interaction between the vertically propagating planetary waves and the isolated high potential vorticity region known as the 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. 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 <ref>M.P. Baldwin and T.J. Dunkerton. 'Stratospheric Harbingers of Anomalous Weather Regimes , Science Magazine.</ref> and may even be responsible for the cold European winters of the 1960s. Stratospheric warming of the polar vortex results in its weakening. When the vortex is strong, it keeps the cold, high-pressure air masses contained in the Arctic; when the vortex weakens, air masses move equatorward, and results in rapid changes of weather in the mid latitudes. Life Bacteria Bacterial life survives in the stratosphere, making it a part of the biosphere. In 2001, dust was collected at a height of 41 kilometres in a high-altitude balloon experiment and was found to contain bacterial material when examined later in the laboratory. Birds Some bird species have been reported to fly at the upper levels of the troposphere. On November 29, 1973, a Rüppell's vulture (Gyps rueppelli) was ingested into a jet engine above the Ivory Coast. Bar-headed geese (Anser indicus'') sometimes migrate over Mount Everest, whose summit is . Discovery In 1902, Léon Teisserenc de Bort from France and Richard Assmann from Germany, in separate but coordinated publications and following years of observations, published the discovery of an isothermal layer at around 11–14 km, which is the base of the lower stratosphere. This was based on temperature profiles from mostly unmanned and a few manned instrumented balloons. See also Le Grand Saut Lockheed U-2 Overshooting top Ozone depletion Paris Gun (projectile was the first artificial object to reach the upper stratosphere) Perlan Project Project Excelsior, world record for highest recorded jump 1961-2012 Red Bull Stratos, world record for highest recorded jump 2012-2014 RQ-4 Global Hawk Service ceiling Upper-atmospheric lightning References External links Current map of global winds and temperatures at the 10 hPa level. Atmosphere Atmosphere of Earth Meteorological phenomena