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Tornado: Meaning and Definitions
A severe but short-lived thunderstorm is called Tornado. According to the National Weather Service (NOAA) “A tornado is a violently rotating column of air extending from the base of a thunderstorm down to the ground. Tornadoes are capable of completely destroying well-made structures, uprooting trees, and hurling objects through the air like deadly missiles. Tornadoes can occur at any time of day or night and at any time of the year. Although tornadoes are most common in the Central Plains and the southeastern United States, they have been reported in all 50 states.”
It is very small in size, but extremely violent and the surface wind speed in the small area affected by the storm has been recorded as about 320 km per hour. The word ‘Tornado’ originates from the Spanish word ‘tronada’. In North and Central Africa the word ‘tornado’ is used only for lightning storms. But in meteorology it is used for a special type of extremely severe storm.
Critchfield termed Tornado as the most violent thunderstorm in the lower troposphere. According to him, a tornado is a tight vortex that revolves around an extremely low pressure center.
According to Byers, tornadoes are the most powerful atmospheric disturbances, yet their horizontal extension and area of influence are so small that they never appear on synaptic charts or daily weather maps.
According to Petsern, a tornado is a vortex with small horizontal extension and high intensity, which spreads downwards from the lightning cloud. Generally, this looks like a funnel shaped cloud with a wide part hanging in the cumulonimbus clouds and the narrow tubular part at the bottom keeps hanging towards the ground.
The lower part of a tornado cloud is surrounded by dust and debris. These objects are sucked from the surface and are again thrown out by the centrifugal force of the vortex. Initially this funnel is almost vertical. But as it moves ahead of the cloud located above, the upper part of the vortex bends and sometimes gets separated from the cloud.
Structure of a Tornado
A tornado is a phenomenon characterized by high-speed winds. Its diameter spans from 30 meters to 1.5 kilometers, with most tornadoes falling within the range of 150 to 600 meters of diameter. The ground speed of a tornado typically ranges between 32 and 64 kilometers per hour. The path length varies from a few kilometers to approximately 320 kilometers, and the width can exceed 800 meters. Tornadoes may occur either in isolation or separately.
The swiftly rotating winds within a tornado have never been accurately measured due to the destruction of any wind speed gauges placed in their path. The estimated wind speed, ranging from 320 to 800 kilometers per hour, is determined based on the damage inflicted by these powerful storms.
Upon the arrival of a tornado, there is an abrupt drop in air pressure. Generally, a decrease of 25 millibars is observed, but sometimes a more significant change is recorded. The substantial drop in air pressure from the periphery towards the center generates immense destructive power within the tornado. In 1904, a tornado in Minnesota recorded a 200-millibar drop in air pressure.
The sudden drop in atmospheric pressure creates a significant pressure disparity between the inside and outside of large buildings. Consequently, buildings may collapse and be destroyed in the resulting explosion. In tornadoes in the Northern Hemisphere, the winds circulate counterclockwise around the center. Notably, the wind speed and intensity in tornadoes are exceptionally high, surpassing that of any other storm.
Characteristics of Tornado
Tornadoes bring about atmospheric conditions similar to thunderstorms, exhibiting symptoms such as strong storms, torrential rain, hail, thunder, and lightning. The high wind speed accompanying tornadoes produces a distinctive loud roar. Despite their strength, these storms are relatively short-lived, lasting for approximately 8 minutes, and, in some instances, concluding almost instantly.
The genesis of these powerful winds occurs in regions where tropical and cold polar air masses converge. Tornadoes are closely associated with the cold air of temperate cyclones. The spring and early summer months are particularly conducive to the formation of these storms. Tornadoes can also emerge from highly intense, scattered lightning storms, although such tornadoes tend to have brief lifespans and follow irregular paths.
Geographical Distribution of Tornadoes
Tornadoes can form anywhere in the world, excluding the polar regions and the extremely cold northern parts of continents during winter. However, they are notably prevalent in the United States of America and Australia. In the U.S., a substantial number of tornadoes originate in areas east of the Rocky Mountains and east of the Andes Mountains. Eastern India also experiences the impact of tornadoes. The geographical distribution of these storms underscores the significant role played by the vertical distribution of temperature, atmospheric humidity, and the influence of mountains on air currents in their origin.
