Four hundred and twenty-two nights of middle atmospheric temperature observations were obtained in the High Arctic at Eureka (80 ¿N, 86 ¿W) during six wintertime measurement campaigns from 1992--1993 to 1997--1998 by using a lidar and meteorological balloons. The measurements reveal that temperature changes of greater than 30 K occur in the stratosphere within the space of a few weeks. In a comparison with maps of wind in the Northern Hemisphere, it is clear that much of the thermal variability is associated with movements of the wintertime stratospheric vortex over the measurement site. Measurements in the vortex core show a persistently cold lower stratosphere and warm upper stratosphere. Conversely, outside of the vortex altogether the lower stratosphere is relatively warm and the upper stratosphere is cold. By separately examining measurements obtained inside the vortex core, a 25¿2 K annual warming of the upper stratosphere in late December is apparent. This annual vortex core warming is distinctly different from planetary-wave-driven sudden stratospheric warming and the seasonal march of temperatures due to changing insolation. ¿ 2000 American Geophysical Union