Per occurrence excess of loss

December 1999: Lothar crosses northern France on the 26th, Martin the center and southwest on the 27th and 28th, and counting one event or two decides how many retentions the cedant bears. A per occurrence excess of loss treaty aggregates every loss arising from a single cause before comparing the total to the retention. A windstorm damaging four thousand roofs is one occurrence, and the cedant bears the retention only once, however many policies are hit. The definition of an occurrence therefore becomes the heart of the contract, and is settled by the hours clause, which bounds the window within which losses attach to one cause. The problem it solves is accumulation: a portfolio perfectly balanced risk by risk can still be wrecked by a peril that strikes thousands of policies at once, and no per risk cover answers that, since no individual claim reaches the retention. This is the form used for catastrophe reinsurance, whose upper layers are priced by catastrophe model rather than by experience, the record being far too short for a two hundred year return period.