The Mississippi Valley in the United States is particularly infamous for tornado activity, recording an average annual count of 145 tornadoes. Noteworthy states where these storms occur most frequently include Kansas, Iowa, Texas, Arkansas, Oklahoma, Missouri, Alabama, Mississippi, and Nebraska. Consequently, the highest concentration of tornadoes is observed in the Central West of the United States, with the southeastern states taking the second spot. Tornadoes do not originate west of the Rockies.
Tornadoes originating in the United States are linked to cold fronts, characterized by warm air near the surface and a layer of dry air above it. The temperature gradient between these converging air masses leads to convective instability in the atmosphere above the temperature inversion level, giving rise to these intense storms and lightning events.
Weather Conditions Associated with Tornadoes
Extensive research on tornadoes has been conducted in the United States under the guidance of the U.S. Weather Bureau. The findings have unveiled consistent atmospheric conditions across all types of tornadoes:
- A temperature inversion is identified above the layer of warm air near the surface, causing a significant reduction in the rate of loss within the dry air mass.
- The air column ascends to considerable heights.
- A belt of strong winds is situated between 3,000 and 6,000 meters, where wind speeds exceed 64 kilometers per hour.
- Moisture is concentrated in a specific narrow belt within the layer of humid air.
- The axis of fast winds, descending downward, intersects with the axis of the flow of moist air.
- The level of free convection in the atmosphere is relatively low, and convective instability permeates the entire air column.
According to Taylor, these factors have proven effective in tornado forecasting in the United States, with commendable efforts from the Meteorological Department of the United States Air Force. However, considering the intricacies of tornado forecasting, Sutton, O.G. expresses a viewpoint acknowledging the complexity of the task. He notes that despite the strong inward wind flow at the surface, certain questions, such as the existence of a partial vacuum at the tornado’s center, lack entirely satisfactory answers in the current understanding of modern meteorologists. The absence of a reliable theory further complicates the prediction of tornado origin and paths.
Tornado – a Devastating Storm
Tornadoes, categorized as severe storms, inflict extensive damage on both property and lives. The destruction caused by these storms manifests in three primary ways:
High-Speed Surface Wind – Buildings, trees, and other exposed structures collapse due to the tremendous speed of surface winds.
Sudden Drop in Air Pressure – The abrupt decrease in air pressure leads to a substantial disparity between the air pressure inside and outside buildings, causing explosions.
Strong Winds at the Center – The winds circulating near the center of minimum air pressure are so powerful that they can blow off the roofs of houses, objects within the tornado’s grasp are lifted to significant heights and then dropped some distance away.
The path of a tornado results in the collapse of everything in its wake, including buildings, trees, electrical wires, and telephone poles. Illustrating the ferocity and destructiveness of tornadoes, the storm that swept through the states of Missouri, Illinois, and Indiana on March 18, 1925, claimed 685 lives and caused an estimated property damage of 16.5 million dollars.
Notably, February 19, 1984, is recorded as the stormiest day in United States history, with a total of 60 tornadoes reported across the country, primarily in the southern states. This event resulted in about 10,000 deaths, extensive destruction of buildings, and significant loss of life and property, as documented by R. de C. Ward.
What distinguishes a Tornado Watch from a Tornado Warning?
Tornado Watch: Be Prepared!
Tornadoes are a possibility in and around the designated watch area. It is advised to review and discuss emergency plans, check supplies and the safe room, and be prepared to act swiftly if a warning is issued or if there’s suspicion of an approaching tornado. Taking early action significantly contributes to saving lives. The Storm Prediction Center issues watches for counties where tornadoes may occur, covering a broad area that often includes numerous counties or even states.
Tornado Warning: Take Action!
This warning indicates that a tornado has either been sighted or detected by weather radar, posing imminent danger to life and property. Individuals are urged to move to an interior room on the lowest floor of a sturdy building, avoiding windows. If in a mobile home, vehicle, or outdoors, seeking shelter in the closest substantial structure is crucial to protecting oneself from flying debris. Warnings are issued by local forecast offices and typically encompass a much smaller area, approximately the size of a city or a small county. These warnings are triggered when a forecaster on radar or a trained spotter/law enforcement identifies a tornado threat in the specified region.
