ABSTRACT
In each season the maximum winds in the stratosphere or mesosphere (wind speeds in winter approaching double those in the tropospheric jet streams) are attained at heights between 50 km and about 70 km above the Earth (see, for example, MURGATROYD 1957, 1965). This high-level circumpolar vortex is subject to occasional major disturbances
~ sudden warmings '} in winter, accompanied by spectacular rises of temperature at the levels concerned and leading in extreme cases to a period of easterly flow, rather as in summer. The stratospheric circulation, with its disturbances and the characteristic seasonal reversal, extends up to about 80 km and down to 15-20 km; but, because of the low density of the atmosphere at those heights, this deep layer represents only 5-10% of the atmosphere by mass. The momentum carried by the entire stratospheric wind system when near its full seasonal development is also probably at most times only a tenth to a twentieth of that developed in the troposphere, though the ratio clearly varies and may sometimes be rather greater. Any dynamical effects of the stratospheric circulation upon the troposphere are unlikely to be great unless they be cumulative during some persistently anomalous stratospheric regime (see SUN C H U CHI N G et al. 1964) or tip the balance between two possible courses of tropospheric development when this is delicately poised.
