ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) image showing land temperatures in Washington, USA, on 25th June 2021. The city of Seattle is at centre. Red indicates temperatures between 30 degrees Celsius (C) and 40 degrees C. Dark red indicates temperatures between 40 degrees C and 50 degrees C (black). During late June a heatwave in the Pacific Northwest led to all-time record temperatures in a number of US and Canadian cities. It was caused by a strong flow of air moving from east to west, pushing cool ocean air away from the land. The flow was due to exceptionally high pressure inland and low pressure near the coast.