Indian geography 4

ATMOSPHERIC ELECTRICITY

Overview

Aurora Borealis seen over Canada

• The Earth’s surface, the atmosphere and the ionosphere combine to form a global atmospheric electrical circuit
• Free electricity is always present in the atmosphere. It is usually positive
• The intensity of atmospheric electricity is usually greater in the middle of the day than in the morning or at night. Also, it is greater in winter than in summer
• Atmospheric electricity increases with altitude
• The Earth’s surface is negatively charged, while the atmosphere is positively charged
• Benjamin Franklin was the first to prove electrical phenomena of the atmosphere in 1752

Variation of atmospheric electricity

• The primary cause of variation in atmospheric electricity is the thermodynamics of radiation
• Atmospheric electricity is maximum in January and minimum in June
• Humidity increases atmospheric electricity in the cold months but decreases it in hot months

PHENOMENA OF ATMOSPHERIC ELECTRICITY

1. Auroras
   1. Auroras are natural light displays observed in the night sky, especially in polar regions
   2. Auroras occur when the Earth’s magnetic field traps solar wind in the atmosphere resulting in a collision between the solar wind and atmospheric molecules leading to release of energy
   3. They are most prominent closer to the magnetic poles because of longer periods of darkness and strength of the Earth’s magnetic field
   4. The Aurora Borealis refers to auroras in the northern hemisphere. The corresponding auroras in the southern hemisphere are called Aurora Australis
   5. Auroras occur most often near the seasonal equinoxes: from September to October and from March to April
   6. Auroras have maximum intensity during the intense phase of solar cycle when coronal mass ejections increase the intensity of solar wind
2. Static electricity
   1. Static electricity is the build of electrical charge on the surface of objects
   2. The static charge remains on the object until it either bleeds off to the ground or is quickly neutralized by a discharge
   3. Lightning is caused by discharge of static electricity
3. St. Elmo’s Fire
   1. St. Elmo’s Fire is a bright blue or violet glow appearing from tall, pointed objects
   2. It is a phenomenon in which plasma is created when the electric field around the object causes ionization of air molecules
   3. Sharp objects tend to create more plasma because electrical fields are more concentrated in areas of high curvature
4. Lightning
   1. Lightning is an atmospheric discharge of electricity
   2. Occurs during thunderstorms, volcanic eruptions and dust storms
   3. The average lightning bolt can reach temperatures of 30,000 C (about 3 times the temperature of the sun) and carry around 100 million V of electricity
   4. This extreme temperature compresses surrounding air and creates a supersonic shock wave called thunder
   5. In addition to light, lightning has been shown to emit radio waves, X-rays and gamma rays

TYPES OF LIGHTNING

Lightning strikes can carry up to 100 million Volts and reach temperatures of 30,000 C

The lightning that is most commonly observed is called streak lightning. This is just the visible part of the lightning stroke – the majority of the lightning occurs inside clouds, so it is not visible from the Earth.

1. Cloud-to-ground lightning
   1. Second most common form of lightning
   2. b. Poses greatest threat to life and property since it strikes the ground
2. Cloud-to-cloud lightning
   1. Lightning occurring between two clouds is called inter-cloud lightning
   2. Lightning that occurs between two areas of the same cloud that have differing electric potential is called intra-cloud lightning
3. Ground-to-cloud lightning
   1. Lightning discharge between ground and cloud, in the upward direction
   2. Very rare
   3. Occurs when negatively charged ions from the Earth’s surface rise up and meet the positive ions in the cloud
4. Heat lightning: lightning that occurs too far away for the sound of thunder to be heard
5. Dry lightning
   1. Dry lightning is lightning that occurs without precipitation at the surface
   2. This is the most common natural cause of wildfires
   3. Occurs as a result of extreme surface temperatures when convection from the hot surface to cooler atmosphere leads to lightning
6. Positive lightning
   1. Occurs when positive charge is carried on the top of clouds
   2. Very rare
   3. Around 10 times more powerful and longer lasting than regular negative lightning
   4. Very dangerous to life and property
   5. At present, aircraft are not designed to withstand positive lightning
7. Sprite
   1. Large scale discharges occurring high above a thundercloud
   2. Occur about 80 km to 150 km above the Earth’s surface
   3. Reddish-orange or greenish-blue in colour
   4. May account for aircraft accidents at altitudes above thunderstorms
8. Blue jets
   1. Occur at lower altitudes than sprites, but still above thunderclouds
   2. Occur about 40 km to 80 km above surface
   3. Blue in colour
9. Elves
   1. ELVES stands for Emissions for Light and Very low frequency Electromagnetic pulse Sources
   2. Occur in the ionosphere, about 100 km above surface
10. Rocket-triggered lightning
    1. Lightning can be triggered by rockets carrying spools of wire into thunderstorms. When the wire unwinds, it provides a path for lightning to conduct to the surface
    2. Lightning can also be triggered by space shuttle launches and aircraft flight
11. Volcanically triggered lightning
    1. Extremely large volcano eruptions which eject gases and material high into the atmosphere can trigger lightning

