从气象学来说,每五日为一“候”。当平均气温每候高于22℃时才是夏季的到来。每候平均气温等于或大于30℃才算酷热期,三伏天正值酷热期,所以俗语说:“热在三伏”。三伏天在我国最热的地区是长江流域,那里七月初梅雨期一过便处于太平洋副热带高气压控制之下。在这个“大火炉”地区,高温日数普遍有20~30天(气象部门常以最高气温在35℃以上的日数来表示某地区炎夏日子的多少),如南京17天,武汉22天,重庆33天。其实这“三大火炉”并非全国最热的 From meteorology, every five days as a “waiting ”. It is summer when the average temperature is higher than 22 ℃. When the average temperature is equal to or greater than 30 ℃ is considered hot, dog days are hot, so the saying goes: “hot in three volts.” The dog days are the hottest areas in the Yangtze River basin, where the Meiyu period in early July is under the control of the Pacific subtropical high. In this “stove ” area, high temperature days generally 20 to 30 days (the meteorological department often to the maximum temperature above 35 ℃ in the number of days to indicate how much a region), such as 17 days in Nanjing, 22 days in Wuhan, Chongqing 33 days. In fact, this “three furnace ” is not the hottest in the country