Between 30 and 60 people are struck by lightning each year
reached causing condensation and the formation of a cumulus cloud. For the cumulus cloud to develop into a thunderstorm continued uplift must occur in an unstable atmosphere. As the vertical extension of the cumulus increases, it grows into a cumulonimbus cloud. Cumulonimbus clouds can reach heights of 20km above the earth’s surface. Severe weather associated with cumulonimbus clouds includes hail, strong winds, thunder, lightning, heavy rain, waterspouts and tornadoes.
Lightning formation Within a typical thunderstorm cloud a chaotic turmoil of wind, water and ice exists in an environment in which temperature decreases with height. Small particles are carried upward by the wind while large particles move downward under the dominant influence of gravity. The various ascending and descending particles exhibit different velocities depending on their size. Particles that move at different
velocities collide with one another, and out of these interactions emerge (lighter) positively charged particles which move upward and heavy negatively charged particles which move downward. (Figure 4 below)
The initial low-luminosity discharge (called the stepped leader) searches out the ground as a path to discharge it’s energy. (Figure 5) The branches step down at 100 metre increments and will usually be attracted to the tallest object in the area. (Figure 6)
The tremendous charge coming to
ground creates an opposite charge on the earth’s surface. When the stepped leader reaches the ground or is contacted by an upward-moving discharge some tens of metres above the ground, the leader becomes highly luminous and the ‘return stroke’ is formed from the ground up. (Figure 7) The temperature of the lightning channel is approximately five times that of the surface of the sun and it heats the air rapidly as it passes through. Thunder is the noise caused by this rapid explosive expansion of the air at the time of the return stroke. The total discharge is called a ‘flash’ and lasts about half a second. A flash may be made up of three or four strokes, hence the flicker that we sometimes see. Lightning often strikes as far as 10 miles away from any rainfall. Many deaths from lightning occur ahead of the storm because people try and wait to the last minute before seeking shelter. You are in danger from lightning if you can hear thunder. If you can hear thunder, lightning is close enough that it could strike your location at any moment.
Lightning injuries can lead to permanent disabilities or death. On average, 10% of strike victims die; 70% of survivors suffer serious long term effects.
Outdoors is the most dangerous place
A lightning discharge begins with the separation of charge within the cloud. Generally, positive charge masses at the top of the cloud and negative charge at the bottom of the cloud.
to be during a lightning storm. When lightning is seen or thunder is heard, or when dark clouds are observed, quickly move indoors or into a hard-topped vehicle and remain there until well after the lightning storm ends.
Metal in objects such as phones can direct the current into the body Next time you find yourself talking on your mobile phone in the middle of a thunderstorm you may want to cut the conversation short. UK doctors have warned of the danger of lightning strikes when using mobile phones outdoors during stormy weather. In the British Medical Journal, they highlight the case of a teenager left with severe injuries after being struck by lightning when talking on her phone. When a person is hit by lightning, the high resistance of human skin causes the lightning charge to flow over the body - often known as an ‘external flashover’.
But some of the current can flow
through the body. The more that flows through, the more internal damage it causes.
Conductive materials in direct contact with the skin such as liquid or metal objects increase the risk that the current will flow through the body and therefore cause internal injury.
What to do during a thunderstorm
• Avoid using the phone - telephone lines can conduct electricity.
• Avoid using your mobile phone.
• Avoid using taps and sinks - metal pipes can conduct electricity.
• If outside avoid water and find a low- lying open place that is a safe distance from trees, poles or metal objects.
• Avoid activities such as golf, rod fishing or boating on a lake.
• Do not shelter under trees
• If you find yourself in an exposed location it may be advisable to squat close to the ground, with hands on knees and with head tucked between them. Try to touch as little of the ground with your body as possible, do not lie down on the ground.
Figure 5: Stepped leader searches for ground
Figure 6: Upward-moving discharge from tall objects travels toward main leader.
Figure 7: The leader becomes highly luminous and the return stroke is formed
• If you feel your hair stand on end, drop to the above position immediately.
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