To a person of average height on level ground with an unobstructed view, the horizon is some three miles distant. Viewed from a loftier vantage point, say 100ft above the surface, it recedes to 13 miles or thereabouts. And when stout Cortez, a la Keats, "with eagle eyes, stared at the Pacific, silent upon a peak in Darien", his horizon may have been some 40 miles away.
Of course, it is sometimes possible to see considerably further if we are looking at a very large object projecting some distance above where the horizon ought to be - as, for example, if Cortez had fixed his gaze on an adjacent, but relatively distant, peak in Darien. Indeed, if the air was perfectly clear visibility would be in the region of 150 miles.
But the air is never really clear. Rays of light heading towards an observer from some distant object undergo a continuous process of attenuation caused by impurities or water droplets suspended in the atmosphere.
The impurities, for the main part, are tiny solid particles which, in this part of the world, commonly result from domestic or industrial pollution. However they can also occur naturally, comprising perhaps specks of desert sand, particles of arid soil advected from afar or ash from some volcanic eruption in a distant land.
Whatever their source, if these particles are present in sufficient numbers they produce a haze.
Their proliferation or otherwise is highly dependent on local weather conditions.
If there is little upward or downward motion of the atmosphere, the obscuring particles remain in the stratified layers of air near the ground and reduce the visibility. Where there is vertical movement of the air, however, the particles are wafted up and down, and dispersed throughout a very deep layer of the atmosphere. This makes visibility is good.
Depressions, by and large, are regions where there is a great deal of upward movement of air. In the warm sector - the zone between the cold and warm fronts - visibility is often reduced by drizzle or rain, but in the north-westerly airflow behind the cold front, the air is usually very clear.
There is often a dramatic improvement in visibility as the cold front passes; the air becomes pleasantly clear and pure as if one's view of the world was now through a window which had just been cleaned.
Anticyclones, on the other hand, despite their fine settled weather, are regions where there is very little movement of the air in the vertical. For this reason, visibility is often reduced and even nearby hills obscured by a blue, smoky haze.