Premise: Cavalry Sunday is the annual parade of the Combined Cavalry Old Comrades Association which takes place in Hyde Park each year. Retired members of the cavalry regiments march to a service and commemoration around the band stand in the SE corner of Hyde Park and the nearby memorial to the bombing which took place there. The old troopers typically wear bowler hats and march with furled umbrellas.
Hypothesis: It normally rains on Cavalry Sunday, which is why everyone brings umbrellas