The mention of tornadoes tends to conjure up all kinds of violent and scary images in mind. Tornadoes have caused major destruction in the past, however affecting mostly the continent of North America, particularly the United States. But because of their mysterious appearance and extreme destruction, tornadoes have always been a subject of discussion around the world.
In fact, not just tornadoes, weather phenomenon like cyclones, hurricanes, and typhoons all have known to cause much damage and news. However, cyclones are typically strong tropical storms that have also known to affect western coast of India. Typhoons and hurricanes are the same, only known by different names in different parts of the world. Hurricanes and typhoons form over water and are huge, while tornadoes form over land and are much smaller in size. Hurricanes occur in the Atlantic, and typhoons in the Pacific.
What is a tornado?
A tornado is a rapidly spinning vertical column of air that develops from a thunderstorms and forms above the sea level. It gradually moves down towards the ground. No other weather phenomenon matches the wildness and destructive power of tornadoes. Tornadoes are often strong enough to destroy large buildings, leaving only rubble in its place. They have the power and velocity to lift cars weighing tons of kgs in steel and hurl them meters away. At the same time they also have the precision to pick up straws of grass and twist them around tree branches. Around 800 tornadoes are reported each year, resulting in many deaths and injuries.
Tornadoes tend to occur at the event of a violent thunderstorm and caused by two main factors: 1) Instability in the atmosphere
2) The occurrence of wind shear
Instability in the atmosphere
Instability occurs in the atmosphere when extreme hot and humid conditions combine with cold and dry ones. It is a unique phenomenon and it's natural to wonder how it might happen. The unusually warm and humid conditions take place in the lower atmosphere, and the cooler than usual conditions arise in the upper atmosphere. In the United States, warm, wet winds travel north from the Gulf of Mexico in spring and summer, where they meet cold, dry, and south-moving Canadian fronts.
Warm air always rises up, but when the two conditions (warm and cold) meet, the cold air can trap the warm air beneath it. Because the warm air cannot move upward, it begins to rotate. As the sun heats the ground, more warm air continues to rise, until finally the mass is strong enough to push through the cold air barrier. The rising warm air pushes the cold air beneath it, creating a rotating column that can span up to 15km, while its twisting speed exceeds 350km an hour. The spinning air may remain unseen until it picks up enough dust and debris for its shape to be visible. Tornadoes can last for a minute or an hour, and they can cause immense damage within this time frame within the distance they stretch.
The occurrence of wind shear
Wind shear is also a combination of two opposing forces. Wind shear occurs when the wind direction changes and the wind speed increases with height. An example would be a southerly wind of 15 mph at the surface, changing to a southwesterly or westerly wind of 50 mph at 5,000 feet altitude. Tornadoes are produced when two differing air masses meet. When cooler polar air masses meet warm and moist tropical air masses, the potential for severe weather is created.
Since instability in the atmosphere as well as wind shear are necessary for tornado formation, sometimes weak tornadoes can occur when the wind shear conditions are strong, but the atmosphere is not very unstable. This is known to have happened sometimes in the US state of California in winter, when a strong low pressure system comes ashore.
Similarly, weak tornadoes can occur when the atmosphere is very unstable, but there is little wind shear. For instance, Florida (which reports more tornadoes than any other state in the US) has many weaker tornadoes of this type. Naturally, the most violent of tornadoes occur when both strong instability and strong wind shear are combined and present. This occurs in the middle part of the country mostly during the spring and to a lesser extent during fall.
The deadliest of tornadoes recorded till date is the 'Tri-State Tornado' that killed 695 people and injured 2,027. It traveled more than 300km, with winds circulating at the speed of over 400km/hr through Missouri, Illinois and Indiana on March 18, 1925. It was rated an F5 at the top of the Fujita scale (like Richter scale for earthquakes), the worst in history.
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