The production of cirro-cumulus and cirro-stratus sometimes spreads across the sky with astonishing speed, and this rapid advance of the edge of the cloud may lead to quite mistaken ideas as to the velocity of the wind at that altitude. In the case of cirro-cumulus, or cirro-macula, it is easy to fix attention on a single cloudlet. If this has the usual ball-like form, it can only be regarded as floating in the air and moving with it. Meanwhile new cloudlets may be forming and growing denser, so that the cloud patch as a whole may be apparently advancing at a much greater rate. Careless observation would then lead to the idea that the cloud was moving much faster than it really is, but if the attention is rigidly fixed on a particular cloudlet the mistake is impossible. If the cloud is a variety of cirro-stratus, it is not always easy, or even possible, to distinguish between the advance of condensation and the movement of the whole, but it can nearly always be done if the cloud shows any definite features upon which attention can be fastened. Sometimes none sufficiently marked can be seen, and when that happens it is still possible in most cases, by watching the edge of the cloud-mass, to see whether new cloud is being added to that edge. The wave-like forms present a special case, which will be dealt with in a later chapter, after the general principles of cloud formation have been discussed in connection with the great clouds of the lower air, whose causes and conditions are far better understood. CHAPTER IV ALTO CLOUDS