More than 2500 people are dying due to lightning strikes in India every year. About 90% of them are those engaged in agricultural activities. Also, large number of livestocks are getting electrocuted. It is mainly due to lack of knowledge about the lightning phenomena and actions to be taken to escape from getting struck by lightning.
Also, there is a worldwide increase in lightning activity due to global warming. Global warming is causing more lightning incidents. One degree Celsius rise in surface temperature may lead to rise in frequency of lightning strikes by 12%. According to annual lightning report prepared by the Indian Meteorological Department (IMD) there has been 34 % rise in lightning strikes in India. Accordingly fatalities due to lightning strikes also increased.
For early warning dissemination on thunder storm and lightning, National Disaster Management Authority (NDMA) released a protocol in July 2021.
Indian Meteorological Department (IMD) is made nodal point for prediction and issue of thunder storm and lightning warnings. IMD issues four time bound forecasts. Long range forecasts are for more than 5 days in advance. Medium range forecasts are for 1 to 5 days in advance. Short range for 24 hours in advance and very short range for 3 hours in advance. Accuracy of these forecasts increases with time closeness to the events. These forecasts are prepared and transmitted every 3 hours intervals. These location specific forecasts are provided to state control rooms, disaster management units, media personnel and various NGOs. They are further disseminated to more interior and remote areas by local control rooms. Closer to the occurrence of the event information is communicated through DAMINI mobile app. Auto notifications are provided to users of DAMINI and other mobile apps MAUSAM and MEGHDOOT.
National Oceanic and Atmospheric Administration (NOAA), Govt of USA undertook lightning strike awareness programs every year in the month of June commencing from the year 2001. It is a week long program starting on the third Sunday of June every year and dedicated to education about the dangers of lightning and actions to be taken after receiving lightning strike predictions and alerts from meteorological department. Fatalities due to lightning strikes are dramatically declining year after year, which can be attributed to response for awareness programs. According to the data collected by National Weather Service Safety, fatalities due to lightning reduced from more than 100 before 2001 to less than 15 in 2021 in the USA.
In an interview conducted by Times of India Director General, Indian Meteorological Department, said that there is still lot of scope for improving the accuracy of weather and lightning predictions. IMD is almost accurate in the warnings and from 2019 started issuing thunder storm lightning alerts. Efforts are on to augment weather monitoring network. From 4 Doppler radars in 2006, the department now has 30. Another 26 will be set up in 2 to 3 years time covering the entire country.
Now, meteorological department is technically resourceful enough in predicting lightning strikes up to an hour ahead of time. It is up to people for taking precautionary measures, paying attention to IMD predictions.
Further action to be taken to bring down lightning strike fatalities in India is to start awareness programs like those being conducted in USA. These programs can be initiated by regional agricultural research centres and related academic institutions. They can be conducted in regional languages to reach maximum people engaged in agricultural activities.
Physics behind lightning
Lightning is often not straight. It takes a zigzag path. Lightning is flow of electric charge. It takes least resistance path as it travels to the ground. Air is not a perfect mixture. The concentrations of water vapour, dust particles and other pollutants vary from point to point in the atmosphere offering different resistances. Hence, lightning takes least resistance path which is zigzag.
Lightning and thunder
The travelling speed of lightning is 1x 10^8 m/sec. That is about 1/3rd of 3×10^8m/sec, which is the speed of light. Depending on the height of the cloud above the surface of ground and angle of travel, length of lightning path can be anywhere from 1 to 5 Kms and duration of travel time of lightning can be from 10 micro seconds to 100 micro seconds. Human eye sight cannot register minute difference in time and the whole process appears like a high intensity single flash. Lightning carries 30,000 amps of current.
This current raises temperature of the air around its path to as high as 50,000 deg F, which is 5 times hotter than the surface of the Sun. Air is expanded within a minute fraction of a second due to this immense heat and immediately cooled down and compressed creating high intensity sound wave. Travelling distances of sound from different segments of lightning path to observer vary. Because of vast difference in velocities of light and sound same lightning phenomena appearing as flash for fraction of a second can be heard as thunder for several seconds. Thunder production time is the same as flash occurring time. But because of path difference between sections of lightning path and observer, thunder is stretched for several seconds.
Cumulonimbus clouds and electrical polarisation
Cumulonimbus clouds are responsible for heavy rain and lightning strikes. For formation of cumulonimbus cloud moisture, unstable air mass and lifting force are required.
The average thunder storm cloud has a 24 Km diameter and a height of 13 Kms . Depending on the weather conditions, it takes nearly 30 minutes to form. During formation of storm cloud itself electrical polarisation takes place. Top of the cloud acquires positive charge and cloud bottom negative charge. This negative charge on the cloud’s bottom induces positive charge on the ground below by pushing electrons to lower depths.
The charge on the ground is influenced by the charge built up in the clouds. This causes the surface of the ground and objects (men and material on the ground) acquire positive charge.
