ose main
groups, but that's enough to begin on. The general idea of the
classification 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 Cumulus 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 class 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 certai