Long range infrasound propagation strongly depends on the state of the stratosphere. Infrasound can be efficiently ducted between the earth's surface and the stratopause under a favorable wind and temperature structure between 40 and 50 km altitude. Understanding infrasound propagation under variable stratospheric conditions is of importance for a successful verification of the Comprehensive Nuclear-Test-Ban Treaty, where infrasound is used as a verification technique. Inversely, infrasound observations can be used in acoustic remote sensing of the upper atmosphere. In previous studies, attention has been paid to the strength and direction of the circumpolar vortex wind. In this study, an analysis is made of the temperature effect in the stratosphere on infrasound propagation. A case study is presented from an explosion during a Sudden Stratospheric Warming. During such conditions, the size of the classical stratospheric shadow zone (200 km) appeared to be reduced by a factor of two. The occurrence of such conditions is quantified by evaluating ten years of atmospheric specifications. It unexpectedly appeared, that the size of shadow zone can become smaller than 100 km which is confirmed by evaluating infrasound detections from mining blasts in southwestern Siberia, Russian Federation. These results are valid over a latitudinal range of 20N to 60N, which is determined by the stratospheric surf zone.