From AMS Glossary
A device used to obtain data on the size distribution and mass of hailstones.
A hailpad usually consists of a plastic foam panel covered by aluminum foil or white latex paint and set in a frame that is hammered into the ground. Hail that impinges on the pad leaves dents in it. The dimensions of the dents are analyzed to obtain the hailstone size and mass data.
Long, A. B., et al. 1980. The hailpad: materials, data reduction and calibration. J. Appl. Meteor.. 19. 1300– 1313.