III. Weather Phenomena

12. Special weather phenomena in Taiwan

Taiwan is surrounded by sea; it is close to the East Asian Continent at its west, while the vast Pacific Ocean is at its east. It is located in the subtropical region, with milder seasonal differences than the northern continent. However, in winter when the cold air from Siberia arrives there will often be days with temperatures below 10 °C. Snow can be found in the high mountains. In summer, under the influence of the subtropical Pacific high pressure, it can be very hot. Temperatures of 35 °C or above are common on those days. At times Taiwan will also be struck by typhoons from the tropical ocean. Therefore, it is not an exaggeration to say that Taiwan's weather is varied. The special geographic conditions of the island, which are influenced by the Central Mountains from north to south acting as a natural barrier, also add complexity to Taiwan's varied weather. From this, during different seasons, some special weather phenomena are found in Taiwan. A number of these phenomena have been observed by our ancestors and given fitting and sometimes funny names, remaining until now. Some of these are: "burning wind", "down mountain wind", "northwest rain", and "mad dog wave", and so on. In the following paragraphs, these phenomena will be introduced from the meteorological perspective.

a. The hot and dry 'burning wind'

On the 7th of May 1988, there was a very deep low pressure system over the East China Sea; a strong west wind was blowing across the Taiwan sky. At noon the Central Weather Bureau weather station in Taitung recorded a temperature of 33.5 °C, with a relative humidity of 56%. Starting at one p.m. the temperature began to rise rapidly until two p.m. when the temperature reached 39.7 °C, breaking and marking the highest temperature recorded in 91 years. At the same time the relative humidity had also decreased to 26%. In mere two hours, the temperature had risen by 6.2 °C. At the same time, on the other side of the Central Mountains, the temperature of Taichung was only 33.8 °C, a total of 5.9 °C lower then Taitung. This phenomenon was especially rare as it occurred during the Mei-Yu season. In summer when a typhoon passed through the northeast and north of Taiwan, a similar situation also happened in Taichung. When this phenomenon occurs, the Taitung region will experience hot and dry winds which give people a burning sensation which is why local residents call it the "burning wind".

Taitung is located on the lee side of west wind; the wet air from South China is forced to ascend when it is blocked by the 3000m high Central Mountains. The water vapor in the air condenses and becomes clouds or rain on the windward side due to cooling through expansion and the remaining dry air climbs over the mountains and descends. It is then super-heated in an adiabatic compression process and becomes the "burning wind". A similar phenomenon also takes place in other countries; in the Alps it is called a Foehn, while in the Rocky Mountains, U.S. it is called a Chinook.

b. The dashing "downmountain" wind

In winter, when the northeast monsoon is prevalent, the area from Fangshan, Fonggang to Hengchun in Pingtung County, often experiences continuous strong gales. For example, from the 28th of November to the 1st of December, 1987, under a severe cold surge, the Hengchun region experienced strong gales for four consecutive days. The maximum wind speed reached 37.2 m/s on the 29th (a scale 13 on the Beaufort scale). Electricity posts toppled, causing power and water cuts. Over 70% of the crops were lost, either by being blown down by the wind or by withering. The upper layer observation data of the period recorded by the Hualien weather station showed that eastern Taiwan had experienced an east wind from ground level to 2000 m above. In terms of the geographic situation of Taiwan, the Central Mountains extend from the north all the way south to Hengchun Peninsula. The terrain begins to drop from Fangshan to the south, where average height is below 1000 m. The east wind layer is thick enough not to be blocked by the terrain and passes through the south part of the Central Mountains, dashing down the west side of the mountain and creating a strong downslope wind in the area from Fengshan, Fenggang to Hengchun. The locals call this the "downmountain" wind. When the "downmountain" wind occurs, the temperature in Hengchun doesn't significantly increase but instead shows only signs of decrease, and it is a different kind of phenomenon from that of the "burning wind" in terms of formation.

c. The "Huoyan Mountain weather dividing line"

In winter, the East Asian Continent is under the influence of the Mongolian high pressure system. Cold north wind or northwest wind blows north of the Yangtze River, changing direction to the northeast when it reaches south of the river. Taiwan is located on the southeast edge of the Mongolian high pressure system and so it often experiences northeast wind, which is also known as the northeast monsoon. From the upper wind observation in the past, the thickness of the northeast monsoon that affects Taiwan in winter varies according to the change in strength of the Mongolian high pressure system. When the northeast monsoon is strong it can reach from the ground to an altitude of 3 km. When it is weak, it is less than 1 km thick, its average thickness being about 1.5 km.

People who live in the north Taiwan will notice that in winter there are significantly cloudier and rainier days. But in central and southern Taiwan, every year from October to the Mei-Yu season of the next year, the sky is always clear, with rarely a rainy day. The weather of the north contrasts greatly with that of the south. Drivers who have driven on the highway in winter might have had this experience: as the car moves south from northern Taiwan, the sky is always cloudy and gloomy. Especially in Sanyi, Miaoli County, the sky will be covered with heavy clouds and fog. However, as the car passes Huoyan Mountain and goes downhill to the Da-anxi Bridge, the warm sun will suddenly show and the sky will brighten and reveal a refreshing blueness.

