The summer of 1783 is the hottest on record and a rare high-pressure zone over Iceland causes the winds to blow to the south-east.

The poisonous cloud drifts to Bergen in Denmark–Norway, then spreads to Prague in the Kingdom of Bohemia (now the Czech Republic) by June 17, Berlin by June 18, Paris by June 29, Le Havre by June 22, and Great Britain by June 23.

The fog is so thick that boats stay in port, unable to navigate, and the sun is described as blood colored.