NASA image sourced from National Oceanic and Atmospheric Administration ( 

There were a series of high-impact weather events in the Northern Hemisphere in June and July 2021.

An extreme heatwave affected western North America, sparking wildfires and causing deaths and economic losses.

Heavy rainfall in western Europe resulted in flooding, leading to loss of life and property damage.

Parts of China experienced record-breaking amounts of rainfall that paralysed economic hubs.

These events, while extraordinary in their impact, were caused by a common atmospheric mechanism: Rossby waves

These waves are the main drivers of weather outside the Tropics (including in the Southern Hemisphere).

They form as undulations in the jet stream, a band of strong winds in the upper atmosphere. High- and low-pressure systems are carried along by the jet.

However, when Rossby waves grow very large they break, much like ocean waves, causing highs and lows to remain stationary over one location for some time.

This can contribute to strong heating of the Earth’s surface or flooding from stalled rain systems.