There are, on average, about 1,800 thunderstorms in progress at any one time around the world with 100 lightning strikes every second. A lightning bolt travels at about 14,000mph and heats up the air around it to 30,000°C - five times hotter than the surface of the sun. The chance of being hit by lightning is about one in three million.
Lightning is dangerous. Currently,between 30 and 60 people are struck by lightning each year in Britain of whom,on average, three may be killed. This compares with about 75 deaths in the much larger USA. The number of people killed by lightning each year has varied markedly. For example, the worst year in recent decades was 1982 when 14 people were killed, whereas there were no deaths in 2000 or 2001 - the first years without lightning fatalities since 1937.
There are around 300,000 ground strikes by lightning every year in Britain.On average (based on a ten-year period),this means that someone is struck once every 6,000 strikes and someone killed once every 100,000 strikes. A 'thunderstorm day' may produce up to 10,000 ground strikes although the exceptional day of 24 July 1994 produced 85,000 ground strikes.
What causes thunderstorms and what should you do to protect yourself during one?
Thunderstorms can form when moist, unstable air is lifted vertically into the atmosphere. This lifting can occur by:
(1) Convective lifting When there is unequal warming of the surface of the earth and the atmosphere is unstable, parcels of air can be lifted rapidly into the atmosphere forming isolated thunderstorm cells. These storms are usually localised and while they can be severe, their life span may be as short as 15 minutes. (Figure 1).
(2) Orographic lifting Warm moist air is forced up against the side of a mountain or range of mountains in unstable atmospheric
conditions. (Figure 2).
(3) Frontal Lifting Warmer air is uplifted at the leading edge of a frontal system. (Figure 3) Immediately after lifting begins, the rising parcel of warm moist air begins to cool due to adiabatic expansion. At a certain elevation the dew point is reached causing condensation and the formation of a cumulus cloud. For the 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.
|Figure 1: Formation of a convective thunderstorm||
Figure 2: Formation of an orographic thunderstorm
|Figure 3: Formation of a frontal thunderstorm||Figure 4 A lightning discharge begins with the separation of charge within the cloud.|
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 above) A lightning discharge begins with the separation of charge within the cloud.
|Figure 5: Stepped leader searches for ground|
Generally, positive charge masses at the top of the cloud and negative charge at the bottom of the cloud.
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)
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
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 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 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 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 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 lowlying 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.
• If you feel your hair stand on end, drop to the above position immediately.