What is the jet stream?

Jet streams are ribbons of very strong winds high in the atmosphere, typically found around 9 to 16 kilometres up near the boundary between the troposphere and the stratosphere. They form where large masses of warm and cold air meet, and the greater the temperature difference, the faster they blow, often exceeding 160 kilometres per hour and sometimes far more. There are usually two main jet streams in each hemisphere, the polar and subtropical jets, generally flowing from west to east. They steer weather systems, so shifts in the jet stream can bring storms, heatwaves, or cold snaps. Airlines use them to their advantage, riding a tailwind eastward to save fuel and time while avoiding them when flying west. Jet streams were dramatically discovered by high-altitude pilots during the Second World War.