III. Weather Phenomena

10. Thunderstorm

a. Introduction

A thunderstorm is a severe weather phenomenon formed under extremely unsteady atmospheric conditions. It usually carries with it strong gusts of wind, torrential rain, lightning strikes, thunder, and sometimes is even accompanied by hailstones and tornados. That's why it often creates disaster.

There are generally two types of thunderstorm; the first is a frontal thunderstorm and the other an air mass thunderstorm. Thunderstorms in Taiwan begin to pick up in frequency each year in March and reach a maximum in July or August. Thunderstorms between March and June are mostly frontal thunderstorms whereas those from July to September are air mass storms.

Frontal thunderstorms are caused by the dynamics of air, they are formed when warm and wet air is lifted up by the front and generates strong convection. Thunderstorms usually appear near the front, and sometimes will appear ahead of a front. There is no specific time of occurrence. It can happen at daytime or nighttime. During the Mei-Yu season in Taiwan, when the Mei-Yu front is very active, there are often severe thunderstorms, which can last as long as a few hours. Therefore this kind of thunderstorm can also bring floods. The 'May 28 Flood' in 1981 and 'June 3 Flood' in 1984 are two examples.

An air mass thunderstorm is also called a thermo thunderstorm. This is because air mass thunderstorms usually happen on summer afternoons at around 2 to 3 p.m. They are mainly formed by thermo effects. In summertime, Taiwan is under the influence of the maritime tropical air mass; daytime sunlight will cause convective instability in some regions, and therefore cause thunderstorms. This kind of thunderstorm only affects a local area and is not as destructive as a frontal thunderstorm.

b. Thunderstorm formation

A single thunderstorm cloud (i.e. cumulonimbus) has a very short life span, usually 1 to 2 hours. This period can again be divided into 3 stages: developing stage, mature stage, and dissipation stage (Fig. 4). In the first stage an updraft is formed and carries warm and wet air to the sky. The uplifted air is cooled and the water vapor in it condenses into clouds. When the air flow continues to ascend, the clouds will also accumulate in height and reach an even cooler sky. At this time the water drops in the clouds will be heavier and bigger. Some water drops will be frozen into snow, some will simply get bigger and bigger, until a point when they can no longer be supported by the updraft and begin to fall. Also descending with the rain or before the rain is the downdraft, which generates cool gusts, a sign of a coming thunderstorm.

Why these rapidly moving air currents generate static electricity in the thunderstorm cloud is still not completely known. Part of the reason may be due to the fraction and dissipation of water drops. What we do know is that positive charges will be at the top of the cloud while negative charges will be at the bottom of the cloud, attracting the positive charges on the ground. The air between the cloud and the ground acts as an insulator and in a short period of time prevents the two charges from contacting each other and forming electric currents. However, when the voltage generated by the two charges is large enough to breakthrough the blockage of insulating air, lightning will be created. (In some thunderstorms, the static electricity voltage can measure up to several million volts.) Lightning can take place inside a cloud or between two clouds; it can also move upward from a lower region to a higher one or downward from the cloud to the ground. On a global basis, the frequency of a thunderbolt hitting the Earth is about 100 times per second.

Most lightning in fact does hit the same place twice, and in a row too. The first strike is the "preceding lightning strike". This is an invisible bolt of electrically charged air which acts as the pioneer and goes down near the ground. The charged air is like a wire making way for the second strike. The moment the preceding strike approaches the ground, a "returning strike" will jump along the current established by the first strike. It will generate visible lightning and loud thunder; this is the second strike. This returning electric current has an electrical core and is surrounded by a tube-like, white-hot air. This air will emit light, expand, and explode, resulting in what we know as thunder. We can see the lightning almost the exact moment as the explosion happens, but can only hear the thunder afterwards. That is because the speed of sound, (340 m/s) is much slower than the speed of light (300k km/s).

The lightning will gradually balance the unevenly distributed electricity in the cloud. Likewise, the downdraft formed by rainfall will slowly reduce the temperature differences between the hot and cold air. It will also cool down the ground surface, thereby stopping hot air from rising. Therefore after a short summer thundershower the weather will quickly become clear.

The development of air mass thunderstorm

Fig. 4: The development of air mass thunderstorm;left: developing stage; center: mature stage; right: dissipation stage.

c. How to prevent lightning strikes

According to scientific research, the energy released in a thunderstorm is far more powerful than an atomic bomb. It's no wonder then that during thunderstorms there are often incidences of lightning strike damage. What should we do in times of thunderstorms to avoid being struck by lightning?

i. Individuals

Don't remain near isolated tall buildings, towers, electricity poles or chimneys, etc. Stay away from empty high ground or tall trees; these isolated and high standing objects are the primary target of lightning bolts. Don't swim in a river or sail a boat on a lake, because human beings or boats on the water surface are also protruding objects and may be an obvious target for lightning. Moreover, metal and wet objects are very good conductors. Therefore you should stay away from wires, water pipes, brass and iron ware or other conductive objects; you should also avoid wearing sweat or rain soaked clothes or getting near damp walls.

ii. Buildings

The installation of lightning rods on tall buildings can effectively reduce the chance of lightning strikes. A lightning rod is a metal rod, pointed on the top and the base is linked by brass wire to a brass plate buried underground. When the thunderstorm cloud approaches the ground, the conductivity generated by the ground will concentrate electric charges at the point of the rod, through which electricity will be gradually discharged. Thereby the danger of a drastic electric discharge can be avoided.

iii. In forest scenic areas

We all fear being struck by lightning. Although lightning striking people and animals is often fatal, the fact is that only very few people are actually killed by lightning. On the other hand lightning strikes causing forest fires are hugely destructive and should be regarded at least as seriously as human lightning strikes. For example, on April 7, 1973, a bolt of lightning struck the Chinese cypress forest beside Xianglin Elementary School in Alishan and started a fire, which turned out to be massively extensive and caused disastrous damage. Many large tall old trees in Taiwan forest areas are often struck by lightning, so if we want to protect these treasures we should install them with lightning rods.

iv. Electricity facilities and other public facilities

Electricity facilities are obvious targets of lightning strikes. Lightning strikes will suddenly overload the electric circuits and cause damage to electricity posts, transformers, and power lines. Therefore the power company should reduce the load of electric circuits in areas with frequent lightning.