- Brazilian researchers recently succeeded in taking pictures of positive upward discharges of electricity from lightning conductor rods in São José dos Campos.
What is Upward Lightning?
- Upward lightning is a phenomenon whereby a self-initiated lightning streak develops from a tall object that travels upward toward an overlaying electrified storm cloud.
- For this to happen, storm electrification and the resulting presence of a cloud charge region are enabling factors.
- The vertical elevation of a tall object accentuates the electric field locally on the ground, resulting in conditions favourable for the initiation of an upward streak (called a leader) from a tall object, which can also develop in response to an electric field change created by a nearby preceding lightning flash.
Lightening and its Strike Procedure
- Lightning is a very rapid — and massive — discharge of electricity in the atmosphere, some of which is directed towards the Earth’s surface.
- These discharges are generated in giant moisture-bearing clouds that are 10-12 km tall.
- The base of these clouds typically lies within 1-2 km of the Earth’s surface, while their top is 12-13 km away.
- Temperatures towards the top of these clouds are in the range of minus 35 to minus 45 degrees Celsius.
- As water vapour moves upward in the cloud, the falling temperature causes it to condense. Heat is generated in the process, which pushes the molecules of water further up.
- As they move to temperatures below zero degrees celsius, the water droplets change into small ice crystals.
- They continue to move up, gathering mass — until they are so heavy that they start to fall to Earth.
- This leads to a system in which, simultaneously, smaller ice crystals are moving up and bigger crystals are coming down.
- Collisions follow, and trigger the release of electrons — a process that is very similar to the generation of sparks of electricity.
- As the moving free electrons cause more collisions and more electrons, a chain reaction ensues.
- This process results in a situation in which the top layer of the cloud gets positively charged, while the middle layer is negatively charged.
- Electrical difference between two layers:
- The electrical potential difference between the two layers is huge — of the order of a billion to 10 billion volts.
- In very little time, a massive current, of the order of 100,000 to a million amperes, starts to flow between the layers.
- Enormous heat release:
- An enormous amount of heat is produced, and this leads to the heating of the air column between the two layers of the cloud.
- This heat gives the air column a reddish appearance during lightning. As the heated air column expands, it produces shock waves that result in thunder.
- Reaches Earth:
- While the Earth is a good conductor of electricity, it is electrically neutral.
- However, in comparison to the middle layer of the cloud, it becomes positively charged. As a result, about 15%-20% of the current gets directed towards the Earth as well.
- It is this flow of current that results in damage to life and property on Earth.
- Lightning strikes more on:
- There is a greater probability of lightning striking tall objects such as trees, towers or buildings.
- Once it is about 80-100 m from the surface, lightning tends to change course towards these taller objects.
- This happens because air is a poor conductor of electricity, and electrons that are travelling through air seek both a better conductor and the shortest route to the relatively positively charged Earth’s surface.