The "Cloud" That Buried Pompeii Was a Pyroclastic Flow—Not a Cloud

What engulfed Pompeii on August 24–25, 79 CE was not a cloud but a pyroclastic density current (PDC)—a ground-hugging, gravity-driven avalanche of superheated gas, ash, pumice, and rock fragments.

Physical Characteristics

• Temperature: 300–600°C (up to 1,000°C near vent).
• Speed: 100–700 km/h (60–430 mph).
• Density: Particle-rich base (concentrated flow) + dilute ash cloud surge (overriding).
• Extent at Pompeii: Deposit 4–6 m thick over ~60 ha. Flow reached ~15–20 km from vent; surge cloud traveled further.

Sequence (Plinian Eruption of Vesuvius)

1. Phase 1 (Hours 1–18): 30-km high Plinian column → pumice fall (lapilli) buries Pompeii ~2.5 m. Many flee.
2. Phase 2 (Early Aug 25): Column collapses → six PDC pulses. First surges (dilute, hot) asphyxiate/incinerate remaining residents (~1,150 bodies found). Later dense flows entomb structures.
3. Herculaneum (closer): Hit by hotter, denser PDCs first (400–500°C); instant vaporization of soft tissue.

Why "Cloud" Is Misleading

The overshooting ash cloud (co-ignimbrite plume) rose to stratosphere, spread across Mediterranean. But the killing agent was the ground-hugging flow. Victims died from thermal shock (instant rigor, pugilistic pose) and asphyxiation—not "ash suffocation" alone.

Scale

Total magma erupted: ~1.5–2.5 km³ (DRE). VEI 5. One of history's most documented eruptions (Pliny the Younger's letters).

Pompeii was not "covered by a cloud." It was swept away by a hurricane of fire and stone.