Cloud Studies

by Arthur William Clayden

Now, the more even the distribution of temperature on the ground the less the probability of coarse interconvection, and the same is true of any higher stratum of air, provided it is free from disturbing influences from outside. If, therefore, we have large currents near the ground, ending, as they must, in cumulus, it has already been explained that these clouds stop the action, and the general system of large currents will be restricted to the region in which they occur. At some distance above the lower clouds the only difference will be that water vapour has been brought up to their level in great abundance. Smaller systems of interconvection can then exist, and so we may have the spectacle of several layers of cloud—cumulus capping the great currents of lower regions, alto-cumulus forming the summits of the smaller currents of intermediate regions, and cirro-cumulus floating far above both.

Frequently it happens that before the ascent of vapour has gone quite far enough to produce a cloud, other causes co-operate, and the cloud makes its appearance suddenly over considerable patches of sky. The most potent of these is a fall of the barometric pressure, which is brought about by some of the air far above the region of even the highest clouds flowing away to some other district. The air at all lower levels being thus relieved of the superincumbent pressure, immediately expands, and is thereby cooled throughout. Consequently, if at any level it was near its point of saturation, it will be carried beyond that point, and cloud will rapidly make its appearance over a large part of the sky, possibly at more than one level. Stratiform arrangements will be the rule; but if interconvection is going on at the time, its presence will be betrayed by a granular or cumuloid structure. Interconvection clouds should then be most frequent, and best formed when the air as a whole is still or moving slowly (so as not to create great eddies), when the temperature is rising rapidly, and when the barometer is making a sudden fall. All these conditions are met in thunder weather, and at the time when a summer anticyclone is giving way. It will be remembered that many of the most beautiful forms have been described as forming under one or the other of these very conditions.

A second contributing cause, and one which tends to make the condensation in patches or long broad bands ranged roughly at right angles to the direction in which the air is moving, has been referred to earlier. It is the passage of the air over an undulating country; the up-and-down movements of the lower air being transmitted upwards to great altitudes, as ever broadening and flattening waves. If the upper air is flowing more rapidly than the lower, these broad waves may be far ahead of their real cause, which will, therefore, quite escape recognition, but the phenomenon is constantly to be detected in the arrangement of the lower clouds. Two instances in the writer’s experience will suffice. It was desired one morning to measure the altitude of some small clouds which were passing from the north-west at a height of probably between 2000 and 4000 metres, over a hill only about 150 metres higher than the valley in which the apparatus was fixed. In order to make the measurement, it was necessary for the cloud to cross the valley and appear in the same field of view as the sun, according to the method that will be described further on. But in order to cross the valley the air had to descend, and so, of course, had the cloud stratum, though to a less extent. But small as the descent was, it was enough to dry up the clouds entirely, and for more than a couple of hours the clouds came sailing over the hill, disappearing entirely, and then reforming so far beyond that no measurement was possible, since not one single fragment came near enough to the position of the sun, which remained shining brightly through a broad clear gap between two patches of cloud-strewn sky.

On another occasion considerable preparations had been made for some photographic observations during an eclipse of the sun. The observatory stands on the eastern side of the valley of the Exe, which is flanked on its western side by a long ridge of hills going up to 800 feet above the sea. Beyond these hills lies the deep, narrow valley of the Teign, and beyond that the granite ramparts of Dartmoor, 1000 feet above the sea. The wind was blowing gently across the two valleys, and shortly before the eclipse began a broad strip of thin cloud formed above and rather towards the eastern side of the Exe valley, just where the sun was, while at the same time the sky was practically clear half a mile further east, and bright sunlight was streaming down on the ridge between the two rivers a few miles towards the west. The cloud was never thick enough to quite hide the sun, so that the eclipse was easy to watch with the naked eye; but in spite of fairly rapid movement of the cloud masses as they drifted before the sun, they kept on forming in just the same place, and completely prevented the carrying out of the programme planned. It is almost certain that the phenomenon was brought about by an upward moving wave marking the place where the level of approaching saturation was upheaved by the disturbance caused by crossing the two valleys and intervening ridge.

These two instances are not quoted as examples of a rare occurrence, but as definite simple instances of a phenomenon which may be constantly observed, and as proof that the conformation of the ground does exercise an influence upon the distribution of cloud.

But no irregularities of the ground will suffice to explain the minute waves and ripples which have been described at the beginning of this chapter. These must be due to wave disturbances in the air itself. They have been explained as due to two different currents of air, either a warm damp current flowing over a cold one, or vice versâ. Now, such an occurrence as a warm damp current flowing over a cold one must be very rare, though it is impossible to deny that it might occur. The immediate contact of a cold current above a warm damp one is equally unlikely, unless the general atmospheric condition were greatly disturbed, which is the same thing as saying that wave clouds would not occur. They are most frequent at just those times when interconvection has freest play, and this is amply sufficient to account for a plane of saturation without any necessity for a hypothesis of two layers of air at different temperatures all but producing cloud at their junction. No convincing evidence of cloud production by such means has yet been adduced, and it is better to rely upon causes which we know do operate than to call in theories as to what might possibly happen. This is one of those points in the study of clouds which need investigation, and until proof is forthcoming it is better to say that the admixture of two strata of air might conceivably produce cloud, but most forms can be accounted for by other causes of which we have more positive evidence.

Still, the wave clouds are due to waves, and there seems no other way of accounting for them than the supposition of gentle differential currents. But if such currents occur the ripples and waves will not be limited to a definite surface, so to say, of contact, but will be propagated upwards and downwards for considerable distances from the level of greatest disturbance. Whether, therefore, the level at which the natural operation of interconvection has produced saturation is high or low in this region, the result will be the marshalling of the ascending and descending elements of the convection system in the characteristic waves.

The differential currents, then, which cause the waves must not be conceived as producing those waves at a surface of contact, nor must the currents be thought of as separated by any definite surface, but rather by a region of variable but usually considerable depth, in which the air is disturbed by a series of small slow eddies and oscillatory movements. When the waves are parallel straight lines the air currents may be really portions of a whole, having the upper part more rapid than the lower. In such a case the direction of movement should be at right angles to the cloud lines. If the upper current differs in direction as well as velocity, the direction of movement of the clouds will be intermediate, and will resemble that of the upper or lower current, according to their relative distances from the plane at which the clouds are formed.