III. Weather Phenomena

3. Tornado

A tornado is a violently rotating column of air in a small range with huge power and extreme damage capability. The diameter of a tornado can range from tens of meters to several hundred meters. In average it is about 250 meters. When seen from afar, it looks like a dark grey funnel or a trunk. From the cloud bottom to the ground, the whole funnel shaped air column rotates violently in a counterclockwise direction around a nearly perpendicular axis, and it also moves forward at the same time. Sometimes the column of air will hover in the sky; sometimes it will descend to the ground, causing heavy damage to the area it passes. The length of a tornado path is on average around 5 to 10 km. However, there is also a record of a 300 km tornado path. Some tornados last less than one minute, some can last over a few hours, but on average, the life span of a tornado is not more than 10 minutes.

Since the wind that comes from a tornado is so strong, normal wind speed measuring devices are incapable of withstanding the wind force and will be totally destroyed. That's why it is difficult to obtain credible records. Looking at structural damage of buildings and the striking power of flying objects in a tornado we can estimate that the wind speed is around 100 m/s and can reach as high as 200 m/s. Severe typhoons that come to Taiwan are extremely rare, but can possess a maximum wind speed higher than 80 m/s which illustrates just how powerful a tornado is.

The factors that contribute to tornado formation are still not completely known. What is known is that they usually happen around strong cold fronts and squall lines (a thunderstorm area in front of a front), and they are usually accompanied by hurricanes. Tornados sometimes occur in Taiwan during spring and summer. Fortunately their ranges are usually small and their paths short; seldom do they create serious damage to Taiwan. But in the American Midwest, tornados are a severe weather phenomenon. Because tornados often appear in groups and bring strong wind and a sudden drastic drop of pressure, they are extremely destructive and fearful.