Book 10
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The signs in the sky which betoken the weather may be due to mere coincidence of the seasons, as is the case with signs from animals seen on earth, or they may be caused by changes and alterations in the air. For neither the one explanation nor the other is in conflict with facts,
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and it is not easy to see in which cases the effect is due to one cause or to the other.
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Clouds may form and gather either because the air is condensed under the pressure of winds, or because atoms which hold together and are suitable to produce this result become mutually entangled, or because currents collect from the earth and the waters; and there are several other ways in which it is not impossible for the aggregations of such bodies into clouds to be brought about. And that being so, rain may be produced from them sometimes by their compression, sometimes by their transformation;
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or again may be caused by exhalations of moisture risingLucr. vi. 519. from suitable places through the air, while a more violent inundation is due to certain accumulations suitable for such discharge. Thunder may be due to the rolling of wind in the hollow parts of the clouds, as it is sometimes imprisoned in vessels which we use; or to the roaring of fire in them when blown by a wind,Cf. Lucr. vi. 271-284. or to the rending and disruption of clouds, or to the friction and splitting up of clouds when they have become as firm as ice.
