To a person of average height who stands on level ground with no obstructions - by the seashore, for example - the horizon is a mere three miles away. Viewed from a loftier vantage point, some 100 feet above the surface, it recedes to 13 miles or thereabouts; and seen from the top of a mountain of 1,000 feet, the distance to the horizon is more than 40 miles.
But obviously the distance one can see depends on other factors, too. If, for example, the object of one's scrutiny is a high mountain, then this "elevated horizon" will potentially be visible from a much greater distance. The limiting factor in such circumstances is likely to be the "turbidity", or transparency, of the atmosphere.
It is reckoned that even if the air were perfectly clear, the maximum visibility would be only about 150 miles, because the molecules of the air itself attenuate the light. But such an extreme is very rarely reached; even when it is not raining there are always tiny particles suspended in the air - particles so small that they settle only at extremely low wind speeds. If these particles are dry - smoke or dust - and if they are present in sufficient size and numbers, they produce a haze.
If they consist predominantly of water droplets, they produce a mist, and if the mist thickens to the extent that visibility falls below 1,000 metres, meteorologists define it as fog.
The proliferation or otherwise of these small particles is largely determined by the amount of vertical motion in the atmosphere at any particular time. If there is little vertical motion, the obscuring particles remain in the layers of the atmosphere nearest the ground where they are produced, thus reducing visibility. If there are plenty of rising currents of air, the particles are dispersed throughout a very deep layer of the atmosphere, and visibility is good.
Depressions, by and large, are regions where there is a great deal of upward movement of the air, particularly in the north-westerly airflow behind the cold front. Indeed the improvement in visibility with the passage of such a front is often quite dramatic; the rising air currents produce showers, but the air is pleasantly clear and pure.
Anticyclones, on the other hand, despite the fine settled weather normally associated with them, are regions where there is very little movement of the air in the vertical. For this reason visibility is often disappointingly poor, and on warm sunny days the far-off hills are often obscured by a thick, blue haze.