The "official photographer" pulled out, from a bulging inside pocket, a large bundle of photographic prints and spread them on the table. The collection included both the pictures Ralph had taken with his new lens and some of the old ones intensified in the way that his visitor had showed him. They made a striking contrast, in their vivid black and white, to the cloud pictures, printed in a pale blue, issued by the Weather Bureau. "I think Ralph's pictures are away ahead of the Weather Bureau ones," declared Fred. The Forecaster shook his head. "Some of them are prettier pictures," he said, "but the Weather Bureau sheet is chosen to help observers classify the clouds. If you notice that blue sheet of cloud forms that Washington has issued, you'll notice that they are very carefully selected and that you really can tell the various types of cloud from them. At the same time, clouds are hard to classify, because, at any given time, you're looking at a stretch of sky--counting the separate layers of cloud--several hundred square miles in extent, and, generally, there are many different types of cloud in the sky at the same time." "How many kinds of clouds does the Weather Bureau name?" asked Anton. "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 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."