Forgot to mention, pyroclastic flows sometimes push a wave of heat in front of them. They're moving so fast 100km\\h - 700km\\h that they behave like a giant fan, pulling the heat from the cloud itself, and pushing it in front of it.
The higher the ash plume, the more danger. https://twitter.com/keawallace_/status/1380954114868326404

In the 1902 Martinique eruption only 1 person escaped in the direct path of the flow, and he was in an underground prison cell with poor ventilation, he peed on his clothes and covered the holes in the he barely survived, and was severely burned.

Pliny the Elder died, after the Mt. Vesuvius Pyroclastic Flow, which destroyed Herculaneum and Pompeii, travel across the literal Bay, pushing a wall of steam (the heat wall I mentioned = the ocean), in front of it, and reached the beach where he was trying to rescue his friend.