"Well, those are the heralds of a thunderstorm. We've only seen those since my nephew began talking about the hurricane, about an hour ago. Away off on the horizon, though, you can see a bigger bunch of those festoons, dropping from the five-mile height of the cirro-stratus and condensing away down lower. This heat that we're now feeling will diminish, just as soon as that cloud covers the sun, not because the sun is hidden, but because of a change of wind." "But the storm's coming up at right angles to the wind," said Tom, "the wind's a little east of south." "It'll blow from the north-east presently," declared the Forecaster oracularly. "Directly opposite to the storm?" ejaculated the kite expert in surprise. "Certainly," was the answer, "that's a part of the thunderstorm formation. You can see now," he continued, "how the thunder heads of cumulo-nimbus are beginning to show, leaden in color below, with the white billowy tops. They're very thick, those masses of cloud, perhaps two miles thick, and the gray rain curtain trails along behind them. Well, Tom, what is it?" he added, turning to the boy, who was claiming his attention. "The wind's shifting," answered the lad. "To the eastward? Of course. It'll be north-east in a minute or two, as I told you. It's got to be." "But why, sir?" asked Tom. "I don't see why a surface wind should have to blow up against a storm." "That," said the Forecaster, "is quite easy. If the rain is falling, it brings down a mass of cold air with it, displacing the warm air that lies before the advancing storm. The warm air is driven forward, but, at the same time, the descending cold air requires warm air to replace it in its turn, and the warm air, therefore, curves backward and flows into the upper portion of the storm cloud, where its moisture is condensed as rain. So, my boy, a little distance in advance of a thunderstorm there are three currents of air, an upper current of cold air, traveling in the same direction as the storm, and driving the cirrus clouds before it; a current of warm air, going in the opposite direction to the storm and pouring a torrent of warm air into the cloud; and the cold squall, which drives out from under the thunder-cloud and which comes in violent gusts." "But I thought," said Fred, "that thunder and lightning came from two clouds banging together. If most of the thunder storms travel from the west, where does this banging come in?" "It doesn't come in at all," the Forecaster replied; "thunder and lightning do not result from clouds striking each other. It's not quite so simple as that.