When the height of burst is lowered, as is the case with balloon shots and with many tower shots, these afterwinds are strong enough at the surface to entrain loose dust and carry it aloft. A visible stem is formed which trails the cloud and which is practically free from radio- activity. Eventually, however, the top of this stem will merge with the lower portions of the cloud. Most of the stem will rapidly fall back to the surface, because the entrained particles are too large to remain aloft for a long time. It follows, further, that the incorporation of radionuclides in balloon shots is quite similar to that in airburst particles, and that the same is true for many of the tower shots. In the latter case, however, portions of the tower are also incorporated in the particles as though the tower, or at least the upper part of it, were part of the device. At still lower heights of bursts (as in many tower shots) no trailing of the stem is observable, and more and more of the stem mixes with the debris cloud. Some of the stem particulates form a substrate to which still hot debris particles become attached, and onto which some of the more volatile species become attached. Thus, the debris cloud becomes more diluted with inert and slightly radioactive particles as the detonation point gets closer to the surface and the mean particle size increases. At some rather low height of burst, soil particulates reach the fireball before cooling is complete and some of the particulates become partially or totally fused. The effect is that the mean particle size increases further and that more particles incorporate radionuclides, either within their volume or on their surface. Fusion of the soil as a direct result of the interaction of the fireball with the surface does not occur until the height of burst is quite low. This is because as the result of shock wave reflection at the surface, the lower boundary of the fireball does not touch the surface until the detonation point is at a scaled height of perhaps 25 feet per (kiloton)!/3, At lower heights of bursts, the events may be considered surface shots. In surface shots, a large amount of soil debris enters the cooling fireball at an early time. Thus, much soil becomes completely fused, or partially fused. As a result, the radioactivity is distributed over a wider range of particle sizes, although particles resembling airburst particulates in size as well as shape are still found in abundance in the range below a few micrometers. In addition, fused ejecta particles are also found. These particles may be quite large, well over 1 mm, and often possess typical teardrop shapes. They may be presumed to have originated from the molten lining of the incipient crater. How these observations relate to the specific fallout characteristics, particularly size distribution and radioactivity behavior, will become clearer in the following sections. 553