‘Sanjiu’ (literally ‘the third nine’) refers to the coldest period of the year in traditional Chinese seasonal culture, typically spanning from the 19th to the 27th day after the Winter Solstice. There’s a common saying in China: ‘The coldest days fall in Sanjiu,’ reflecting the widespread observation that this period experiences the lowest temperatures annually. But why is Sanjiu the coldest?From a meteorological perspective, although the Winter Solstice marks the shortest day and longest night in the Northern Hemisphere—with the sun at its lowest angle—the Earth’s surface continues losing more heat than it gains for some time afterward due to thermal lag. Even as daylight gradually lengthens after the solstice, solar radiation remains weak, and net heat loss persists. By the time Sanjiu arrives, the cumulative heat deficit reaches its peak, resulting in the year’s lowest temperatures.Moreover, cold air masses frequently surge southward from Siberia during this season, especially in East Asia, driven by the strong Siberian High pressure system. Combined with dry air and minimal cloud cover, nighttime radiative cooling intensifies, leading to large diurnal temperature swings and a sharper chill felt by people.Thus, the saying ‘coldest in Sanjiu’ is not just folk wisdom—it aligns well with scientific principles. During this time, people traditionally take extra precautions to stay warm and nourish their bodies to endure the harsh cold.
“三九天”是中国传统节气文化中对一年中最寒冷时段的俗称，通常指冬至后的第19天到第27天（即“三九”期间）。民间有“冷在三九”的说法，意指这段时间是一年中最冷的时候。那么，为什么三九天最冷呢？从气象学角度看，虽然冬至是北半球白昼最短、黑夜最长的一天，太阳高度角最低，但地表热量的散失具有滞后性。冬至之后，尽管白天逐渐变长，但太阳辐射仍较弱，地表持续散热多于吸热，导致气温继续下降。到了“三九”前后，热量收支达到最低点，因此气温降至全年最低。此外，冬季冷空气活动频繁，尤其在东亚地区，西伯利亚高压带来的强冷空气南下，进一步加剧了寒冷程度。加上此时空气干燥、云量少，夜间辐射冷却强烈，昼夜温差大，体感更加寒冷。因此，“三九天最冷”不仅是民间经验的总结，也符合科学规律。人们常在此时注意保暖、进补养生，以应对严寒。
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