The UK is famous for its changeable weather, so that we have just basked in the sunniest, driest April since records began suggests something is afoot. And according to experts, the answer to what held back the famous April showers is simple: a blocked weather pattern.
'Boulder' in the jet stream Usually, weather systems cross the Atlantic from west to east as they go around the Northern Hemisphere, and bring a variety of conditions (in the UK, mostly rain and wind). That's thanks to the jet stream, the current of wind about six miles above the Earth that moves "like a river of faster flowing air", steering weather around the world, said the BBC. When that river speeds up or slows down, high- and low-pressure areas form, which "shape the changes in our day-to-day weather".
High-pressure systems mean the atmospheric pressure is higher than its surroundings, while low-pressure systems are the opposite. High pressure in the summer usually means hot, dry spells. In the winter, when clouds get trapped under high pressure, we end up with "unrelenting dull days" – the so-called "anticyclonic gloom". But for the past few months, the jet stream has been "amplified", said the Met Office. That means it "meanders", which can lead to areas of high and low pressure.
For the last few weeks, there has been "a stubborn area of high pressure" above the UK, said climate experts Simon H. Lee and Matthew Patterson, from the University of St Andrews, on The Conversation. That has "diverted the usual flow of mild, moist air from the North Atlantic like a boulder in a river". This a "blocking weather system" – when the weather effectively gets "stuck".
Contested role of climate change Blocked weather was "a driver" of the UK's 1976 summer drought, said Carbon Brief, and of the "deadly" European summer heatwave of 2003. But the important question is how global warming might be affecting the "frequency and severity" of blocking patterns. Trends are extremely difficult to track because there is "no set definition" of a blocking event.
What we do know, said the BBC, is that global warming has made the weather caused by blocking patterns "more intense", leading to more wildfires, droughts and, in low-pressure patterns, "devastating flooding". |