The tropopause is the transition zone between the troposphere and the stratosphere, marking the level where the vertical temperature gradient changes sign: temperature stops decreasing with altitude and begins to increase. Its height varies with latitude and season: it lies at 8–10 km over the poles, 10–12 km at mid-latitudes, and 16–18 km over the equator, where intense tropical convection pushes the boundary higher. The tropopause acts as a natural barrier to convection: most weather phenomena—clouds, precipitation, thunderstorms—remain confined within the troposphere. Only the most powerful supercells and cumulonimbus towers manage to penetrate it, forming the characteristic anvil shape as they spread laterally upon encountering stratospheric stability. Near the tropopause flows the jet stream, with speeds that can exceed 300 km/h. This strong wind belt generates wind shear and clear-air turbulence (CAT), one of the main hazards for aviation. Weather models carefully analyse the tropopause because changes in its height or temperature affect the dynamics of weather systems, including the intensity of tropical cyclones.