Just as cumulus may be divided into heat cumulus and the clouds of the rear of a cyclone, so cumulo-nimbus may be divided into the same two groups. In the case of the heat thunder-clouds the instability of the air is effected by the rapid heating of its lower layers in contact with the ground, those lower layers being so quickly warmed that there is not time for them to become mixed with the overlying air in which the rate of decrease is normal. If there is much wind we rarely get cumulo-nimbus, because the heated air is mixed mechanically with the overlying parts, and the rate of decrease is approximately normal throughout. Calm air and hot sun are then one set of necessary conditions for the production of instability. But it is well known that thunder-showers and lesser examples of cumulo-nimbus are by no means infrequent in the rear of a cyclone, and such storm clouds are usually attended by considerable wind. They are, as a rule, much smaller than those produced by heat, but they have the same form, and are evidently due to instability in the lower part of the air, and the question is how can that condition be produced. In order to find the answer it is necessary to refer to the temperature phenomena of a cyclonic area. If a cyclone be divided into four quadrants by two lines drawn through its centre, one in the direction in which the system is travelling and the other at right angles to it, then the front right-hand quadrant is the warmest and the rear right-hand quadrant much colder. The cumulo-nimbus clouds of a cyclone are limited to the first part of this cold quadrant, that is to say, to the portion of the storm in which a great volume of cold air is flowing over a district which has just been warmed and wetted by the preceding part. The result is that, the air being warmed by contact with damp ground at a temperature many degrees above that of the air itself, we have produced exactly the same unstable state at a low temperature as we have at a high temperature in the case of heat storms. The lower temperature of the whole is enough to account for the smaller volume of the cloud, and that in turn explains why cyclone thunderstorms are, generally speaking, on a much smaller scale than heat storms.