In the central valley of California, raisins are dried in the fall. In September, Thompson seedless grapes are harvested and laid on paper trays, which are essentially sheets of brown paper placed directly on the ground in the vineyards, and allowed to dry. As long as it does not rain, everything is fine; the grapes will dry into raisins, and then they will be ready to be gathered up for cleaning and packaging. But a single shower can ruin a whole years effort in an afternoon..