“Indian summer” (Autumn Tiger in Chinese) refers to the short period of hot weather after the Start of Autumn, typically in late August or early September, lasting for approximately 7 to 15 days.

The northward shift of subtropical atmospheric pressure (known as the subtropical high) over the western Pacific which is supposed to gradually move southward contributes to the formation of “Indian summer”. Influenced by the subtropical high, the temperature rises again with sunny days, cloudless sky and fierce sunshine. China has a vast territory so the manifestation and duration of the “Indian summer” differs accordingly. Sometimes the “Indian Summer” comes back and goes. Generally speaking, during “Indian summer” the temperature is comparatively high, but the air is dry with plenty of sunshine and it is neither too hot nor too cold in the morning and night.

Although the majority of people regard the sunny, warm weather in autumn as “Indian summer”, others disagree.

The “Autumn Tiger” ought to be “Summer Tiger”: In other words, the “Indian Summer” does occur in the summer. Judging from the daily average temperature rule that the first day of five successive days in which the daily average temperature is between 10℃ and 22℃ is considered the initial day of autumn, before the End of Heat (the 22nd and 23rd of August), many regions of China are still in summer. The 15 days of the Start of Autumn are within the summer dog days, a time when sweltering heat is normal.

Furthermore, “Indian summer” ought to be weather that is first cool and then hot. As defined by the Dictionary of Atmospheric Sciences, “Indian summer” is known as the recurrence of hot weather in a short period after the Start of Autumn. The key term is the weather’s change from cool to hot. Indeed, every year after the End of Heat, the scorching heat lessens. But due to the influence of the subtropical high, the temperature temporarily rises again. The cooling down process in the southern part of China is not as evident as it is in the north. The Dictionary also mentions that the “Indian summer” typically occurs in late August and early September, lasting for several days to half a month, sometimes even longer.

In many years, it is hot in both the Start of Autumn and the End of Heat periods. When this long time of heat happens, more attention should be paid to the “Indian summer” period. One should also keep in mind measures to prevent heatstroke.

So far, there is no unified and measurable criteria for “Indian summer”. According to the experts of Beijing Observatory, the highest temperature of the “Indian summer” period is usually over 33℃ and lasts for days. However this is only a reference point.