Jet streams

Background and evolution

Jet streams were noticed in the early twentieth century, when Japanese meteorologist Wasaburo Ooishi tracked fast upper winds using balloons in the 1920s, but they became widely known during the Second World War, when high-flying bombers met unexpectedly strong head winds. They form because of temperature differences between air masses: where warm and cold air lie side by side, pressure surfaces slope steeply aloft, producing strong winds, which Earth's rotation turns into west-to-east flows. The main jets are the polar front jet in mid-latitudes and the subtropical jet near 30° latitude, both westerly, along with seasonal tropical easterly jets.

For India, the most important feature is the seasonal behaviour of the subtropical westerly jet. In winter, the jet splits around the Tibetan Plateau, with one branch flowing south of the Himalaya across northern India. This branch steers western disturbances from the Mediterranean region towards north-west India, bringing winter rain and snow. In early summer, as the plateau heats up, the jet's southern branch weakens and shifts north of the Himalaya. This shift is associated with the sudden onset and rapid advance of the south-west monsoon. A tropical easterly jet then develops at high levels over peninsular India and the Indian Ocean, and its strength is linked to monsoon activity.