来自wiki百科全书的解释:An Indian summer is a meteorological phenomenon that occurs in the autumn, in the Northern Hemisphere. It refers to a period of considerably above normal temperatures, accompanied by dry and hazy conditions, after there has been a killing frost.
其实楼上的解释,小阳春,是正解,就是气候温和,有点干燥,常出现在严霜之后。
以下是wiki更多的解释,
The modern use of the term is when the weather is sunny and clear, and above 21 °C (70 °F), after there has been a sharp frost; a period normally associated with late-October to mid-November.[1]
In some regions of the south-eastern United States, 'Indian summer' is colloquially used to describe the hottest times of the year, typically in late July or August. But in the South, as elsewhere in the US, this period is more commonly known as the dog days, in reference to the position of Sirius, the 'Dog Star' and brightest star in the sky other than the sun. In the desert south-western United States, where frost is rare, the term is sometimes used to refer to a brief period of hot dry weather which occurs after the hottest months and before the onset of winter rains, typically in October or November. It may also be used to refer to any unseasonably warm weather during the first few weeks of the rainy season, before the approach of spring.
The term 'Indian summer' is also used metaphorically to refer to a late blooming of something, often unexpectedly, or after it has lost relevance. This is comparable to the use of the term renaissance in the sense of 'revival', but it carries the added connotation that the revival is temporary. A famous use of the phrase in American literature is Van Wyck Brooks' New England: Indian Summer, a sequel to his Pulitzer Prize-winning The Flowering of New England
参考资料:wikipedia