Why does the jet stream sometimes get stuck and park the weather over one place?

The jet stream effectively gets stuck when a large pressure pattern along its path becomes nearly stationary, called a block, and a block can leave the same weather over one region for days or weeks. Jet streams are fast flowing, narrow air currents in the atmosphere. Each hemisphere has a polar jet around its polar vortex at about 9 km altitude, typically moving around 180 km/h and often much faster.

The jets are the product of solar heating, which drives the planet's big circulation cells, and the Coriolis force acting on the moving air. The path of a jet is not straight. Each large meander of the jet stream is a Rossby wave, caused by changes in the Coriolis effect with latitude. Because the jet's path steers storms at lower levels, forecasters watch it closely. In 2007 and 2012, Britain experienced severe flooding because the polar jet stayed south for the summer.

When a pressure pattern stops moving, meteorologists call it a block. Blocks are nearly stationary large-scale patterns that redirect migratory cyclones, and they can remain for several days or even weeks, giving the affected areas the same kind of weather for an extended period. Some unusually intense summers, such as the 2003 heat wave in Europe, were the result of entrenched highs that became detached from the jet stream for a prolonged period and allowed warm, dry air to build in place.

There is a human story behind our knowledge of all this. The Japanese meteorologist Wasaburo Oishi detected the jet stream in the 1920s by tracking pilot balloons near Mount Fuji, but his work went largely unnoticed outside Japan because it was published in Esperanto.

Why blocks start and end is still not well captured in numerical weather forecasts, and it remains an open area of research.