A large volcanic eruption cools the planet, though not by the mechanism most people assume. Ash falls out of the atmosphere within weeks and does little. What matters is sulphur dioxide injected above the weather, where it forms droplets that reflect sunlight and persist for a year or two. This is why the height of an eruption column and its sulphur content predict the climate effect far better than the volume of material erupted: Pinatubo in 1991 cooled the surface by a few tenths of a degree, while larger eruptions with less sulphur have passed without a measurable signal. The distinction matters for anyone reconstructing past climate from geological records of eruption size.