"Ten," was the reply. "There are lots of variations in those main groups, but that's enough to begin on. The general idea of the cla.s.sification is by the heights of clouds, the Cirrus group being the highest, from about six to ten miles, the Alto group, ranging from two to six miles, and the c.u.mulus and Stratus groups below that. Here," he continued, picking out a photograph that showed only a few faint specks of white, "is a true Cirrus. It is the highest of the clouds, and, as you can see from the photograph, it is delicate and fibrous. This one, that looks like the ghosts of feathers, is another form.

"Cirrus clouds always appear to move slowly, because they're so high up.

As a matter of fact, they fly along at the rate of from one hundred to two hundred miles an hour, and generally in an easterly direction. This photo that looks as if the clouds were a whole pile of spiders' webs, all mixed up, is the second cla.s.s of clouds, known as Cirro-Stratus. Did you happen to notice, Ralph, whether there was a halo round the sun when you took this?"

"Yes, sir, there was," the boy answered, "but it hasn't showed up on the plate. I've got some halo pictures at home, but I didn't think of bringing them along. I just brought my cloud stuff this time."

"Well," said the Forecaster, "suppose you put one of those in here as an example of cirro-stratus. There couldn't be a halo without it. All the upper clouds are made of ice crystals and it is the refraction of the sunlight through these ice crystals that forms most halos. By the way, boys, don't confuse a halo with a corona. They're quite easy to tell apart, because a halo, unless it is one of the unusual white ones, always has red as the inside color and a corona always has the red on the outside."

"How can I tell them apart on a photograph plate, sir?" asked Ralph.

"That doesn't show any colors."

"By their distance from the sun," the meteorologist replied. "Halos are seldom seen except at distances of about twenty-two degrees and forty-six degrees from the sun. There are lots of others, but they are rare. You'll soon learn to catch those distances by eye. Coronas are usually much smaller.

"I think one of the most striking forms of cirro-stratus is the polar 'band,' which stretches from one side of the sky to the other, like a wide white road."

"Ah knows that one, Mistah Levin," put in Dan'l. "Noah, he done stretch that road for the animals to get out of the Ark."

The Forecaster glanced at the aged darky.

"You certainly did manage to pick up a lot of queer superst.i.tions in that article of yours, Dan'l. I've heard that cloud called a Noah's Ark cloud, but I never knew why."

"Yas, suh; oh, yas, suh," Dan'l repeated earnestly, "Noah, he done make that cloud, jest like the rainbow was made to convince Noah that there weren't goin' to be no more floods."

"A high cirro-stratus which looks as if some cream had been poured on the blue sky and hadn't mixed properly yet," the Forecaster continued, "is cirro-nebula. It's very hard to photograph, and even when you do get it on a plate, it doesn't look like much.

"Now the third one in the cla.s.sification is very familiar. That's the well-known mackerel sky. What's the rhyme about that, Dan'l?"

Proud at being thus appealed to, the darky quoted triumphantly:

"Mackerel scales and mares' tails, Make lofty ships carry low sails."

"That's correct," said the weather expert, "because those clouds foretell wind. Sometimes the cloud flakes are less solid and look like the foam in the wake of a steamer.

"Beneath them come the alto clouds, which are made up of drops of moisture instead of crystals of ice. The fourth cla.s.s, called alto-stratus, is a thick sheet of gray or bluish color, sometimes thin enough to let the sun shine through. When lower and in heavy roundish ma.s.ses it's called alto-c.u.mulus, which is the fifth on the list, and when it is lower still and looks like a lot of great blue-gray footb.a.l.l.s wedged closely together it is known as strato-c.u.mulus."

He shuffled the prints rapidly, selecting types of clouds as he did so, and pencilling on the back the character of the cloud.

"Then comes the c.u.mulus, the big round cloud, that looks like ma.s.ses of fluffy cotton wool piled on top of each other. These are the 'woolpack clouds,' which, in summer time, throw deep shadows on the gra.s.s. It is this cloud which, when it comes between you and the sun, gives rise to the old saying that 'every cloud has its silver lining.'"

"Those aren't the thunder clouds, sir, are they?" the photographer asked.

"No," the Forecaster answered. "The thunderstorm clouds are called c.u.mulo-nimbus. They're heavy ma.s.ses 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 Cla.s.sification, 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.

"Yes. When the air is full of water vapor, it hasn't the same readiness to absorb it. When you perspire on a dry, hot, windy day, the air absorbs it right away, but on a day that's humid or muggy, the air can't hold any more, so it doesn't evaporate and the perspiration trickles down your back and into your eyes. A moist climate feels hotter in the summer and colder in the winter than a dry one, although, in reality, it isn't as hot or as cold. Every moist climate is a cloudy climate, and Ireland--which is called the Green or Emerald Isle because there's so much rain that none of the vegetation ever dries up--has some of the most beautiful clouds in the world."

"Is there any place in the United States without clouds?" asked Ralph.

"There's no place in the world that's absolutely cloudless," was the answer, "but clouds in some deserts are few and far between. There's one well known hotel, in the Southwest, that advertises 'free board every day that the sun doesn't shine.' It's a safe offer, too, for last year they only lost two days on it. There are some clouds there, but not such as to obscure the sun.

"In a cloudless country, boys, there are great extremes of temperature, as much as forty to fifty degrees between noon and midnight. You'll get sunstroke in the early part of the afternoon and shiver under blankets in the evening. That's because there are no protecting layers of clouds to equalize the radiation. The air, especially high up, is very cold.

Don't forget that the upper clouds are all made of ice crystals."

"I've been wondering," said Anton, "how you can find out that it's so cold high up in the air if no one can live up there?"

"Balloonists have often pa.s.sed through clouds of ice crystals and snow,"

the Forecaster answered, "though, of course, they've not been as high as the upper clouds. Many observations have been made by releasing small sounding balloons with an instrument attached, letting them go as high as they could, until they burst and fell to the ground. But much of our upper-air exploring has also been done with kites."

"Kites? Like Franklin's?"

"Not quite," said the Forecaster; "our weather kites aren't built like that. They look more like a box. I'm expecting one here, every day."

"Here?"