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Abstract Yilan is located in northeastern Taiwan, in the northeast monsoon zone. From October to February every year, moisture air brought by the northeast monsoon causes heavy rains in the area. This study analyzes observational data from two Central Weather Bureau weather stations in Su-ao and Yilan from 1982 to 2009, and from 13 telemetry automatic weather stations in mountains or near mountain areas between 1996 and 2009. All stations in the study were divided into two regions, depending on their location north or south of the Lanyang River, to compare their seasonal variation and their interannual variation during the northeast monsoon period; and in so doing, explore the climatic differences between the two regions. The results of the seasonal change show that no significant differences in mean annual temperature could be detected between Yilan and Su-ao. The Su-ao station’s annual precipitation, precipitation days, precipitation intensity, and average wind speed were all higher than those of the Yilan station, while the average annual relative humidity was higher in Yilan than in Su-ao. The annual precipitation for the telemetry station south of the river was higher than that of the one north of the river. The difference was more obvious during the northeast monsoon period, and there were more precipitation days south of the river from October through April the following year. The interannual variations in the northeast monsoon period are as follows: Su-ao’s precipitation and number of precipitation days were higher than those of Yilan and showed a declining trend. The declining trend in precipitation and precipitation days was more obvious in Yilan than in Su-ao. The precipitation intensity was higher in Su-ao than in Yilan; both places displayed a slight trend of increased precipitation intensity and had a trend of rising temperatures, but Yilan’s rate of increase was higher than Su-ao’s. Both places’ average relative humidity was gradually decreasing year by year. Before the year 2000, the relative humidity was higher in Yilan; after 2000, it was higher in Su-ao, showing that the trend of declining temperature is stronger in Yilan. The average wind speed was higher in Su-ao and showed a slight increasing trend, while the increasing trend in Yilan was more apparent. The measurements at the southern region telemetry station showed that rainy days and precipitation intensity were higher than in the northern region, and showed a declining trend. The distribution of rainfall at the automatic telemetry stations in the southern and northern regions of the Langyang River were as follows: The most rain in the northern region fell in the mountain area close to the west side of the Xueshan mountain range; south of the river, the mountain area south of the Lanyang plane was where the most rain fell. In the northern region, the number of precipitation days increased with higher altitude, while in the southern region this relationship was not obvious. There was little difference in precipitation days and total precipitation between all stations within the two regions, while their differences in precipitation intensity and rainfall were quite great.
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