere with (scatter) the passage of light from the distant object.
What conditions give the 'clearest' visibility?
The "cleanest" air, at least in maritime mid-latitudes, will potentially occur with ex-polar air masses, where the absolute and relative humidities are low, and are brought along on a reasonably brisk breeze to mix the lowest layers and reduce or eliminate any surface-based potential for visibility reduction. Although showers may lead to temporary (& often marked) reduction in visibility, the showers usually pass swiftly, allowing a return to the prevailing excellent conditions. Also the flow is often tending to greater instability (cold air flowing over relatively warmer surfaces), and this enhances the vertical mixing.
... And what conditions produce the poorest visibility?
Air masses moving poleward (i.e. from the south in the northern hemisphere) are tending to stability and damped vertical mixing: in the extreme case (anticyclonic inversions in winter), no vertical mixing occurs over a large area. Moderate to poor visibility due to man-made processes are often a feature of 'blocked' anticyclonic episodes, where the flow aids transfer of the pollutants from nearby or distant sources, and the associated inversion of temperature (highly stable environment) works against vertical mixing of low-level air, trapping the dust, particulates etc., within the lowest few 10's of metres of the troposphere. In extreme cases, the concentration of pollutants can reduce the visibility to fog limits [< 1000 m], though the relative humidity is <90%.
Air masses arriving from a south or south-easterly direction (in NW Europe) often bring a high 'dust loading', given the likely source region in the arid areas of the Mediterranean & North Africa. This is particularly so when the source regions are subject to intense convective storms - e.g. the Sahara in the 'summer-half' of the year. These storms lift huge quantities of very fine sand particles into the air which are carried along for many hundreds of km, and can markedly reduce the visibility some considerable distance from the source region.
For astronomers, what constitutes 'good' visibility (or 'seeing') is of course directed away from the meteorologists (or aviators) horizontal bias (except of course for celestial objects close to the horizon): As a basic requirement, absence of cloud (or deep fog) is required, but even with 'clear skies', viewing of deep space objects (which are effectively point-source light-emitting objects) require a steady atmosphere - this means that light must not be subject to the distortion caused by high winds (jet stream turbulence) in the upper atmosphere. Surprisingly, this means that immediately to the rear of a swiftly-clearing cold front, whilst cloud cover may be minimal & the stars are shining strongly, images of distant star clusters will appear indistinct due to the presence of the polar jet to the rear of the front. For objects within the solar system, the effect of atmospheric turbulence is not so important.
Assessing the visibility in the past relied upon the observer knowing the distance of known objects from the observing point, and then determining the furthest of these objects that could be seen clearly. Provided that the visibility did not vary markedly from the value found (i.e. due to adjacent mist / fog), then the distance of this object would be the declared visibility. In recent times, most reports for aviation work have used the concept of 'prevailing' visibility where the most prevalent (rather than the minimum) value governs the issued & recorded figure.
With greater use of automatic weather stations [AWS], visibility is more likely now to be determined by a 'transmissometer' (or similar), where the attenuation of a known-strength fine beam of laser-light over a short base-line can be accurately measured and converted into a visibility figure. An added refinement to assess the effect of precipitation is to 'fire' pulses of light energy within the immediate vicinity of the observing point & then detect the amount of scattered light returned. By combining the two methods, a highly accurate readout of local visibility is achieved, but it should be remembered that these systems cannot assess reduction in visibility due to 'adjacent' factors such as passing showers or a fog bank. However, provided they are calibrated & maintained properly, they give a much more consistent record of visibility than that based on a variety of human observers.