The Yushan area is located in the center of subtropical Taiwan. However,due to the mountainous terrain the elevation varies from 300 meters to 3,952 meters. At higher elevations, the temperatures are lower. Thus, Yushan National Park has three climatic zones: subtropical, temperate and frigid.
Information regarding temperature, precipitation, wind, typhoons, atmospheric pressure, humidity, vaporization, frost and snow is collected from the weather station on Yushan's North Peak (elevation 3,850 meters), from the weather station at Alishan (elevation 2,408 meters) and from Lu-lin Mountain.
The average annual temperature is about 20 degrees Celsius at an elevation of 1,000 meters. At an elevation of 2,500 meters, the average annual temperature is about 10 degrees Celsius. At elevations of 3,500 meters or more, the average annual temperature is only five degrees Celsius. For the past ten years, the average yearly temperature at this elevation has been 3.6 degrees, which is about the same as the ten years before that. The temperature is the highest in July, but has not exceeded 7.9 degrees at this elevation. In January and February, temperatures may drop to –1.2 degrees Celsius, which is somewhat colder than the previous ten years.
The lowest recorded temperature at this elevation was -18.4 degrees Celsius (January 31, 1970). In the recent ten years, the lowest temperature recording has been –13 degrees Celsius (February 6, 1995). At elevations of 3,500 meters and higher, the temperature is usually below zero during the months of January and February. In March, November and December, temperatures are usually below five degrees Celsius. At an elevation of approximately 2,500 meters, the summers are cool. (See Table 2-6 for Alishan and Yushan weather conditions, Table 2-7, Figure 2-9 for precipitation and other weather data). From comparisons of weather data from the past 20 years, the average annual temperature has dropped 0.6 degrees. The highest temperatures recorded have increased by four degrees, but the lowest temperature recordings have decreased by 14 degrees.
This indicates that there is great variability in temperature and weather during the year with increasing instability. The Alishan area shows similar but less severe trends.
Average annual rainfall in the Yushan area is about 3,600mm. It rains an average of 140 days per year, mostly between May and August.
From May until the first part of June is "plum rain season." In June, July and August, there are typhoons and summer storms. The precipitation is heaviest at mid elevation on the western side of Lu-lin Mountain and along the eastern bank of the Laku Laku River, reaching 4,200mm per year. Yushan and Hsiu-ku-luan Mountain are at elevations of about 3,500 meters. Rainfall is less at these higher elevations, averaging 2,700mm per year. Over the past ten years,the annual precipitation in the Yushan area has declined from 3,200mm to 2,700mm. In the Alishan area, annual precipitation has decreased about 200mm from 3,800mm. In both areas, winters are dry and summers wet, so there is a sharp demarcation between seasons. During the dry period in November and December, precipitation is only 1/8 the precipitation in August.
Wind and Typhoons
Most of the Yushan area is at high elevation, and there are strong winds all year round. Average annual wind velocity in the high mountain areas is about 5.1m per second. Lu-lin Mountain and Alishan areas are surrounded on all sides by hills and mountains, which block some of the wind. Thus, average annual wind velocity in these areas is only 1.07m per second.
There are no significant differences in wind velocity between months or between areas. In summer and fall, southeasterly and southwesterly winds prevail. In the winter and spring, the strongest winds are mostly from the west. Typhoons are a source of wind and rain. They typically approach Taiwan from the south or east. As a typhoon passes over the Central Mountain Range, much of the wind is reduced and the storm loses strength as it batters against the high mountains. Alishan and Lu-lin Mountain areas may see heavy rains of up to 1,000mm. Although the mountains and forest block much of the wind associated with a typhoon, wind velocity can still reach 41m per second.
Atmospheric pressure decreases with increasing altitude. For every 100-meter increase in elevation, atmospheric pressure decreases by 7.1mm. At elevations of 3,500 meters and above, average atmospheric pressure is 482mm, or about 64% of the atmospheric pressure at sea level. At elevations of about 2,500 meters, average atmospheric pressure is about 570mm. Atmospheric pressure does not change markedly between months.
Humidity and Vaporization
In the Central Mountain Range, in the Yushan area, there are tall mountains and good airflow. In addition, most of this area is above the vapor collection zone, so the average annual humidity is only about 80%. The remaining areas have a humidity of 85% or higher. There is not much change in humidity level between months. Only the period between November and January is drier due to low precipitation. During this period, the humidity is 75% or lower. Much of the area experiences low temperatures with fog and mist. Due to the large amount of mist, vaporization is about 1,000mm, with little variation between months. Vaporization level is less than precipitation level, thus, there is surplus water.
From September to April, the Yushan area is often covered with frost. However, due to strong wind, the frost level is not high, except in the valleys. At elevations above 2,000 meters, there is snow. At elevations of 3,000 meters or more, there are four consecutive months of snow accumulation. The first snow may appear in October and completely melts by May. Snow falls 24.3 days per year on average on Yushan, which is less than in the previous ten years. At lower elevations,snow may fall only 0.6 days per year. Snow mostly falls in January and February.