Direct Lightning Stroke
A lightning stroke is an electric discharge between an electrically charged cloud and earth surface (earth lightning), between two or more clouds and each other or between parts of one cloud (cloud lightning). Just a small percentage of strokes happens be tween the surface and the clouds. The lightning strokes originate in the „storm cells“, which stretch average out up to few kilometers. Every storm cell is active for up to 30 minutes and generates from two to three lightning strokes per minute. The storm cell often reaches the height of over 10 kilometres, whereas the bottom visible part of the clouds is usually at the height of one to two kilometres. In the centre of the storm cell there exists a strong rising air flow, which causes separation of positive and negative charges. The positive charge is usually bind ed on the frazils at the top of the storm cell, while negative charge is usually binded on water drops at the bottom of the cell. Nearby the earth the cell is charged with positive charge which is usually caused by discharge especially from forest. Beyond the storm cells originating from the summer heat there are storm cells originating from the frontal cloudiness as a result of big air masses movement. The storm frequency depends on the season.
In summer months (July–August) there are on average 5 times more storms than in winter months (December–February). The environmental heating up supports the storm creation. In autumn warm water near the seacoast gives the necessary energy for the storm creation. According to IEC 1312-1:1995 and IEC 62305 it is possible to describe lightning charges by five basic parameters:
Another important parameter, which is presented during the classification of storm activity is so-called intensity of storm activity or the frequency of the lightning strokes per km2/year. The frequency in our geographical latitudes varies from 2 to 8 strokes/km2/year, but in subtropical and tropical climate it varies from 30 to 70 strokes/km2/year.