Twisters: Formation, Lifecycle & Distribution

• Context (HT): A massive EF3 twister struck the Clayton area of St. Louis, USA.

About Twister

• Commonly known as a tornado, it is a rapidly rotating column of air that extends from a cumulonimbus cloud (usually in a thunderstorm) to the ground.
• Forms a funnel-shaped cloud and is often visible due to condensation, dust, and debris.
• With wind speeds exceeding 480 km/h, these are among earth’s most destructive weather phenomena.
• Types of (Twisters) Tornadoes:
  • Supercell Tornadoes: Most intense; develop from rotating thunderstorms.
  • Landspouts: Form from surface-level convergence; weaker.
  • Waterspouts: Tornadoes over water bodies; common in tropical regions.
  • Non-supercell Tornadoes: Associated with squall lines or sea breezes.
• Intensity Measurement (Enhanced Fujita Scale): Classifies by estimated wind speeds and damage:
  • EF0: Light damage, winds 105–137 km/h
  • EF1 to EF2: Moderate to considerable damage, winds 138–217 km/h
  • EF3: Severe damage, winds 218–266 km/h
  • EF4 to EF5: Devastating to incredible damage, winds 267 km/h and above.

Formation of Twister (Tornado)

• Requires a precise combination of thermodynamic instability and atmospheric dynamics:
  • Warm, moist air rises from low levels (oceans or plains).
  • Cool, dry air from higher altitudes (mountains or polar) overlays it, causing atmospheric instability.
• Wind shear (change in wind speed/direction with height) creates horizontal spinning air. Strong thunderstorm updrafts tilt this horizontal spin vertically, forming a rotating storm core called a mesocyclone.
• The rear-flank downdraft (descending cool air) tightens the rotation, focusing it towards the ground.
• When the rotating column extends from the cloud base to the surface, a tornado forms.

Lifecycle of a Tornado

• Development: Rotation begins within the storm (mesocyclone stage).
• Maturity: Funnel cloud touches the ground; tornado reaches peak strength and damage potential.
• Dissipation: Updrafts weaken, rotation slows, and the tornado fades.

Geographic Distribution

• United States: The highest tornado frequency is mainly in “Tornado Alley” (Texas, Oklahoma, Kansas, Nebraska) due to the clash of warm, moist Gulf air with cool, dry Rocky Mountain air.
• Other tornado-prone regions: Bangladesh (site of the deadliest tornado in 1989 with 1,300+ deaths), Canada, Argentina, and Australia.
• India: Rare but have caused fatalities and damage in Bihar (2021) and West Bengal (2020).
  • Indo-Gangetic Plains & parts of Northeast India are more vulnerable due to local atmospheric factors.

Also Read > Tornado.