The ground-based microwave radiometers GROMOS and MIAWARA at Bern (Switzerland) continuously measure ozone and water vapor profiles from 20 to 70 km altitude. A major sudden stratospheric warming occurred around 19 February 2008 with minimal temperatures of 189 K at 40 hPa and maximal temperatures of 300 K at 4 hPa. During the stratospheric warming the Swiss ground-based radiometers observed a depletion of ozone and an enhancement of water vapor while NASA's CALIPSO satellite instrument measured a large PSC area over Europe. Ozone depletion in the lower stratosphere is explained by transport of ozone poor air from the cold polar vortex. The depletion of upper stratospheric ozone is caused by a sudden temperature increase of about 50 K. A simulation of a chemical box model confirms that a major fraction of the observed decrease of the ozone mixing ratio at 4 hPa can be explained by the effect of the increasing temperature on the ozone chemistry. The chemical ozone destruction is dominated by a catalytic NO<inf>x</inf> cycle, which is more efficient at higher temperatures. The water vapor enhancement can be explained by transport processes. The rather unusual occurrence of a PSC and a sudden stratospheric warming at midlatitudes suggest that further monitoring of the Earth's middle atmosphere is required for the timely detection of unexpected problems due to ozone loss and climate change. Copyright 2009 by the American Geophysical Union.