1413. Thunder-clouds are those clouds which are in a state fit for producing lightning and thunder. The first appearance of a thunder-storm, which usually happens when there is little or no wind, is one dense cloud, or more, increasing very fast in size, and rising into the higher regions of the air. The lower surface is black, and nearly level; but the upper finely arched, and well-defined. Many of these clouds often seem piled upon one another, all arched in the same manner; but they are continually uniting, swelling, and extending their arches. At the time of the rising of this cloud, the atmosphere is commonly full of a great many separate clouds, which are motionless, and of odd whimsical shapes; all these, upon the appearance of the thunder-cloud, draw towards it, and become more uniform in their shapes as they approach; till, coming very near the thunder-cloud, their limbs mutually stretch towards one another, and they immediately coalesce into one uniform mass. Sometimes the thunder-cloud will swell, and increase very fast, without the conjunction of any adjoining clouds; the vapours in the atmosphere forming themselves into clouds whenever it passes. Some of the additional clouds appear like white fringes, at the skirts of the thunder-cloud, or under the body of it; but they keep continually growing darker and darker, as they approach to unite with it. When the thunder-cloud is grown to a great size, its lower surface is often ragged, particular parts being detached towards the earth, but still connected with the rest. Sometimes the lower surface swells into various large protuberances, bending uniformly downward; and sometimes one whole side of the cloud will have an inclination to the earth, and the extremity of it will nearly touch the ground. When the eye is under the thunder-cloud, after it is grown large and well-formed, it is seen to sink lower, and to darken prodigiously; at the same time that a number of small clouds (the origin of which can never be perceived) are seen in a rapid motion, driving about in very uncertain directions under it. While these clouds are agitated with the most rapid motions, the rain commonly falls in the greatest plenty; and if the agitation be exceedingly great, it commonly hails.
