Vertical profiles of radar reflectivity have been measured in the cores of New England thunderstorms and correlated with surface weather conditions reported by an extensive network of cooperating observers. Although the reflectivity differences between hailstorms and rain-thunderstorms are slight at low altitudes, they are significant in the middle and upper portions of the storms. Tornado-producing thunderstorms reveal even more striking anomalies in high-altitude reflectivity. The maximum reflectivity of the profile, the height of the maximum, and the reflectivity ratio (maximum aloft to surface) all increase with the severity of thunderstorm weather. The experience of 233 profiles measured during two years is the basis for an estimate of hail and tornado probability as a function of reflectivity at an altitude of 30,000 ft and the profile shape parameters. These indices provided tornado warning times of one to nearly three hours in two multi-tornado squall lines. The extremely high values of reflectivity observed aloft in a few thunderstorms are interpreted as indications of large hail, having diameters in excess of the radar wavelength of 3.2 cm and in concentrations of 10 g per m3 or more, occurring in the very small core region and in a limited height range about 20,000 ft. The persistence of large reflectivity ratios in unusually severe thunderstorms provides evidence for the storage of hailstones near the altitude of updraft maximum in a convection cell, followed by a gradual release of particles as a new cell becomes active and begins to grow and store a new supply of hailstones.