Thunderstorms have occurred frequently in the south recently! How to avoid danger when encountering thunderstorms?

□ Luo Mingjun

Recently, thunderstorms have occurred frequently in various parts of the south. Not long ago, the 600-meter-high Guangzhou Tower was struck by lightning many times, and the scene of the tower tip and the "fire dragon" intimately connected was even more shocking. So, what is a thunderstorm? Why do thunderstorms mostly occur in summer? Which places in my country are prone to thunderstorms? What are the advantages and disadvantages of thunderstorms?

What is a thunderstorm

Thunderstorms, also known as "convective storms," ​​are a type of weather system with vigorous convection, common in summer. When the air on the ground is heated and rises, strong convection occurs with the upper cold air, and water vapor accumulates to form cumulonimbus clouds. The constant convection and frequent friction between air and water vapor causes a large amount of electric charge in cumulonimbus clouds. The collision between ice crystals and water droplets in the clouds causes the charge to separate. Usually the positive charge is at the top of the cloud, the negative charge is in the middle of the cloud, and the rising air currents in the lower part of the cloud are positively charged. When the difference in electric field strength between clouds or between clouds and the ground is large enough, it will break through the air and discharge, producing cloud-to-cloud lightning or cloud-to-ground lightning.

A lightning bolt can be thousands to hundreds of kilometers long, with a voltage of 100 million to 1 billion volts and a current of 30,000 to 300,000 amperes. Lightning generates extremely high temperatures of 17,000 to 28,000 degrees Celsius, causing the air and water vapor to expand rapidly after being heated, instantly generating a shock wave. If the shock wave is large enough, it will make a loud noise, which is thunder.

Thunder is heard as a sharp crack near a thunderstorm, and as a rumble in the distance. Lightning travels much faster than the sound of thunder, so we see lightning first and then hear thunder. Continuous lightning and thunder are called thunderstorms. Thunderstorms usually last for a short time, and a single thunderstorm usually does not exceed 2 hours.

Why do thunderstorms occur more often in summer?

In summer, due to the high temperature in the eastern monsoon region of my country, the saturated content of water vapor in the air is relatively high. The ground is exposed to sunlight for a long time, and the thermal air flow is more likely to rise to form altocumulus clouds and thunderstorm clouds. In winter, the air is cold and dry, and the solar radiation is weak. It is not easy for the air to form strong up and down convection, so it is difficult to form thunderstorm clouds. Thunderstorms in my country mainly occur from April to September, and there is almost no thunderstorm weather from November to February of the following year. Thunderstorms caused by thermal convection often occur in the afternoon and evening.

In spring and summer, low-latitude areas are hot and rainy, and the air is in a warm, humid and unstable state, which is prone to thunderstorms. Hainan Island and Leizhou Peninsula are the thunderstorm centers in my country. From March to May, frontal troughs are frequent in South China, and Hainan Island and Leizhou Peninsula are prone to spring thunder. From May to August, Hainan Island and Leizhou Peninsula are on the edge of the western extension ridge of the subtropical high pressure in the western Pacific Ocean, and on the ground they are often located in front of the southwest trough. From the ground to the high altitude, southerly airflows prevail, and water vapor is abundant, so thunderstorms often occur.

Warm and humid air is forced to rise on the windward slopes of the mountains, forming orographic thunderstorms. "The thunder is strong, on the south mountain," the words in the Book of Songs describe orographic thunderstorms. The high incidence of thunderstorms on the Qinghai-Tibet Plateau is also related to the topography. The Qinghai-Tibet Plateau, with an average altitude of more than 4,000 meters, has a more significant surface heating effect in summer due to its complex and changeable topography. A large number of local small circulations often occur, resulting in convective weather. The Qinghai-Tibet Plateau has become the area with the most thunderstorm days at the same latitude in the Northern Hemisphere.

What are the pros and cons of thunderstorms?

Thunderstorms promote precipitation and help alleviate high temperatures and droughts. Thunderstorms can clean harmful gases and particulate matter in the atmosphere, and produce negative oxygen ions to improve air quality. The electrical energy of thunderstorms can charge the ionosphere like a charger, allowing it to quickly return to normal. Under the high temperature and high voltage conditions caused by thunderstorms, air molecules will be ionized, and nitrogen and oxygen will combine into nitrite and nitrate molecules, which will dissolve in rainwater and fall to the ground, becoming natural nitrogen fertilizer. After a thunderstorm, crops grow more vigorously.

But at the same time, thunderstorms and lightning can also pose a threat to humans and objects. All of us, especially young people, should have the awareness of respecting nature and cherishing life. Do not stay on the platform of a high building during a thunderstorm, do not hide under a big tree to avoid thunderstorms, and do not enter isolated sheds; stay away from metal objects such as water pipes and gas pipes outside buildings and electrical equipment; do not approach broken high-voltage wires at will; avoid using telephones or electrical appliances with plugs. In addition, thunderstorms may be accompanied by severe disastrous weather such as heavy rain, strong winds, hail, tornadoes, etc., and affect flight operations. We should pay attention to thunderstorm warnings and take precautions against derivative disasters such as mountain torrents, landslides, and mud-rock flows.

(The author is a teacher at Longcheng Senior High School in Longgang District, Shenzhen)