Experiments in several locations with seeding of continental convective clouds with micrometer-size hygroscopic aerosol suggest precipitation enhancement is possible by this technique. Initial numerical simulations of such seeding for the most part supported this concept. However, very recent numerical simulations for a wider variety of atmospheric vertical buoyancy and wind profiles have shown that in some situations hygroscopic seeding apparently may produce precipitation enhancements, and in others decreases. A review of past cloud condensation nuclei (CCN) observations and new observations obtained in 2004 shows that considerable variability in CCN characteristics can occur at continental sites. It is concluded that guidance for hygroscopic seeding operations should include observations and forecasts of both vertical profiles of stability and winds, and CCN in cloud inflow.