The summer and winter monsoon is very obvious in Sanyi. Due to geological features and landform, heavy fog occurs quite often. Therefore, it is also known as “fog town” and “city of wood sculpture”. Apart from Dadu Terrace, plains dominate the south of Huoyan Mountain and extend to the Jianan Plain. The north side of Huoyan Mountain is the Miaoli hill area. When the southwest monsoon blows northward, it passes through the plains and the Huoyan Mountain is the very first elevated area the monsoon hits. The updraught is accompanied by temperature drop, and results in the foggy and humid climate in the Sanyi area. Drivers on the freeway can feel this weather condition the most. While it’s sunny weather and clear skies in the south, one has to slow down his car speed and even put on some more clothes, because the area is usually covered with heavy fog with limited visibility. 

Temperature

Because of the landform, there is temperature difference at the south and the north of town. The temperature in Liyutan Village at the south is one to two degrees higher than the north part of town. It is colder in the north in the winter. The temperature normally drops below 10℃ and the average temperature in January is 15℃. The hottest season is July, with the average temperature of 28℃. The year-round average temperature is 22℃. The winter weather in Huoyan Mountain varies, and the mountain itself is actually the parting line for the weather in Taiwan. The north of Huoyan Mountain is cold and humid in winter, while it is mild and comfortable at the south. 

Precipitation:

Due to summer and winter monsoon and the influences from geological features, the average precipitation in Sanyi ranges from 1500 to 2000 mm. Precipitation days are mostly in the period from January to June, and actually concentrated in April. There is less rain in November. From March to September is the rainfall period. Because of the strong southwest monsoon that brings much rain from the beginning of June to the end of September and its coincidence with typhoons in the same period, there are often heavy rains and various disasters occur. On August 7, 1959, heavy thunderstorms resulted in flash flood, and caused severe damages to properties. It claimed 31 lives and is one of the most serious flooding in recent years. 

Wind Direction:

The climate in Taiwan belongs to island-type climate. The wind direction is greatly influenced by the temperature differences between land and ocean. The southwest monsoon in summer often brings thunderstorms, while the northeast monsoon in winter brings strong winds to the area. The wind is especially strong at the Bogongkeng Section of Jianfeng Highway (Provincial No. 13) due to its geological features. It is a very typical monsoon climate zone.