LIGHTNING IN EVERYDAY LIFE

World map showing the frequency of lightning strikes. Lightning strikes most frequently in the Congo

Lightning conductor

• It is a metal rod or conductor used to protect a building from lightning
• It is mounted on the top of the building and connected to the ground using a wire
• When strikes, it will preferentially strike the rod and be conducted harmlessly to the ground
• Lightning rods are usually made from good conductors of electricity such as aluminium or copper

Lightning protection on aircraft

• On aircraft, an electrical circuit is established on the aircraft’s outer surface
• Aircraft made from aluminium naturally act as good conductors of electricity. When the aircraft is made of carbon composites, a layer of conductive fibre is embedded to ensure conductivity
• When the aircraft is struck by lightning, current travels on the outer surface of the aircraft, with the interior remaining unaffected
• Proper shielding is provided to ensure lightning does not affect cockpit electronics, fuel tanks and radar and other avionics
• Aircraft also use static dischargers to prevent buildup of static electricity

Trees and lightning

• Trees are natural conductors of lightning. They provide connection for lightning to reach the ground. However, the outer layer of trees (bark) is not a good conductor
• Trees get burnt from lightning because lightning travels on the outer surface of the tree, burning away the bark.
• Usually, trees can recover from damage to the bark. However, sometimes the damage is too severe for recovery.
• Oak and elm are two trees most frequently struck by lightning. Teak provides the best conducting connection for lightning
• By attracting lightning towards them, trees prevent damage to nearby buildings. However, for the same reason, it is not safe to seek shelter under trees during lightning

Shelter from lightning

• To get shelter from lightning, there needs to be an electrical connection through the exterior surface on to the ground. The connection must ensure that people do not get in contact with the electricity
• Best lightning shelters: houses, buildings, closed-roof cars, closed-cabin boats etc
• Worst lightning shelters: trees, tents, open barns, open-roof cars, open boats etc
• It is unsafe to use radios, cellphones etc during lightning strikes

MEASUREMENT OF ATMOSPHERIC ELECTRICITY

1. Electrometer
   1. Simple instrument for measuring atmospheric electricity at ground surface
   2. Developed in 1700s by Alessandro Volta
   3. Consists of a glass jar with a pointed metal rod, whose lower end is attached to two straws. Electricity in the atmosphere cause the two straws to recede from each other, the amount of divergence indicating the intensity of electricity
2. Weather balloons
   1. A balloon which carries instruments aloft to send back information regarding temperature, humidity etc
   2. The device that does the actual measuring is called radiosonde
   3. The radiosonde is an inexpensive device, and it is lost when the life of the balloon expires
3. Lightning rocket
   1. It is a device that measures electrostatic and ionic charge in the atmosphere
   2. Consists of a rocket launcher which is in communication with the detection device on the ground
   3. This system controls the time and location of a lightning strike
   4. Uses solid (cesium salts) or liquid (calcium chloride) propellants to produce exhaust gases that act as a conducting pathway between the clouds and the ground