Areas of high and low pressure determine our weather. When plotted on weather maps, they form circles or ovals. Their movement over land and sea is what makes our climate so varied. However, sometimes areas of high pressure remain in one location for days on end, obstructing normal weather patterns. The scientific name for this meteorological phenomenon is a “block” or “blocking high,” explains Henning Rust, professor for statistical meteorology at Freie Universität Berlin.

Omega Blocks

An area of high pressure can sometimes be found just north of an area of low pressure. In other cases, two areas of low pressure can accompany a long-lasting blocking high, flanking it to the southwest and southeast. This phenomenon is called an “omega block” and takes its name from the shape these areas of pressure form when plotted on weather maps, i.e., that of the uppercase Greek letter omega (Ω).

A blocking high such as this means many days of blue skies and sunshine. However, these sunny days are more dangerous than you might think. “Blocks like these in summer may lead to a heat wave in one place, but torrential rain in another,” says Rust. This is because the risk of extreme weather is growing. In summer, high pressure leads to high temperatures, dry periods, and even droughts. Under these conditions, both humans and animals can suffer heat stress.

The situation is similarly dangerous in the areas of low pressure that accompany the highs: torrential rain and heavy storms are just two examples of what can occur. In extreme circumstances, they can lead to flooding and landslides. Germany recently experienced these weather dynamics first-hand in the form of the devastating floods that swept areas such as the Ahr valley in summer 2021.

If blocking highs occur in winter or spring, the temperature plummets. The sudden onset of cold weather can be dangerous for many people, especially the homeless. Cold spells in spring can also kill off sensitive plant shoots and buds.

However, not all cases of extreme weather are a result of blocks. For example, the winter storms we experience every year in Germany are the result of moving areas of low pressure – the complete opposite of blocks.

The Jet Stream

Researchers are still investigating how areas of high pressure come to stay in one place and turn into blocks. “Unfortunately, predicting when this phenomenon will occur or even end is difficult,” explains Rust. One thing is certain: the jet stream plays an important role in the process. High above the northern hemisphere, strong westerly winds thread their way around the world like a rippling ribbon. This wind current, known as the jet stream, keeps the air masses in the atmosphere – including atmospheric highs and lows – moving, thus continually changing the weather. Occasionally, the jet stream splits up, with the result that the winds flow around areas of high and low pressure. With no winds to move them, these areas of pressure then remain in one place for a long period of time – after five days, meteorologists call this a block.

Blocks most frequently occur over large continents, for example North America, Europe, or Asia. High levels of solar radiation and high temperatures mean that the high pressure systems remain intact – in the case of an omega block, this effect is further compounded by the areas of low pressure on either side. These high pressure systems are so stable that they redirect the jet stream.

The situation is made more difficult due to the fact that climate change is changing the atmosphere as we know it. Many researchers believe that, in the future, the jet stream may be increasingly negatively affected and may even become weaker. If this happens, scientists are unsure what effect it will have on blocks. Will they become more common? Will they last for longer periods of time?

Henning Rust has already been able to gain some interesting insights into the subject. The meteorologist and his research team analyzed historical weather data as part of a study that was published recently. The aim of the study was to automatically collect data on the frequency of blocks in Europe from 1990 to 2019 and to analyze them using statistical models. This led the team, comprised of both the Theoretical Meteorology and Statistical Meteorology working groups at Freie Universität, to make some important discoveries. The analysis showed that the total number of blocks in Europe has not changed much over the past decades; however, researchers were able to observe some changes when they looked at how blocks develop over the seasons. The seasonal blocks that used to occur in November and December started occurring in spring. “What’s more, we are seeing more and more omega blocks during the summer that simply will not budge,” says Rust. More omega blocks could lead to more heat waves.

“In densely populated cities, inhabitants are particularly affected by heat waves such as these,” claims Rust. “Few can tolerate exposure to this much heat, especially the elderly. Furthermore, cities offer few possibilities for citizens to cool off. The climate is already changing; politicians need to act now.” On a local level, creating a heat action plan for Berlin would be an important first step toward dealing with the effects of prolonged blocks.

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This article originally appeared in German on July 2, 2022, in the Tagesspiegel newspaper supplement published by Freie Universität Berlin.