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5. Điều kiện hình thành sét


Conditions for lightning around the world

Conditions for lightning around the world  

There are about 50,000 thunderstorms around the world every day. It is estimated that there are about 100 lightning discharges every second. Most of this lightning is summer lightning. Roughly speaking, thunderstorms are concentrated in Southeast Asia, Central Africa, and from Central America to the north of South America, at a latitude of around 30º (north to south).

Generally, there is not much lightning activity over the oceans.

In contrast with summer lightning, winter lightning is a special phenomenon and occurs only along the Japan Sea coast and the west coast of Norway.

Geographical distribution of thunderstorm days

Geographical distribution of thunderstorm days  

A map which shows the geographical distribution of thunderstorm days in a year is called an IKL map (Isokeraunic Level map). An IKL map for Japan is displayed at left. Areas of frequent thunderstorms in Japan are the Hokuriku (northern) region, the mountain areas of northern Kanto, the areas around the Suzuka mountain range of the Kinki region, and those parts of Kita-Kyushu around the Hita basin. (Based on data published by Japan Meteorological Agency, 1968, giving an average over ten years between 1954 to 1963.)


Ground lightning strike density

One example of the relationship between IKL and the ground lightning strike density Ng (number of lightning strikes per square meter per annum) can be expressed in the following equation.

Ng = 0.1 x IKL (strikes/ km2/ year)
λ: latitude



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