Non semper ea sunt quae videntur, said the Roman fable writer Gaius Phaedrus: "Things are not always what they seem". Our atmosphere, which at times looks pristine pure and clear, always contains a myriad of tiny particles indeed heavily polluted air may contain anything from 10,000 to 50,000 of them in every cubic inch. It is a sobering thought to realise that were it not for the fact that the air is regularly "washed" by rain, causing many of these dusty motes to be carried earthwards by raindrops or by drifting flakes of snow, the atmosphere would quickly become clogged with a well nigh impenetrable and suffocating haze.
Meteorologists have a special name for the "shell of dust" embodied in our atmosphere: it is called the konisphere. Because of it, even the cleanest air is anything but pure. With every breath we draw, we; inhale a host of micro sticks and micro stones, and it is only the extreme modesty of their dimensions that makes them barely noticeable. Most of the time they are too diluted to bother us.
These motes are of many sizes, shapes and origins. Every wind that sweeps across the desert scoops up many tons of pulverised rock to scatter far and wide. The soils of the world are littered with tiny fragments of vegetable fibre, lifted by the gentlest breeze to be wafted here and there. Conifers, ragweeds and a thousand different plants and trees contribute pollen to the atmosphere. Spores and microbes are everywhere, and from our kitchens, factories and forest fires come soot, smoke and other tiny particles.
As if all this were not enough, every now and then the earth itself explodes in a great volcano, to spew tons of powdered rock into the air where it drifts away for years before it dissipates. Even the ocean, when its spray evaporates, attempts to make a salt mine of the air we breathe. And to cap it all, in an almost literal sense, our atmosphere collects a daily dose of ashes, so to speak - the remains of millions of incinerated meteors or shooting stars.
Thus does the earthy accumulate its konisphere. Its constituents play an important part in the formation of our weather, since many of them act as "condensation nuclei" by providing the surfaces on which water droplets form. The most favourable nuclei for fog and cloud formation are those that are "hygroscopic" - substances like salt and acid particles that attract and absorb the moisture from the air. The resulting liquid spheres are large enough to scatter sunlight, and make the ensemble visible as cloud.