Meteorologist: Indian summer is the autumn equivalent of "cold gardeners"

Meteorologist Dr. Dariusz Baranowski explained in an interview with PAP that Indian summer, a period of warm days following the first autumnal cooling, is related to the cyclical dynamics of the so-called Rossby wave. A similar phenomenon in spring is responsible for the cold spells during the "cold gardeners" period.

Atmospheric physicist Dr. Dariusz Baranowski from the Institute of Geophysics of the Polish Academy of Sciences noted that what is known in meteorology as Indian summer is not the same as the golden Polish autumn . "What we have in September – the golden autumn – is simply part of summer," he clarified. He explained that although at our latitude, most solar energy arrives in June, due to thermal inertia, the land and oceans accumulate heat and release it with a significant delay. Therefore, even around the autumnal equinox, it is still warm.

Indian summer, on the other hand, is a period of warm weather following a significant cool down, such as the first frost. It usually occurs in October or November. This concept is known throughout Europe – from Serbia to Finland – and also in North America (as Indian Summer).

This phenomenon is associated with Rossby waves, a result of the Earth's rotation. These are large-scale, undulating meanders in airflow occurring in the upper atmosphere. Typically, they form several interconnected waves (usually three or four) around an entire hemisphere, acting as a boundary zone separating cold air from the north from warm air from the south. Rossby waves influence the movement of local pressure systems.

"One of its most visible manifestations is the jet stream," said the atmospheric physicist. He specified that it is a narrow band of extremely strong wind (reaching speeds of hundreds of kilometers per hour) at high altitudes. It is thanks to this stream that airplanes flying, for example, from America to Europe can achieve very high ground speeds.

Rossby waves occur in the mid-latitudes of Europe, North America and northern Asia, as well as at corresponding latitudes in the Southern Hemisphere, where, however, these areas are mostly inhabited by small populations.

As the seasons change, the Rossby wave pattern undergoes a reorganization. In autumn, as the wave moves southward, or in spring, as it travels northward, it can break and reconfigure. At this point, the wave can "stall" for several days, which meteorologists call a circulation block. The normal west-to-east movement of weather systems is then suspended.

When such a blockage occurs in autumn, stationary pressure systems such as the Siberian High, the Azores Low, or the Greenland High gain a dominant influence on the weather in Central Europe. In our part of the continent, the Siberian High usually plays a key role, bringing in dry and, at this time of year, still warm continental air. Because the land is warmed from the summer, and the air within the high is dry and free of strong winds, pleasant weather occurs for a few days – and this is what we call Indian summer.

"The same mechanism in spring is responsible for the May cooling, or 'cold gardeners,'" the researcher added. However, he noted that the reconfiguration of the Rossby wave and the dominance of the Siberian High then bring in air from the continent that's still cool from winter. This is the source of spring frosts, which are so unpopular with farmers.

Interestingly, "cold gardeners" occur with great regularity in early May, while the timing of Indian summer is much less predictable. This is related to weather patterns in distant parts of the world, including the dynamics of the Indian monsoon and the shifting of rainfall zones in the tropics. These global processes influence the precise timing of the Rossby wave's collapse.

"The Indian summer example shows that weather is a physics of interconnected vessels. What happens far away, like the monsoon in India or rainfall zones in Africa, has a real impact on the weather in Poland and its predictability. The fact that cyclical processes occur worldwide means that certain phenomena here also have a cyclical nature," concluded Dr. Baranowski.

Ludwik Tomal (PAP)

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