Last evening I descended into Dublin Airport, having come by aeroplane from central Germany. And the difference was remarkable. When I left, the mountains of the Odenwald, just south of Frankfurt, had been shrouded in a turbid haze, noticeable from the ground but inescapably obvious as a yellow blanket, rather foul to look at, when viewed from certain angles in the air. I was reminded of T.S. Eliot's The Love Song of J. Alfred Prufrock, in which he recalled how . . . The yellow smoke that rubs its muzzle on the window panes, Licked its tongue into the corners of the evening.
But back in Ireland, viewed from above or below, the air was crystal clear. Objects in the far distance stood out as though one could touch them, their edges sharply defined and perfectly in focus. It was as if one's view of the world was now through a window that had just been cleaned.
A haze of the German kind - and of course it occurs in Ireland sometimes too - is a reduction in visibility caused by tiny solid particles suspended in the air, little motes of dust or smoke. It is reckoned that even if the air was 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; there are always tiny particles suspended in the air - particles so small that they settle only at extremely low wind speeds, and most commonly in this part of the world they result from domestic or industrial pollution. But they can also occur naturally - specks of desert sand, particles of arid soil carried from afar, or ash from some volcanic eruption in a distant land.
If these particles are present in sufficient size and numbers, they produce a haze. Pollutants are obviously more prevalent in highly industrialised, continental regions than they are in a country like Ireland, whose prevailing winds have typically spent several cleansing days over the Atlantic Ocean. But the proliferation or otherwise of haze particles is also highly determined by the local weather conditions at the time, most notably by the amount of vertical motion in the atmosphere.
If there is little vertical motion, the obscuring particles remain in the stratified layers of the atmosphere near the ground, and reduce the visibility. If there are plenty of rising currents of air, as is the case on days when scattered showers of rain occur, the particles are wafted up and down, and dispersed throughout a very deep layer of the atmosphere - and visibility is good.