XII
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There are some species of winds which issue from clouds that are rent and pour down their contents. They are called by the Greeks cloud winds (ἐκνεφίας). Their method of formation, as I suppose, is this: among the particles given off by the earth’s vapour and carried aloft there is great inequality and dissimilarity, some being dry and others moist. When the particles have massed in one body there is great discord and internal strife, which probably leads to the forming of certain hollow clouds with narrow pipe-shaped spaces left between, much like a flute in shape. In these gaps there is shut up rarefied air, which, being buffeted about in the confined space and becoming heated, strives to get more room. It expands and rends its envelope, breaking forth in wind, which, as a rule, is squally, since it descends from above and falls on us with fierce vehemence. It is not diffused, nor does it come through a wide open space, but it struggles and opens up its way by main force. As a rule, it is a brief gust. As it bursts through the cloudy receptacle by which it was confined and overleaps the battlements, it comes in tumultuous energy, sometimes not unattended with fire and the sound of thunder in the heavens. Such winds are much more violent and of longer duration if they have taken up in their course other gusts proceeding from a like cause, and thus several have conspired to form one. It is just like the flow of torrents of moderate size, not serious as long as each has its separate course. But when a number of them have combined their streams, they surpass in size regular, constant rivers. The same thing may probably happen in squalls; they are short-lived whenever they are alone. But when they have joined forces, and the air expelled from several parts of the sky at once has all combined in one, both force and duration are added to them.