Typical value for the insulation break down voltage of air is 3 kV/mm. That is 3000 kV/m and 3×10^6 kV/Km. Considering the distance between cloud and the ground as 1 Km, voltage required for the insulation to break down is 3x 10^9 V/Km. The charge building up process in the cloud continue till lightning strikes or acquired charge is dissipated. The tallest point on the ground is more susceptible for lightning to strike as it offers least distance and obviously least resistance path under normal conditions.
Ground currents in lightning strikes
When lightning strikes a point on the ground, energy travels outwards from the point of strike along the ground surface as current. Fatal potential differences are developed between adjacent points along radial lines starting from point of strike. Humans or animals standing, keeping legs along the radial line gets electrocuted. Current enters through one leg and leaves from other leg as shown in the figure before.
Changes in electrical fields in atmosphere and surface of ground during lightning storm
Figure 6 depicts electric field strength during normal fair weather conditions at the surface of the earth. It is 300V/m. When storm is in progress cloud gets electrically polarized. This process continues till the electric field strength reaches up to as high as 3MV/m over the surface of the ground before lightning strike. Shown in Figure 7.
Depending on electrical field strength measurement and ground charge build up, lightning strike early warning systems are built here.
For measuring the field strength, instruments are available from reliable and reputed manufacturers.
These instruments can raise sound alarm when the measured electric field strength exceeds threshold of safety limit. This alarm sound can be amplified and used to alert people in the open areas engaged in their daily work.
Lightning strike early warning system can also be built by monitoring ground charge build up during thunder storm. For sensing charge build up a gold leaf electroscope can be used with slight modifications. Gold leaf electroscope is a demonstration tool used for explaining properties of static electricity. It can be used for sensing induced ground surface charge prior to lightning strike.
A copper disk is attached to copper rod at the top above the cork lid. At the bottom side of this rod two gold leaves are attached as shown in the figure. Copper disk is connected to ground. Under normal weather conditions no electrical charge resides on the surface of the ground. Gold leaves of the electroscope, which are connected to ground remain in folded state.
During lightning storm the negative charge at the bottom of the cloud induces positive charge on the ground. Copper disk, vertical copper rod and gold leaves of electroscope also attain positive charge. Gold leaves diverge due to repulsion and touch two soft copper balls, which are attached to two copper rods. Siren speaker circuit is completed and siren is activated.
This early warning system is positioned inside the shelter shown in figure 10. Battery gets charged by photovoltaic source through charge controller. A 12V, 25W speaker is fixed to the roof of the shelter. A 15 metre metal rod is positioned at a distance of 10 metres from the shelter. The metal rod serves as lightning protection system for the shelter. It provides a low resistance safe passage to ground for lightning charge and saves the surrounding area. Floor of the shelter shed is covered with rubber sheet as protection from ground currents (if any), during lightning strikes. Normally, people working in the open area nearby can rush into the shelter after hearing alert siren.
Warning signs a few seconds prior to lightning strikes
- Just before lightning strike static energy fills the air. Hair will stand up. This is because all hairs acquire same positive charge along with the ground on which the person is standing. As all hairs acquire the same positive charge they repel from each other. This is an indication that lightning is going to strike within a few seconds time.
- Scent of ozone is observed. Air above surface of the ground is ionised producing ozone. Ionised air forms least resistance path for lightning to strike. Ozone gives a pungent odour similar to that of chlorine. It is detectable even at very low concentrations of 0.1 ppm in air. Scent of ozone in the air means that lightning strike is imminent.
- Feeling of metallic taste in the mouth is felt just before lightning strike. Any electrical stimulation can create metallic taste. Same thing is observed when a battery is getting discharged fast. When body along with ground on acquiring large electrical charge can lead to this kind of metallic taste.
- Feeling of dizziness (feeling that surroundings and body are spinning) and sweating is caused by electric charge interfering with nervous system of the body as heart functions on electrical impulses.
- Buzzing and vibrations observed in metallic objects. Air molecules vibrate as the sound moves through the air. The eardrum vibrates when the sound wave reaches it. Similar way metal objects like tin foils, empty metal containers respond to travelling sound wave. Vibrations in the metal objects are an indication for electric discharges in the area. During thunder glass windows of any building located in the vicinity rattle due to high intensity sound wave.
- Tingling feeling on the skin can be
…To be continued
Devanoori Bhanumurthy holds a B. E. degree in electrical engineering from Andhra University Engineering College. He is a Fellow Member of IETE. He has worked in many reputed organisations including Overseas Communications Service; Defence Ionospheric Research Station, DRDO unit in Jammu & Kashmir; Defence Electronics Research Laboratory; and Geological survey of India. Beside being an author of international repute, he is now engaged in R&D activities in Photovoltaic Powered Systems. He may be contacted through: ammaluelectronicshyd@gmail.com.
