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by K. R. Abayasinghe

The first inter-monsoon season follows the northeast monsoon season in Sri Lanka and it normally prevails during mid-March and mid-May. During this inter-monsoon season, we will experience a number of weather conditions, sometimes extremes.

Wind and temperature conditions in this season enhance the development of the cloud type called cumulonimbus and sometimes "anvil cloud" because of its similarity to the anvil used by the blacksmith. This cloud produces lightning, tornado and hailstorms, which are equally hazardous when they are generated by a well developed cumulonimbus clouds. These clouds develop between the altitudes 500m and 12km under the influence of atmospheric conditions with strong upward movement of air associated by the convection.

The lower part of a cumulonimbus cloud mostly consists of water particles and drops but the region above about 5 km consists mainly of super-cooled water drops and ice.

Since rain starts with the falling ice particles, we sometimes may experience rain with ice (hailstorms). But fortunately we are living in a tropical country where the low level temperatures are very much above the freezing temperature of water and for this reason, unlike in colder parts of the world, the ice tends to melt before reaching the earth saving us and our property from damages caused by hailstorms with large ice balls.

Vigorous convection leads to development of small-scale cyclone type systems usually called tornado which have a very short lifetime like 10 minutes but they are rich with very strong gusty winds which are capable in resulting considerable damage in a small area in a very short duration. Therefore one has to expect hazards caused by a number of events like heavy sharp rain, lightning, strong winds and hailstorms during the prevailing inter-monsoon season.

Precautionary steps to reduce or minimize hazard by all these events have to be seriously considered. But as lightning is the most hazardous product of cumulonimbus clouds, let us look into the precautionary measures for reducing hazards by lightning.

Lightning is merely an electrical current reaching the earth from the base of cumulonimbus clouds, which are usually electrically charged. The electric current of a ground lightning flash is about 25,000 Amperes (25,000 A) and the potential difference between a charged cloud and the earth is about 100 million Volts. Therefore the energy of a lightning flash bringing 5 Coulombs to ground is about 500 million Joules.

A lightning flash, after travelling a distance about half a kilometre from clouds selects a closest conducting path to go to earth before getting neutralised. Therefore the tallest objects of a location are the best supporters for a lightning flash in its way to ground. in a natural environment, tall trees are the lightning-attractors.

Otherwise conducting (metal) structures or objects (like communication tower, telephone and power supply lines and TV antenna) will attract the close by lighting flashes. In absence of such objects, the buildings in the location are vulnerable to direct hit of thunderbolts.

During bad weather conditions with thunderstorms, lightning surges may reach the inside of a building in a number of ways.

Direct hit

It is obvious that the lonely (considerably tall) buildings are vulnerable to lightning flashes. Once it strikes a building the lightning current will pass through the conductive materials of the building (reinforced iron in concrete columns and slabs, wiring, electrical circuits, metal structures etc.) on its way to ground if there is no safe way is provided for the purpose. Selection of the path is decided by a number of factors like specific resistance and the situation etc. of the conducting part within the building.

Side flash

A part or the whole current of a lightning flash, which has hit a close by telephone or power line or a TV antenna, may travel to a building via the electrical (metal) connection between the point of strike and the building.

Also, depending on a number of parameters, part of a lightning flash reached a close by tall object (tree, tower) may spark to the conducting parts (concrete sun shade, concrete chimney roof or aluminium sheet) of a building even without any electrical connection.

Step voltage

Voltage difference between feet resulted by a current spreading from a point of a lightning strike on the ground.

Contact potential

Shock occurs when there is a contact with an object like a tree, building or a conducting wire that acts as a part of a conductor of a lightning current.

Lightning caused hazards could be reduced with a number of precautionary measures. A few important and necessary precautions to prevent buildings from lightning damages are mentioned below:

1) Install good earth for the electric circuit of buildings.

The earth wire and the rod should be properly installed to make it capable to perform its job properly.

According to the recommendations by electricity supplying agencies (internationally or nationally) the resistant of the earth rod should be less than ten (10) Ohms and the connection between the earth rod and the earth wire should be firm.

There are a number of ways of installing earth rods to have its resistance low. Two economical and easy methods are mentioned below:

a) Installing a set of parallel rods.

A number of rods installed in a place are electrically connected with each other with a conducting wire like copper or iron.

b) Installing several number of earth rods around the building and connecting each to the electric circuit of the building at several points (Diagram 1 and 2).

2) Use power surge diverters

Supply lines of power and communication can be interrupted by a gadget called power diverter which is capable in directing high currents intruding a building along supply cables as a result of a lightning flash to the ground without allowing it to come into the interior parts of the building circuit.

3) Installing lightning conductors/arresters

Direct hit of a lightning flash to a building is the most dangerous event when the lightning hazards are concerned. Lonely and tall buildings are the most vulnerable to lightning.

The lightning rod (LR) (or lightning arrester or conductor) is the widely used device for protection of buildings. Continuous research in this field has been useful to apply modifications to the lightning rod to make it efficient in conducting lightning currents to the ground protecting the building and the property inside.

The LR usually consists of three parts namely,

* Air terminal
* Down conductor
* Ground terminal

(Diagram 3)

Following precautionary steps should be practiced by all of us to prevent or at least minimise the hazards caused by lightning strikes and surges.

A) Do no have any conducting connection (like wires) between houses and the close-by trees.

B) In environments with thunderstorms,

a. Keep electrical instruments disconnected from the main power supply.

b. Television antennas should be disconnected from the television sets and connected to an properly installed earth rod. If this is not possible the antenna socket should be placed close to the earth outside the house.

c. As far as possible, avoid handling/touching electrical instruments like refrigerator, electric iron, metal frame, TV and radio.

d. Find shelter in a safe place to avoid exposing yourself to the open air. If the time interval between lightning flash and hearing thunder becomes less than 15 seconds, move quickly to a protected location as there is immediate danger of a lightning strike nearby.

e. Try to avoid loitering in open areas like paddy fields, tea estates or playgrounds. Specially avoid working in open air holding metal tools like mammoty, knife and iron rods.

If this cannot be avoided, crouch down, singly, with feet together . Footwear or a layer of any non-absorbing material, such as plastic sheet, offers some protection against ground currents.

f. Do not seek shelter under or near isolated tall trees and in high grounds. If the vicinity of a tree cannot be avoided, seek a position just beyond the spread of the foliage.

g. By sitting down or lying down reduce the effective height of the body.

h. If in an open boat keep a low profile. Additional protection is gained by anchoring under relatively high objects such as jetties and bridges, provided that no direct contact is made with them.

i. Avoid riding horses or bicycles, or riding in any open vehicle such as a tractor.

j. Avoid swimming or wading.

k. Avoid touching or standing close to tall metal structures, wire fences and metal clothes lines.

l. Limit the use of telephones when a thunderstorm is overhead.

Lightning hazards are not fatal always. The state of the damage depends on the path of the lightning discharge through the body and the intensity of the current. In case of lightning strikes to persons, first aid should be given to the diseased before taking him for medical treatments. Body should be massaged to treat in case of temporary paralysis due to lighting strikes.

If respiration is disturbed, artificial respirations should be tried. This could be done by blowing air into the patients body through his mouth. In many cases, massage and artificial respiration have to be given simultaneously.

* There is no danger in touching, holding or carrying a person struck by lightning. (The writer is Deputy Director, Department of Meteorology, Colombo 7)

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