Weather from Above: What Rossby Waves Are and Where to Watch Them
Rossby waves are giant waves in the jet stream in the upper troposphere (at an altitude of about 9 km), created by the Earth's rotation. They are the main cause of changes in the weather in both the northern and southern hemispheres, often changing on a large scale.
These waves occur in the jet stream, which circles the entire northern and southern hemispheres. The Earth rotates, meaning the air flow cannot move straight; instead, it begins to ripple like a river that meanders through the landscape. This is how Rossby waves are created.
And why are they important? These waves determine where cold or warm air flows, how pressure formations move and how the weather changes. For example, when a wave "gets stuck", it can bring long periods of drought, heat or rain.
Fig. 1 shows a Rossby wave over Europe (see map), representing a meander of the jet stream arched to the north. The jet stream generally meanders more in the northern hemisphere than in the southern hemisphere. In the northern hemisphere, there is a more frequent alternation of continents and oceans, which leads to temperature and consequently pressure differences, and this supports the formation of Rossby waves.
The Rossby wave influences the distribution and formation of pressure systems in the lower parts of the troposphere, which is then reflected in the direction of air flow. The Rossby wave in Fig. 1 caused the advection of very warm air from the south to Western Europe (see map), while colder air from the north reached Eastern Europe (Fig. 2). If this wave persists in one place for a long time, it can lead to long-term high temperatures, which can lead to heat waves and droughts.
In some cases, the Rossby wave can break away from the jet stream and form a separate circulation core, which is referred to as a cut-off low. This circulation formed during September 2024 over central and southeastern Europe (Fig. 3). The cut-off low over this area brought intense rainfall and widespread flooding (see map).
Rossby waves are a key feature of atmospheric circulation and play a major role in shaping the weather in mid-latitudes. They influence where warm or cold air flows, how pressure systems move, and can lead to extreme weather events such as heatwaves, prolonged rainfall, or drought. Observing them, especially on jet stream maps like those available on Ventusky, can help us better understand both current and future weather patterns.
