"No," the Forecaster answered. "The thunderstorm clouds are called cumulo-nimbus. They're heavy masses of cloud rising in the forms of mountains or towers. Isn't there a rhyme about clouds and towers, Dan'l?" "Yas, suh, there's a rhyme," the old darky replied, and he quoted: "When clouds resemble domes an' towers The earth is wet with frequent showers." "That, boys," the weather expert said, "is another true proverb, because the description applies to thunderstorm clouds, when the rain is likely to fall in frequent showers." "It doesn't look like a regular rainy sky, though, Mr. Levin," said Anton. "I thought rainy skies were usually heavy and gray." "They are," the Forecaster answered, "and the Weather Bureau gives all the rain clouds the general name of Nimbus, which simply means a thick layer of dark clouds, without shape and with ragged edges, through which rain or snow falls steadily. Sometimes, when there is a powerful wind in the cloud layer, the lower edges of the clouds are broken apart, or loose clouds are seen traveling fast under the overlying gray. Sailors call this scud." "Mr. Levin, suh," broke in Dan'l, "Ah knows a rhyme for scud, too," and he quoted: "Scud above and scud below Shows there's goin' to be a blow." "Well," said the Forecaster, hesitating, "that's not quite as good as some of the others, because you don't see scud until the wind has already come. As a whole, though, it's right, because it implies that the atmospheric currents are powerful, and if the rain disappears, a wind is likely to follow. I noticed you missed the rhyme about the rain before the wind, in your article, Dan'l," he continued. "Yas, suh!" the darky answered, "Ah don't know that one." "It runs like this," the Forecaster answered: "When the rain comes before the wind, Be sure to take your topsails in, When the wind comes before the rain, You can put them on again." "That's a good one, too, because high winds and steady rain seldom go together. "The last type of clouds, which is Number Ten in the Weather Bureau Classification, is called Stratus. It really looks like a lifted fog, which sometimes it is. Indeed, there is no essential difference between clouds and fogs, anyway, except that fogs are formed at the surface and clouds above it." "All clouds are fogs, sir?" said Anton, in a surprised voice. "Yes, my boy. Clouds are visible water vapor. Their visibility depends largely on condensation, just as rain depends largely on the dew-point." "What's the dew-point, sir?" "The dew-point," the Forecaster explained, "is the temperature at which the air becomes so full of vapor that it can't hold any more without letting it down as rain or snow. It's never the same any two days in succession, because the air can hold more water vapor when it is warm than when it is cold." "Is that why muggy days are so uncomfortable?" asked Ross.