Structure and Maintenance of Dry-type Moisture Discontinuities Not Developed by Subsidence

are frequently observed flowing over moister and colder currents in cases where no subsidence is indicated. For long periods of time, this warni, dry air layer can be found aloft over large sections of the country, particularly over the southern United States, and for this reason its life history presents an exceedingly important problem of synoptic meteorology. The source of this air is a controversial topic (2, 3). Indeed, the present state of the question is such that even the general classification Polar or Tropical cannot be applied with certainty; and in this report no attempt will be made to label this air mass so as to imply a certain source region or to ascribe to it any definite history. I shall be concerned merely with its esistence above moister air; and for the sake of conformity with customary synoptic usage, shall refer to it as Ts air.l The extremely sharp transitions in moisture observed in passing from underlying moist air masses into the Ts currents provide an opportunity to study several interesting questions, one of which is the thermal balance accompanying these transitions. In the case of subsidence inversions there can be litt’le doubt that thermodynamic factors, particularly a t upper levels, play the dominant role in governing the thermal conditions within the inversion layer. On the other hand, the discontinuities wherein Ts is the superimposed air mass seem to be governed in large This naturally implies that