A Day That Should Never Have Happened - volatility, the bell curve and fat tails

The board says nobody knows tomorrow’s weather, only its climate, how wild it usually is, and calls that wildness volatility. This lesson measures the climate, and then meets a day it said could not come.

Before you start. From A Drunk Walk Through Paris - the random walk and the efficient market you need the random walk: a price that moves each day by chance, with no memory, whose spread grows with the square root of time. From Compound Interest, the 8th Wonder you need only that a return is a percentage change, and that a big loss costs more to win back than its size suggests (that lesson calls it variance drag); this one links it rather than retelling it.