Jet streams are fast flowing, relatively narrow air currents found in the atmosphere at around 12 km above the surface of the Earth, just under the tropopause. They form at the boundaries of adjacent air masses with significant differences in temperature, such as of the polar region and the warmer air to the south. Because of Earth's rotation the streams flow west to east in both the Northern Hemisphere and the Southern Hemisphere due to the Coriolis effect. The paths of the flows typically show a meandering shape, and these shapes themselves propagate east, at lower speeds than that of the actual wind within the flow.