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The volcanic cloud is in the lower stratosphere above the 380 K isentropic level, which characterizes the tropopause.

If an isentropic curve crosses the heterogeneous region, the point where it enters this region must, just as for the isothermals, be connected with the point where it leaves the region by another curve.

What has been said concerning the relative position of the border-curve and the isentropic curve may be easily tested for points of the border-curve which represent rarefied gaseous states, in the following way.

As a limiting case we imagine the process to be isentropic.

Let us imagine in the same way all the isentropic curves drawn for homogeneous states.