Overview
Although weather generally changes, in systems known as “heat domes,” an air mass is isolated from the systems around it; hot air remains trapped inside while moisture cannot enter the region. This phenomenon, which was uncommon until recently, can create rainless and cloudless conditions for weeks. Nights that are as hot and oppressive as the days cause fluid loss in plants and animals, cardiovascular problems, and other serious difficulties.
Latest situation
A heat dome is not the same as a period of hot summer weather or a heatwave. Although there is no universal definition of a heatwave, it generally refers to daytime temperatures rising significantly above the seasonal average maximum for at least several days; in UK, at least two days are required for it to be officially designated as such. As global surface temperatures have risen due to human-induced climate change, these heatwaves have become more frequent.
Situation on the ground
A heat dome requires calm, warm air and a large area of high pressure that remains in place for a long time. Rising warm air is pushed downward by high pressure in the upper layers, compressed, and heated adiabatically. Under cloudless skies, the ground gradually heats up and dries out, while nighttime provides very little relief. An “atmospheric block” keeps the stagnant air in place.
Western North timeline
There are two types of blocks, called Omega and diffluent; the Ω-shaped Omega block is common in heat domes. Blocks can last from days to weeks; in UK, one persisted for months during the summer of 1976. The event that affected western North America from late June to early July in 2021 broke many records, including 49.6°C, the highest temperature measured in Canada.