The formation of cumulo-nimbus and cumulus is dependent upon the presence of a large amount of water vapour. It is worth while to consider whether the atmospheric movements which bring about the condensation could exist without moisture. Wherever we find differences of temperature between neighbouring places we must get currents of hot air rising from the warmer spots, and compensating descending currents around them. But we have pointed out that if the rate of cooling as we ascend in the still air is less than the rate at which an ascending current will be dynamically cooled, such a rising current will come to rest. If, on the other hand, the rate of cooling in the ascending current be less than in the still air, the equilibrium will be unstable, and a violent uprush will result. Now, in a climate such as our own, where the lower regions of the air contain large quantities of water vapour, any considerable rise brings about more or less condensation, and that condensation is attended by a liberation of very large quantities of heat, which retard cooling in the ascending current, and so facilitate the production of instability. But if this cause is put aside it is still possible to have a similar circulation. When discussing the causes of instability, it has been pointed out that the prime condition was an unusually rapid rate of fall of temperature in still air, such as may be produced by hot sunshine. Now, these conditions are exactly those which will give rise to the phenomenon of the mirage, and which reach their fullest development in great desert districts when the air is still. Again, it has been pointed out that the causes which bring showers and thunderstorms to an end include the chilling of the lower parts of the ascending column by the descent of cold rain or hail from above. We may also add the shading of the underlying ground by the cloud itself, and the absorption of heat in the partial evaporation of some of the rain during the lower part of its fall. In a desert district the arising currents are so dry that even a very great ascent does not often result in visible cloud; and when it does, the cloud is produced at so great a height that the air is too rarefied to produce anything much denser than thin alto-stratus, from which no falling droplets could reach the earth. It seems, then, that there will be no such automatic check on the growth of the circulating system, and it will go on growing in volume and intensity indefinitely. As a matter of fact, this is not the case. A different check does come into operation, but not until the indraught and updraught have become so powerful as to draw up the dust and sand and generate a sandstorm, the weight and shade of which, in time, destroys the circulating currents which uplifted them.