The clear blue skies have given way to a brief downpour. Your outdoor activity has become a victim of popcorn convection or popup thunderstorms. These are terms used to describe widely scattered showers or thunderstorms that show little organization and can be very difficult to forecast. Usually they form in the afternoon as a result of daytime heating. Sometimes called air mass thunderstorms, they tend to be small and short lived, with little movement. They very rarely are severe, but can produce heavy rain, downbursts and occasionally hail. Since they get most of their energy from heat of the day, they tend to fade quickly after sundown, and often bring temporary relief to the summer heat.
Mary Knapp, Weather Data Library