- Thunderstorms and tornadoes are severe local storms. They are of short duration, occurring over a small area but are violent.
- Thunderstorm is a storm with thunder and lightning and typically also heavy rain or hail.
- Thunderstorms mostly occur on ground where the temperature is high. Thunderstorms are less frequent on water bodies due to low temperature.
- Worldwide, there are an estimated 16 million thunderstorms each year, and at any given moment, there are roughly 2,000 thunderstorms in progress.
How does a thunderstorm form?
- Ground is significantly heated due to solar insolation.
- A low pressure starts to establish due to intense upliftment of an air parcel (convention).
- Air from the surroundings start to rush in to fill the low pressure.
- Intense convection of moist hot air builds up a towering cumulonimbus cloud. [I have explained how condensation occurs in previous post]
- Characterized by intense updraft of rising warm air, which causes the clouds to grow bigger and rise to greater height.
- Later, downdraft brings down to earth the cool air and rain.
- The incoming of thunderstorm is indicated by violent gust of wind. This wind is due to the intense downdraft.
- The updraft and downdraft determine the path of the thunderstorm. Most of the time, the path is erratic.
- When the clouds extend to heights where sub-zero temperature prevails, hails are formed and they come down as hailstorm. Intense precipitation occurs.
- In a matter of few minutes, the storm dissipates and clear weather starts to prevail.
Motion of a thunderstorm
- Path of a thunderstorm is erratic. Motion is primarily due to interactions of its updrafts and downdrafts.
- The speed of isolated storms is typically about 20 km (12 miles) per hour, but some storms move much faster.
- In extreme circumstances, a supercell storm may move 65 to 80 km (about 40 to 50 miles) per hour.
- Downdrafts are referred to as macrobursts or microbursts.
- Macroburst is more than 4 km in diameter and can produce winds as high as 60 metres per second, or 215 km per hour.
- A microburst is smaller in dimension but produces winds as high as 75 metres per second, or 270 km per hour
- They are seriously hazardous to aircrafts, especially during takeoffs and landings.
Types of Thunderstorms
- Convectional, Frontal, Orographic Thunderstorms.
- Isolated Thunderstorms, Multiple-Cell Thunderstorms, Supercell thunderstorms.
- Caused due to intense heating of ground during summer.
- Forceful upliftment of warm moist air parcel when it passes over a mountain barrier creates cumulonimbus cloud causing heavy precipitation on the windward side.
- Orographic ‘Cloud bursts’ are common in Jammu and Kashmir, Cherrapunji and Mawsynram.
- Thunderstorms occurring along cold fronts.
- Single-cell thunderstorms are small, brief, weak storms that grow and die within an hour or so. They are typically driven by heating on a summer afternoon.
- Single-cell storms may produce brief heavy rain and lightning (Very common in India during summers, mostly April, May. In Kerala they are called ‘Mango Showers’ and in Karnataka ‘Blossom showers’).
A multi-cell thunderstorm
- A multi-cell storm is a thunderstorm in which new updrafts form along the leading edge of rain-cooled air (the gust front).
- Individual cells usually last 30 to 60 minutes, while the system as a whole may last for many hours.
- Multi-cell storms may produce hail, strong winds, brief tornadoes, and/or flooding.
A super-cell thunderstorm
- A super-cell is a long-lived (greater than 1 hour) and highly organized storm feeding off an updraft (a rising current of air) that is tilted and rotating.
- Most large and violent tornadoes come from super-cells.
- As water vapour moves upward in the cumulonimbus cloud, decreasing temperatures causes it to condense.
- The heat (latent heat of condensation) generated in the process pushes the water molecules further up.
- As they move beyond zero degrees, water droplets change into small ice crystals.
- As they continue to move up, they gather mass — until they are so heavy that they start to fall.
- This leads to a system where smaller ice crystals move up while bigger crystals come down.
- The resulting collisions trigger the release of electrons, in a process very similar to the generation of electric sparks (this is called as ionization – an electron in the outer shell is pealed out of the atom and the atom become an ion. There are two types of ions based on charge – cation and anion. Cation: A cation is an atom or a molecule which is positively charged, i.e. has more number of protons than electrons.Anion: An anion is an atom or molecule which is negatively charged, i.e. has more number of electrons than protons).
- The moving free electrons cause more collisions and more electrons, as a chain reaction ensues.
- The process results in a situation in which the top layer of the cloud gets positively charged (cations) while the middle layer is negatively (anions) charged.
- The electrical potential difference between the two layers is huge, of the order of 109 or 1010 volts.
- In little time, a huge current, of the order of 105 to 106 amperes, starts to flow between the layers.
- It produces heat, leading to the heating of the air column between the two layers of cloud.
- It is because of this heat that the air column looks red during lightning.
- The heated air column expands and produces shock waves that result in thunder.
Lightining from cloud to Earth
- Earth is a good conductor of electricity but is electrically neutral.
- In comparison to the middle layer of the cloud, however, it becomes positively charged.
- As a result, a flow of current (about 20-15%) gets directed towards the Earth as well.
- It is this current flow that results in the damage to life and property.
- There is a greater probability of lightning striking tall objects such as trees, towers or buildings.
- Once about 80-100 m from the surface, lightning tends to change course to hit the taller objects (guess why very tall buildings have a vertical pole above).
- This is because travelling through air, which is a bad conductor of electricity, electrons try to find a better conductor, and also the shortest route to the relatively positively charged Earth’s surface.
