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.