What are thunderclouds?

In general, thunderclouds are cumulonimbus clouds. These are vertically dense clouds formed by water vapors drifted by powerful air currents. Thunderclouds can be an individual cloud or a cluster, and they are responsible for lightning, tornadoes, hail, and other hazardous activities. The thunderclouds have a variety of cloud particles and precipitates, and the majority of charges present in the thundercloud are negative charges. The density of negative charges varies with different geographical locations. Thunderclouds produce excessive negative charges at higher elevations when the ambient air temperature is between -5 to -15 degrees.

Thunderclouds also have the distribution of positive and negative charges that generate an electric field and electrical activities. The concentration of positive and negative charges remain distributed and separated throughout the cloud. Many proposed models describe the mechanism of distribution of these charges and the overall electrical structure of the cloud. According to researches, bigger and heavier cloud particles acquire negative charges, whereas lighter cloud particles acquire positive charges. The heavier and larger particles tend to fall and get accumulated near the bottom periphery of the cloud, thus separating the positive and negative charges.

Thunderclouds generally consist of amalgamated frozen water droplets, supercooled water droplets, and ice crystals. A collision between such clouds results in a thunderclap and spark. In this article, a generalized introduction has been provided regarding the electrical characteristics of a thundercloud.

Mechanism of lightning

The occurrence of lightning is due to the electrical discharge as a result of an increase in the density of one of the charges. The charges in the cloud increase in density over time. At a point in time, an electrical discharge (lightning discharge) takes place between opposite charges, from charged regions to the atmosphere, or the ground. This lightning or electrical discharge neutralizes the charge density over the cloud region. Two-thirds of electrical discharge occurs within the cloud itself, from cloud to cloud, or from cloud to air. The rest is discharged from the cloud to the ground.

Cloud-to-ground lightning

Cloud-to-ground lightning occurs through four strokes, these strokes are discussed in detail in the present section.

Initial stroke

The flash of lightning is caused due to an electrical breakdown between the positive charges and negative charges. Due to the breakdown, the layer of air at the breakdown site undergoes partial ionization where the neutral atoms and molecules are converted to charged particles. These flashes of lightning carry a current of thousands of amperes and travel with a velocity of 1.5 x 10⁵ m/s. The lightning is characterized by stepped pulses traveling downwards.

Return stroke

When the pulse is about to strike the ground approximately, at a distance of 15 to 50 m, 5 coulombs of charge remain that is distributed along the pulse channel. It induces an opposite charge above the periphery, where the lightning is about to strike. As a result, there is an increase in the electric field in that area. It creates an opposite upward discharge that rises to meet the pulse. It results in a short circuit and a high luminous return stroke with high magnitudes of current.

Multiple return strokes

The intensity of the channel increases as multiple subsequent upward discharges join the main channel. The stroke moves upward to the atmosphere reaching a height where the temperature is 0 degrees.

Dissipation of energy

During the return stroke of the flash, approximately 10⁵ joules of energy is dissipated. This energy is utilized in particle dissociation, ionization, excitation, radiation, and increase in kinetic energy. It takes less than microseconds to convert neutral atoms into charged ions characterized by plasma.

Cloud charge structure and cloud electrification

A thundercloud is primarily characterized by tripole, constituting positive charges at the cloud top and bottom. The negative charges are distributed at the center of the cloud. To determine the electric field due to the charge distribution, the method of images is primarily applied. The electric field above the ground is proportional to the distance from the axis of the tripole.

The overall electric field changes with subsequent flashes and with a continuous stroke for a considerable amount of duration. The negative charges get neutralized during the ground flashes and the dipole charged moments are a function of time. These dipole-charged moments are transferred by the clouds.

Mapping thunder

Mapping thunder means measuring the power of the lightning flash. But initially, the cloud's electric potential needs to be measured. The power of the thunder is estimated to be around 2 gigawatts, which is way more powerful than the power produced by nuclear powerplants. The energy is enough to power the entire New York City for about 26 minutes.

The amount of electrical potential created by lightning flash is enough to accelerate electrons to a speed, and break other atoms to generate gamma rays.

Context and Applications

The topic is taught in many undergraduate and postgraduate degree courses like:

• Bachelors of Technology (Electrical Engineering)
• Bachelors of Technology (Electrical and Electronics Engineering)
• Masters of Technology (Electrical Engineering)

Practice Problems

Q1. Which of the following is the correct position of negative charges in a thundercloud?

a. Top

b. Bottom

c. Center

d. They are all over.

Answer: Option c

Explanation: The distribution of charges in the thundercloud is characterized by the presence of positive and negative charges. The distribution of positive charges is at the top and bottom of the cloud while the negative charges are distributed at the center.

Q2. How many strokes do a typical lightning have?

a. Three strokes

b. Five strokes

c. Six strokes

d. Four strokes

Answer: Option a

Explanation: A typical lightning has mainly three strokes, initial stroke, return stroke, successive return strokes. These strokes are accompanied by huge energy dissipation.

Q3. How much power is possessed by a lightning flash?

a. 1 gigawatt

b. 100 megawatts

c. 2 gigawatts

d. 1.5 gigawatts

Answer: Option c

Explanation: The measurement of thunder power is known as thunder mapping which is done after measuring the cloud's electric field. A typical flash results in 2 gigawatts of power.

Q4. How much energy is dissipated during lightning flashes?

a. 10⁶ joules

b. 10⁷ joules

c. 10⁵ joules

d. 10 joules

Answer: Option c

Explanation: The energy dissipated during the flash of the lightning has an energy of approximately 105 joules. This energy is utilized in dissociation, ionization, increasing the kinetic energy of the particles, and so on.

Q5. During which of the following temperature conditions, a thundercloud accumulates excessive negative charges?

a. At low temperatures

b. At high temperatures

c. Both a and b

d. Never

Answer: Option a

Explanation: A typical thundercloud consists of ice particles, supercooled liquid, and cool precipitates at higher altitudes where the temperature is low. These particles, when collide with other similar clouds develop negative static charges which tend to get neutralized via electrical breakdown.