Turnover, and why autumn smells like that

Notes · no. 1

Through summer a deep lake sits in layers: warm and light on top, cold and dense below, with a sharp gradient between them that resists mixing. The lower layer is cut off from the air, and whatever is decomposing down there uses up the oxygen without replacing it.

When the surface cools in autumn, the density difference closes and the whole column mixes. The bottom water comes up. That is the smell — it is not the lake going wrong, it is the lake doing the one thing it does on a schedule.

Shallow water does this constantly and unremarkably. A deep basin saves it all up for a fortnight and announces it.