Observations of the March 19, 1982 ash eruption of Mount St. Helens, made by the National Weather Service (NWS, Portland, Oregon) on 5-cm radar, were used to estimate the volume of the ash cloud (2000¿500 km3), the concentration of ash (0.2-0.6 g m-3) and the total mass of ash erupted (3-10¿1011 g). The position of the cloud was also tracked by radar. Particle sizes in the ash cloud were estimated from settling velocities suggested by decrease in maximum ash cloud height with time as it moved away from the volcano. The March 19, 1982 ash blanket was samples and mapped. Ash fallout times and accumulation rates were reconstructed from ground observations. Grain size distributions for various samples were used to obtain particle concentration (0.2 gm-3), total ashfall mass (1-3¿1011 g), and radar reflectivity factor (4-5 mm6 m-3) for the ash cloud. Our preferred estimate for total ashfall mass (4¿1011 g) is that obtained from the product of the ash cloud volume determined by radar (2000¿500 km3) and the particle concentration inferred from ashfall data (0.2 g m-3). Previously published ashfall data for the May 18, 1980 Mount St. Helens eruption has been studied using our ashfall inversion technique to estimate 6-hour mean particle concentration (3 g m-3), the size distribution, total ashfall mass (5¿1014 g), and radar reflectivity factors (7-60 mm6 m-3) for the ash cloud. A somewhat higher value (9 g m-3) for particle concentration was estimated from radar observations for an ash cloud formed during the peak eruption rate at Mount St. Helens. The two independent estimates are consistent, given the many uncertainties of the problem. The reflectivity factors for very dense ash clouds (3-9 g m-3) for several orders of magnitude smaller than for severe weather considered routinely detectable by airborne weather radar and dangerous for aviation. Because volcanic ash clouds with particle concentrations of at least 0.2 g m-3 are produced in extremely small eruptions (in terms of total ashfall mass) of duration less than 1 minute, volcanic ash clouds must be considered an extremely serious hazard to in-flight aircraft, regardless of eruption magnitude. These factors should be considered in hazard evaluations for known volcanoes located near air routes. Radar observations and calculations can provide scientists monitoring eruptive activity with significant information for estimating duration of eruption, particle concentrations in ash clouds, total mass of solid material erupted, magma eruption rate, potential ashfall mass, ashfall locations and accumulation rates, and duration and amounts of ashfall. Detailed analysis of ashfall data and NWS radar observations of ash clouds from Mount St. Helens demonstrate that weather radar can yield such timely information during and following volcanic eruptions.