A jet stream is a narrow band of strong winds in the upper troposphere. In our part of the world, it is located at an altitude of approximately 8 to 12 km. Wind speeds there frequently reach 200 to 300 km/h and can sometimes even exceed 400 km/h.
How does a jet stream form?
A jet stream arises as a result of temperature differences between polar and tropical regions. The layers of air close to the ground at the equator are heated much more than those at the poles due to differences in solar radiation. As warmer air is less dense, it has a significantly greater vertical extent. This is also reflected in the tropopause height, which averages 17 km at the equator, around 8 km at the poles and 11 km in our mid-latitudes.
Furthermore, air pressure decreases more slowly with altitude in warmer air than in cold air masses. As a result, at the same altitude (e.g. 6 km), the pressure is lower towards the pole than at the equator (polar trough and equatorial ridge). Differences in atmospheric pressure indicate an imbalance and are counteracted by air currents.
As a result of the Coriolis force, which is caused by the Earth’s rotation, compensatory flows are deflected to the right. If we move northwards with the air, we are therefore deflected eastwards; if we move southwards, we are deflected westwards. This deflection means that the air which is moving from the equator towards the poles in an attempt to equalise pressure differences does not reach the poles, but is deflected eastwards before it gets there.
This gives rise to what are known as circulation cells, which you may well have heard of in geography lessons: the Hadley cell, the Ferrel cell and the polar cell.
The Hadley cell
At the Intertropical Convergence Zone (ITCZ), air rises due to intense heating and flows northwards (and southwards), before sinking again at approximately 30°N as a result of cooling. The air currents at ground level blowing towards the ITCZ are known as trade winds.
The polar cell
The cold, dense air at the pole (surface high) flows southwards and is deflected to the right as a result of the Coriolis force. As it moves southwards, it is warmed again and then rises once more at the polar front, at around 60°N. At high altitudes, the air flows back towards the pole, where it sinks again within the high-pressure area.
The Ferrel cell
This circulation is not caused by temperature differences, but by the two flanking cells (the Hadley cell and the polar cell). This is where the warm, moist westerly winds and the cold, polar easterly winds meet, forming the polar front.
So how and where does a jet stream form?
Jet streams form where circulation cells and different air masses meet. The polar jet stream lies between polar and subtropical air masses, while the subtropical jet stream separates subtropical and tropical air masses. Due to the lack of friction at high altitudes, this results in very strong high-altitude winds with speeds of between 200 and 500 km/h.
Temporal variability in the position of the polar jet stream
The jet stream over the Atlantic and Europe plays an important role in the weather and climate of these regions. It leads to weather situations that favour extreme temperatures or cause moist Atlantic air to flow towards the continent, resulting in flooding. On the other hand, blocked weather situations are also possible, which can lead to droughts. Climate change could alter the characteristics of the jet stream, which would have an impact on extreme weather events. A recent study analysed the strength, tilt and latitude of the Atlantic-European jet stream over the last 600 years on the basis of reconstructions of atmospheric fields. During periods when the jet stream is shifted towards the pole, summer droughts in Central Europe become more severe. If, on the other hand, the jet stream is shifted towards the equator, blocked weather situations are less common, which leads to more frequent flooding in Western Europe and the Alps, particularly in winter. This study also shows that the strength and tilt of the jet stream are strongly influenced by volcanic eruptions. The analysis, which covers a period of 600 years, also indicates that the most recent changes in the jet stream fall within the range of historical fluctuations and cannot be the cause of the increase in floods and droughts observed in recent decades.