How can the weather change so much just passing over a mountain? In winter the northeast monsoon with its wet and cold air is mostly less than 2 km in thickness. In northeast Taiwan it is blocked by the Central Mountains and Xueshan Mountains, and research indicates that by the time it reaches the area around Huoyan Mountain (altitude 600 m), it has already lost most of its strength. Unless the northeast monsoon is especially thick, it will usually become lower cloud or orographic rain and will lose the strength necessary to continue over the mountains to affect the Taichung area. Therefore, Huoyan Mountain becomes the natural boundary dividing the drastically different weather patterns on the south and north side of the mountains.

d. The debating northwest rain

The Taiwanese have a saying: "Northwest rain doesn't fall over the border of the field", which fitly describes the characteristics of the northwest rain. This rain is in fact an air-mass thundershower which occurs on summer afternoons and is caused by the radiation heating of the sun. This type of thermo-thundershower is a meso to small scale convection system in meteorology.

The horizontal coverage of the northwest rain ranges from one-two kilometers, to as large as dozens of kilometers. The duration of rainfall can range from a few minutes to an hour; therefore this type of rain is local and very unpredictable. One person reported this experience when she was in high school: One day when there was a thundershower she saw one side of the Taipei First Girls High School playground was raining where the other side was sunny. What a strange sight! This type of summer air-mass, thermo-thundershower can occur for up to three successive days in Taipei, sometimes the time advancing from day to day, other times with delay day by day. It's an interesting phenomenon.

The term "northwest" in the name "northwest rain" seems to have meaning of direction. Older Taiwanese people call it "falling on the northwest", omitting the word "rain". The practice clearly has its meaning. In the "New Etymology of Taiwanese Language", the term "northwest rain" has an interesting history; an umbrella patch worker claimed that it was a mispronunciation of the term "lion and panther rain", which describes the ferocity of the rain. Yet a farmer gave a different explanation. He thought it came from the term "san ba" (the number three and eight) meaning "unreasonable rain like a woman" because it comes in a crazy way that it resembles an unreasonable woman. However, evidence is lacking in both claims. Another explanation states that the word "west" in "northwest rain" actually describes not the direction of the rain but the direction of the sun at the time a northwest rain occurs. "North" in this explanation, represents water (which conforms to old Chinese believes). In this sense, "falling on the northwest" means the falling of rain (north) at the time of sunset (west), giving it a more reasonable explanation.

e. The unexpected "mad dog wave"

On January 19, 1987, six people who were fishing in Bitoujiao seashore were swallowed by sudden waves, in which three went missing. On September 8, 1987, when the day before typhoon Gerald had hit the south of Taiwan, a similar incident had also taken place in Maobitou seashore in Hengchun. In winter, when a cold front is in the region and with the northeast monsoon being strong, or in the summer when a typhoon hits, the northern seashore will often see sudden waves. Each time there will be 3 successive big waves that will last 2 to 3 minutes, and then everything will be calm again, after which, a while later the waves will come back again; not unlike a mad dog. Therefore the local fishermen call this the "mad dog wave". Since it comes unexpectedly, often without any significant sign at all, when an inexperienced person fishing in coastal areas is hit by this wave, he or she will often be caught by surprise and unable to escape.

The possible cause of the mad dog wave is suggested be related to the long wave induced by a pressure system. With its high propagating speed, the long wave amplitude is enlarged due to the shallow sea bottom when reaching the seashore. However, the real formation reason yet needs to be further investigated.

f. The impassable Yushan wind gap

Yushan is one of the national parks. Mountaineering enthusiasts all set this "mountain of mountains" as the ultimate goal. The Yushan wind gap is the unavoidable passageway on the way to the Yushan weather station on the north peak of Yushan. There once was a weather observer who unfortunately fell to his death while passing by. This is also why the Yushan wind gap is a notoriously dangerous place. Mountaineers who have climbed Yushan probably know that after passing Paiyun Village and reaching 3600 m in altitude or above, the weather becomes so cold and dry that no plants grow in the region. The pathway is all gravel slopes, which requires extreme caution to pass through. The most dangerous time for traversing the area is usually in the winter since the upper wind is mostly from the southwest. This strong southwest wind moves north along the rift valley between Yushan's main peak and west peak over the River Nanzixian, and blows northeast towards the rift valley upstream to Laonong River near Batongguan. When the air current passes through the Yushan wind gap it is almost unstoppable in strength. Maximum wind speed can reach a scale 14 to 17 on the Beaufort scale, which is equivalent to the strong winds of a typhoon. It is impossible to walk upright; one would actually need to crouch. The air is thin around the peak of Yushan; the oxygen amount in the air is less than 7/10 that at ground level. The path from the wind gap to the north peak is very steep, with an inclined angle between 60° to 70° and is 300 m in length. To climb this, one has to face strong gusts of wind that will literally take ones breath away. One will have to stop and rest every few steps before continuing. Taking into consideration that the slope is covered with snow or ice, and that the gravel is very slippery, one can imagine how dangerous this place is, and one can also understand why climbers will try to avoid the wind gap when climbing Yushan in the winter.