<P> The stratosphere is a region of intense interactions among radiative, dynamical, and chemical processes, in which the horizontal mixing of gaseous components proceeds much more rapidly than does vertical mixing . The overall circulation of the stratosphere is termed as Brewer - Dobson circulation, which is a single celled circulation, spanning from the tropics up to the poles, consisting of the tropical upwelling of air from the tropical troposphere and the extra-tropical downwelling of air . Stratospheric Circulation is a pre-dominantly wave - driven circulation in that the tropical upwelling is induced by the wave force by the westward propagating Rossby Waves, in a phenomenon called Rossby - Wave pumping . </P> <P> An interesting feature of stratospheric circulation is the QBO in the tropical latitudes, which is driven by gravity waves that are convectively generated in the troposphere . The QBO induces a secondary circulation that is important for the global stratospheric transport of tracers, such as ozone or water vapor . </P> <P> Another large - scale feature that significantly influences Stratospheric Circulation is the breaking planetary waves resulting in intense quasi-horizontal mixing in the midlatitudes . This breaking is much more pronounced in the winter hemisphere where this region is called the surf zone . This breaking is caused due to a highly non-linear interaction between the vertically propagating planetary waves and the isolated high potential vorticity region known as Polar Vortex . The resultant breaking causes large scale mixing of air and other trace gases throughout the midlatitude surf zone . The timescale of this rapid mixing is much smaller than the much slower timescales of upwelling in the tropics and downwelling in the extratropics . </P> <P> During northern hemispheric winters, sudden stratospheric warmings, caused by the absorption of Rossby waves in the stratosphere, can be observed in approximately half of winters when easterly winds develop in the stratosphere . These events often precede unusual winter weather and may even be responsible for the cold European winters of the 1960s . </P>

Point d is 10 kilometers above earth’s surface. in which layer of the atmosphere is point d located