Origin of Tropical Cyclones

One important point before we begin:
👉 Even today, scientists don’t fully agree on a single “universal” theory of how tropical cyclones originate. But we do understand the broad conditions necessary for their birth.

Earlier, some geographers supported the Frontal Theory – the idea that cyclones form due to frontogenesis (meeting of contrasting air masses). They argued that even in the tropics, land–sea breezes could act like fronts.
But this theory was later rejected because tropical cyclones are not front-based systems. They don’t involve warm vs cold air clashes like temperate cyclones.

Instead, modern understanding sees a tropical cyclone as a heat engine:

• Powered by the latent heat of condensation (when warm, moist air rises, condenses, and releases energy).
• Triggered by thermal low pressure over warm oceans.

For this “engine” to start, six conditions must be met:

✅ Six Requirements for Cyclone Formation

1. Warm Ocean Waters
   • Sea surface temperature must be at least 27°C.
   • This ensures continuous supply of warm, moist air (the cyclone’s fuel).
   • That’s why they form mainly in summer, over oceans.
2. Coriolis Force
   • Essential for giving the storm its cyclonic spin.
   • But Coriolis force is very weak near the equator (within 5°).
   • Hence, cyclones rarely form in the 0°–5° belt, but mostly between 5°–20° latitudes.
3. Inter-Tropical Convergence (ITC)
   • The ITC zone, which shifts northward in summer (up to 30°N), is a breeding ground.
   • It provides the rising, converging winds that help cyclones form.
4. Pre-existing Disturbance
   • A weak tropical disturbance (like easterly waves) often acts as a seed.
   • If conditions are favourable, this disturbance intensifies into a cyclone.
5. Upper Air Anticyclone
   • At about 9,000–15,000 m altitude, there should be anticyclonic circulation.
   • This creates a “suction effect,” pulling air upwards from the surface.
   • This accelerates rising air, further deepening the surface low pressure.
6. Small Atmospheric Vortices
   • Cyclones often form around tiny whirls or vortices inside the ITC zone.
   • These serve as the initial nucleus for cyclonic development.

👉 So, in simple terms: Warm ocean + Coriolis spin + ITC convergence + an existing disturbance + upper air support = Cyclone formation.

🌩️ Weather Conditions Associated with Tropical Cyclones

When a tropical cyclone approaches, the weather changes dramatically. Let’s imagine the sequence step by step:

1. Before the Cyclone Arrives

• Temperature rises suddenly.
• Pressure drops sharply.
• Winds pick up speed.
• High, thin cirrus / cirrostratus clouds appear.
• The sea becomes rough with huge waves.

2. Main Cyclone Phase

• Clouds thicken into cumulonimbus → heavy rains with thunder and lightning.
• Average rainfall: 100–250 mm, but if blocked by mountains, it can reach 1000 mm.
• Visibility = almost zero (sky is pitch dark).
• Conditions are extremely destructive, lasting a few hours.

3. The Eye of the Cyclone

• When the centre (eye) passes overhead:
  • Weather suddenly calms.
  • Sky clears, winds stop, rainfall ceases.
  • But pressure is still very low.
• This deceptive calm lasts only 15–30 minutes.

4. Rear Part of Cyclone

• Weather worsens again:
  • Winds reverse direction.
  • Pressure rises quickly.
  • Torrential rain with thunder returns.
• This second storm can last several hours.

5. After the Cyclone

• Gradually, winds slow down.
• Rainfall reduces.
• Sea surface becomes calm again.
• Clear skies return.

👉 This dramatic “calm → chaos → calm again” sequence is one of the most striking features of tropical cyclones.

✅ Summary for UPSC:

• Tropical cyclones are heat engines powered by latent heat from warm oceans.
• Require 27°C sea temp, Coriolis force, ITC, pre-existing disturbance, upper air anticyclone, and vortices.
• Weather = sudden rise in temperature, drop in pressure, violent winds, dark clouds, torrential rain, followed by calm eye, then second storm, finally gradual clearing