Download The Physics of lightning

 The Physics of lightning Written by: LI Sun­wai September 2011 Lightning is a severe weather phenomenon commonly seen in Hong Kong and could result
in loss of lives and damage in properties. Whenever lightning, and its associated
thunderstorms, affects Hong Kong, the Observatory will issue a Thunderstorm Warning to
alert people and relevant Government Departments to take suitable precautionary
measures. Lightning can occur when Hong Kong is affected by unstable weather systems, such as
perturbations in airstreams, troughs of low pressure or tropical cyclones. Lightning is an
atmospheric electric discharge phenomenon. The large electric current flowing through the
atmosphere generates a huge amount of heat energy and strong flashes. The energy also
heats up the air. Rapid air expansion results in strong shocks and generates loud bangs
which are the thunders that often accompany lightning. The electric discharge that results in lightning is generated by the enormous electric
potential difference between clouds or clouds and the ground surface. Very often lightning
is associated with the cumulonimbus (Cb) clouds. In situ measurement results indicated
that potential differences within or between Cb clouds could reach 105V/m. But how such
huge potential differences can be accumulated between clouds or clouds and the ground? The charge separation processes in the Cb is highly complicated with a number of
mechanisms in action. One of the mechanism accepted by most scientists is that
precipitating particles, e.g. water droplets, ice crystals and hail stones in the Cb are
polarized by the prevailing nature electric field of the atmosphere1 with positive charges
moving to the lower part of these particles and negative charges to the upper. The air
movement inside a Cb is unstable and there are strong upward and downward drafts within
the cloud. Neutral particles in the updraft will collide with the precipitating particles while
moving up. Positive charges at the lower part of the latter, where impact will mostly
happen, will be brought away and carried to the top of the cloud by the upward moving
particles. Thus the upper part of the Cb will become positively charged and the lower
negatively charged. Potential difference will build up between the cloud and the ground
surface as negative charges accumulate at the bottom of a Cb, until it reaches a threshold
at which electricity is discharged, resulting in lightning. (Figure 1) In 2005, the Hong Kong Observatory set up a lightning location network in cooperation with
the Guangdong Meteorological Bureau and the Macao Meteorological and Geophysical
Bureau. The lightning location network monitors lightning activities over the Pearl River
Estuary round the clock. After lightning has been detected, the location where lightning
occurs is recorded and shown graphically on the Observatory's webpage for reference by
the public. Figure 1 Note: 1 – This refers to the "fine­weather electric field". More information is available on P.26 of Meteorological Glossary. References: 1. Hong Kong Observatory website 2. UK Meteorological Office, Meteorological Glossary, Her Majesty’s Stationery Office, 1972
3. 虞吳、臧庚媛和趙大銅，現代防雷技術基礎，氣象出版社，2002