If the westerly wind flow common at our latitude is interrupted over a prolonged period, this is referred to in meteorology as a blocked weather situation. Find out how nearly stationary high-pressure areas, known as “blockings”, arise and why they sustain themselves and can often bring stable weather conditions for more than a week. We explain the three main types of this weather constellation – the omega block, the amplified ridge and the Rex block – and shed light on the far-reaching impacts of blocked situations on weather activity depending on the time of year.
What is a blocked weather situation?
We talk about a blocked weather situation when the prevailing westerly wind flow common at our latitude is interrupted or diverted over a prolonged period of time. This is normally caused by high-pressure areas that remain nearly stationary. A particular characteristic of such highs is that, once the flow pattern has become established, they can effectively sustain themselves and therefore persist for a very long time. Such a “blocking” lasts for around seven to ten days on average but can also persist for two to three weeks in extreme cases.
The three main types of blocking high
A general distinction is made between three types of blocking, based on their structure and the arrangement of the pressure centres.
The most well-known is the omega block, characterised by a strong area of high pressure trapped between two isolated lows. The overall flow pattern is reminiscent of the Greek letter omega Ω, from which it derives its name. The high rotates clockwise, while the two accompanying lows rotate anticlockwise.
A similar, very stable weather situation sets in if a high, or in this case a ridge of high pressure, reaches far to the north and is trapped between two troughs. This situation, referred to in technical jargon as an “amplified ridge”, does not comprise two isolated lows that have broken off from the large-scale flow, but rather two troughs protruding very far south with a ridge in between stretching to the north. This leads to a strong flow circulating from north to south (meridional flow) that practically stays put and thereby ensures stable weather.
A third variant is the so-called Rex, dipole or high-over-low block. With this constellation, an isolated area of high pressure lies immediately to the north of an isolated area of low pressure. The jet stream can be influenced by this in two ways. Either it is split into a northern and a southern branch, which later reunite again, or it is steered in the shape of a reflected S around the dipole – at the northern end around the high pressure to the east, in the middle between the two centres of pressure back to the west, and finally at the southern end around the low back to the east.
In practice, the flow patterns and the arrangement of the pressure centres are rarely as clear-cut as in the illustrated visualisations. Hybrid forms and blending between the different types of blocking frequently occur.
Regions in which blockings occur particularly frequently
Observations show that blocking highs are not distributed evenly, but rather occur more frequently in certain regions. In the northern hemisphere, these are Greenland, Europe (especially Scandinavia), the region surrounding the Ural Mountains and the north of the United States and Canada. The conditions in these regions appear particularly favourable for the formation of stable blocking situations.
Impacts of blocked weather situations
Blocked weather situations lead to distinctive weather phenomena manifested with differing degrees of intensity depending strongly on the time of year and geographical position. The areas under the influence of the high are regularly affected by drought in the spring and summer and additionally by heat waves during the warm season. On the other hand, severe cold snaps can occur in the winter half-year on the eastern flank and partly at the southern edge of the high, which can trigger events such as heavy snowfall around the Mediterranean. At the same time, extreme precipitation events can occur in some cases at the front of the accompanying lows and at the upper edge of the high in the range of the strongest winds.
High forecasting quality during blocked weather situations
As soon as a blocked weather situation has set in and the large-scale flow has come to a standstill, a very reliable forecast can be drawn up for several days that would otherwise not be possible. This is due to the high persistence of blocked weather situations. Interestingly, during a blocked weather situation, forecasting the very next day can on some days be considerably more difficult than forecasting the period five to ten days ahead.
Why are the weather situations shown so stable?
The development and maintenance of blocked weather situations is extremely complex and depends on a range of interacting factors. These include interactions between Rossby waves and different scales, which can lead to deformations of the jet stream or even cause these planetary waves to break. In addition, geographical barriers such as mountain ranges and thermodynamic processes facilitate the formation of such blockings or help to maintain them.
What all blocking situations have in common are characteristic flow patterns that interact to help maintain the pressure constellations. For example, a strongly meridional flow causes the zonal western flow that otherwise prevails to be strongly diverted and the pressure formations to shift only slowly from west to east and in some circumstances to remain stationary.
In addition, warm and moist air is lifted at the front of a low and transported far to the north. When this air rises, it cools and the vapour condenses to form clouds and rain. A large amount of energy is simultaneously released in the form of latent heat. This is referred to as a warm conveyor belt. As well as the precipitation caused by this in the low-pressure area, the release of latent heat also helps high pressure to develop and persist further down the flow.
These explanations have been kept deliberately simple. It should also be mentioned that not all processes contributing to the development and maintenance of blocking situations are yet fully understood.