Vietnam is situated in the centre of the Asian thunderstorm region – one of three thunderstorm centres in the world. This article gives an overview of thunderstorm activity in Vietnam and results from the research on lightning position over the past year in Institute of Geophysics under the Vietnam Academy of Science and Technology.
What is thunder and lightning?
A thunderstorm is a weather phenomenon which creates an atmospheric electrostatic charge – called lightning – and the sound which accompanies the spark – called thunder. . How lightning initially forms is still a matter of debate, however, it is thought that a thunderstorm is formed when hot and humid air reacts with cloud particles in certain ways. A storm can last from 30 minutes to 12 hours, spreading hundreds of kilometres. The force from a thunderstorm could be compared to a small power plant with a power of hundreds of megavats. Electric potential can reach up to one billion volts and an electric current of 10-200 kAmper, lighting a 100 W lamp for three months. According to estimated statistics, there are about 100 electric discharges between electric discharged clouds and the ground globally each second. Its power can reach billions of kW, heating the air at an electric discharge position of up to 28,000 Celsius degree (three times the solar surface temperature).
Lightning causes damage when it strikes the ground. There are two kinds of lightning – negative and positive. Negative lightning (accounting for 90% of all lightning bolts) mainly appears below the cloud and strikes the ground. Positive lightning often strikes the ground from the peak of the cloud. The positive lightning appears suddenly, making it dangerous as people do not expect the strike, as the sky is still clear with no rain. It is also a danger causing damage to people and property. According to a statistics, about 5,000 people in the world are struck by lightning each year.
Thunder is the sound caused due to the lightning channel heating the air. When the air expands quickly, it launches a sound. It is possible to hear lightning from a radius of 20 to 25 kilometres.
Thunderstorm activity in Vietnam
Thunderstorm season in Vietnam is from April to October. The average number of thundery days is about 100 per year with around 250 thundery hours per year. There is a great disparity in thunderstorm activities among regions, Cam Ranh for example sees only 55 hours per year while other regions such as A Luoi district in the central city of Hue see 489 hours per. The disparity is caused by many different factors, including territorial division by high mountains with different aspects, which strengthen thunderstorm activity in one region and limits it in another. These strong thunderstorm regions are the areas which have sharp atmospheric disturbance and favourable terrain for the formation of các dòng thăng.
In Vietnam, about 2 million lightning bolts strike the ground per year. According to limited statistics from the Institute of Geophysics in 2004, 820 lightning accidents occurred in Vietnam over ten years, causing many millions of VND worth of damage and the interruption of telecommunication and electrical services. Lightning accidents also cause much damage to crops and people’s safety, making it a significant problem in peoples lives in several regions such as Co Dung in the northern province of Hai Duong, Son Loc in the central province of Ha Tinh, and the Mekong river delta.
Thunderstorms have direct effects on socio-economic activities. It has useful effects such as providing a rainfall source, a diversified protein source for agriculture, and the nitrate of lightning discharge – all of which help crops. However, its effects can also have negative consequences for the socio-economy of agriculture, such as flooding.
Electric discharge map in 1999 showed that Vietnam was in the world thunderstorm center
Research on thunderstorm and lightning position in Vietnam
Research on thunderstorms aims to observe and study thunderstorm activity over space and time, as well as exploring how and why lightning formulates. Research on lightning position aims to define the density of lightning (number of lightning flash/km2 /year), and also measure other lightning parameters such as the intensity and the slope of the lightning bolt. Lightning density is an important specific quantity for thunderstorm activity in each region. It requires complicated steps to determine a lightning bolts density. This value is different according to region, depending on characteristics in climate terrain and other weather conditions.
To conduct research on thunder and lightning in Vietnam, the Institute of Geophysics installed a network of lightning position machines in several localities. The operating principle of the position machine is to use antenna to receive electro-magnetic waves emitted by a lightning bolt. The rate and time between two peaks of pulse is used to distinguish electric discharge in the cloud and electric discharge to the ground. At the installation point, lightning measuring equipment often gets jammed due to metal objects close to the antenna which absorb electro-magnetic wave and emit radiation. Signals can be also jammed by near-by stations or interaction with the ground terrain when the wave is transmitted from the position of lightning strike to the station. While installing, scientists from the Institute of Geophysics chose optimum positions for the equipment to operate well. When processing data, statistical algorithm is used to eliminate this kind of system interference.
Since the beginning of 2003, the network of lightning position machines has increased to include stations in Thai Nguyen, Phu Thuy and Nghia Do (Hanoi), Moc Chau (Son La), Phu Yen, Binh Thuan and Bac Lieu. As the equipment can measure activity within a radius of up to 400 kilometres, this network covers all Vietnamese territory (Figure 2).
The positions of lightning measuring stations
The results from these stations show that thunderstorms often occur between 2pm to 8pm, depending on each station. Lightning appears soon in Dong Ha and Son La (in April with a frequency of more than 20%). Thunderstorm activity is significant in May in Phu Yen, Binh Thuan and Dong Ha. In Hanoi, the peak of thunderstorm activity is in June and July. The second auxiliary peak of all of the stations across Vietnam appears in September and December.
Lightning activity in one day in several lightning position stations in 2004 Frequency of lightning appearance in a month in 2004
In addition, the network of lightning position machines also helps scientists analyze thunderstorm structure, movement speed and the covering region. Figure 5 shows detailed activities of lightning in Son La on April 2nd 2004 in grid 0.40×0.40 degree. Star marks are positioned where lightning occurred. Figure 6 shows lightning discharge speed (bolts per hour) in this region. This example shows that data on lightning discharge is more detailed than data on thunderstorm hour and month because we can see the intensity of lightning activities in three thunderstorms is rather different.
Lightning discharge speed in Son La on April 2nd 2004 according to statistics of Moc Chau lightning position station
Lightning positioning systems have many applications. They determine lightning density value using norms of lightning prevention, and predict the position of lightning strike for the electricity, airline and meteorology forecast sectors. In the future, Vietnam should invest in improving this lightning position system to conduct basic investigation and to continue to collect information for thunderstorm forecasting prevention purposes.