- Several thousand thunderstorms occur over India every year.
- Incidents of lightning have been showing an increasing trend over the last 20 years, especially near the foothills of the Himalayas.
- People are rarely hit directly by lightning. But such strikes are almost always fatal.
- The most common way in which people are struck by lightning are by ‘ground currents’.
- The electrical energy, after hitting a tree or any other object, spreads laterally on the ground for some distance, and people in this area receive electrical shocks.
- It becomes more dangerous if the ground is wet, or there is conducting material like metal on it.
Prediction and precautions
- Predicting a thunderstorm over a very precise location is not possible. Nor is the exact time that it is likely to strike.
- People are advised to move indoors in a storm.
- Moving under a tree or lying flat on the ground can increase risks.
- Even indoors, electrical fittings, wires, metal and water must be avoided.
The world’s most electric place
- The most lightning activity on Earth is seen on the shore of Lake Maracaibo in Venezuela.
- At the place where the Catatumbo river falls into Lake Maracaibo, October sees 28 lightning flashes every minute — a phenomenon referred to as the Beacon of Maracaibo or the Everlasting Storm.
- The reason probably lies in the topography of the spot: winds blow across Lake Maracaibo — the largest in South America (By volume of water, Titicaca is the largest lake in South America. Lake Maracaibo has a larger surface area, though some consider it to be a large brackish bay due to its direct connection with the sea.) — which is surrounded by swampy plains and connected to the Gulf of Venezuela/Caribbean Sea by a very narrow strait.
- The Maracaibo plain is enclosed on three sides by high mountain sides into which air masses crash.
- The heat and moisture picked from the swampy plains creates electrical charges and, as the air is destabilized at the mountain faces, thunderstorm activity — characterised by almost non-stop lightning activity within clouds — results.
- Direct Strike: Occurs most often in open areas.
- Side Flash (Or Side Splash): Occurs when lightning strikes a taller object and some current jumps on to the victim, who ends up acting as a “short circuit” for the energy.
- Generally occurs when the victim is within a foot or two of the struck object. Most victims are those sheltering under a tree in a rainstorm.
- Ground Current: When an object is struck, much of the energy travels outward in and along the ground surface. This is ‘ground current’, and anyone close can be a victim.
- Ground current affects a larger area than other kinds of current and causes the most lightning deaths and injuries.
- Conduction: Lightning can travel long distances in wires or other metal surfaces.
- Most indoor lightning casualties and some outdoor casualties are due to conduction.
Features of Lightning
- Positive charge accumulates at both higher and lower altitudes.
- Larger and heavier cloud particles charge with a negative polarity.
- Smaller and lighter clouds particles charge with a positive polarity.
- Roughly two-thirds of all discharges occur within the cloud. The rest are between the cloud and ground.
- Lightning createsplasma (ionized gas medium) [30,000 °C].
- The channel pressure greatly exceeds the ambient (surrounding) pressure, and the channel expands at asupersonic rate (speed of sound).
- The resultant shock wave decays rapidly with distance and is eventually heard as thunder once it slows to the speed of sound.
- From severe thunderstorms sometimes spiraling wind descends like a trunk of an elephant with great force, with very low pressure at the center, causing massive destruction on its way. Such a phenomenon is called a tornado.
- Tornadoes generally occur in middle latitudes. The tornado over the sea is called water sprouts.
- These violent storms are the manifestation of the atmosphere’s adjustments to varying energy distribution. The potential and heat energies are converted into kinetic energy in these storms and the restless atmosphere again returns to its stable state.
- Tornado is a small-diameter column of violently rotating air developed within a convective cloud and in contact with the ground.
- Tornadoes occur most often in association with thunderstorms during the spring and summer in the mid-latitudes of both the Northern and Southern Hemispheres.
- These whirling atmospheric vortices can generate the strongest winds known on Earth: wind speeds in the range of 500 km (300 miles) per hour.
- They are often referred to as twisters or cyclones.
Distribution of tornadoes
- Rare in polar regions and infrequent at latitudes higher than 50° N and 50° S.
- The temperate and tropical regions are the most prone to thunderstorms.
- Tornadoes have been reported on all continents except Antarctica.
- United States has the most violent tornadoes.
- Canada reports the second largest number of tornadoes.
- In the Indian sub-continent, Bangladesh is the most prone country to tornadoes.
- At any moment there are approximately 1,800 thunderstorms in progress throughout the world.
- Waterspout is an intense columnar vortex (usually appearing as a funnel-shaped cloud) that occurs over a body of water.
- They are connected to a towering cumuliform cloud or a cumulonimbus cloud.
- Weaker than most of its land counterparts i.e. tornadoes.
- Most waterspouts do not suck up water; they are small and weak rotating columns of air over water.
- They are tornadoes in connection with severe thunderstorms, but simply occurring over water.
Damage caused by thunderstorms and tornadoes
- Many hazardous weather events are associated with thunderstorms.
- Under the right conditions, rainfall from thunderstorms causes flash flooding, killing more people each year than hurricanes, tornadoes or lightning.
- Lightning is responsible for many fires around the world each year, and causes fatalities.
- Hail up to the size of softballs damages cars and windows, and kills livestock caught out in the open.
- Strong (up to more than 120 mph) straight-line winds associated with thunderstorms knock down trees, power lines and mobile homes.
- Tornadoes (with winds up to about 300 mph) can destroy all but the best-built man-made structures.