Cloud
High clouds form at altitudes of in the polar regions, in the temperate regions and in the tropics. All cirriform clouds are classified as high and thus constitute a single genus "cirrus" (Ci). Stratocumuliform and stratiform clouds in the high altitude range carry the prefix "cirro-", yielding the respective genus names "cirrocumulus" (Cc) and "cirrostratus" (Cs). When limited-resolution satellite images of high clouds are analysed without supporting data from direct human observations, it becomes impossible to distinguish between individual forms or genus types, which are then collectively identified as "high-type" (or informally as "cirrus-type" even though not all high clouds are of the cirrus form or genus).Non-vertical clouds in the middle level are prefixed by "alto-", yielding the genus names "altocumulus" (Ac) for stratocumuliform types and "altostratus" (As) for stratiform types. These clouds can form as low as above surface at any latitude, but may be based as high as near the poles, at mid latitudes, and in the tropics. As with high clouds, the main genus types are easily identified by the human eye, but it is not possible to distinguish between them using satellite photography. Without the support of human observations, these clouds are usually collectively identified as "middle-type" on satellite images.
Low clouds are found from near surface up to . Genus types in this level either have no prefix or carry one that refers to a characteristic other than altitude. Clouds that form in the low level of the troposphere are generally of larger structure than those that form in the middle and high levels, so they can usually be identified by their forms and genus types using satellite photography alone.
Mushroom cloud
For ground and low-altitude bursts, the cloud contains also vaporized, melted and fused soil particles. The distribution of activity through the particles depends on their formation. Particles formed by vaporization-condensation have activity evenly distributed through volume as the air-burst particles. Larger molten particles have the fission products diffused through the outer layers, and fused and non-melted particles that were not heated sufficiently but came in contact with the vaporized material or scavenged droplets before their solidification have a relatively thin layer of high activity material deposited on their surface. The composition of such particles depends on the character of the soil, usually a glass-like material formed from silicate minerals. The particle sizes do not depend on the yield but instead on the soil character, as they are based on individual grains of the soil or their clusters. Two types of particles are present, spherical, formed by complete vaporization-condensation or at least melting of the soil, with activity distributed evenly through the volume (or with a 10–30% volume of inactive core for larger particles between 0.5–2 mm), and irregular-shaped particles formed at the edges of the fireball by fusion of soil particles, with activity deposited in a thin surface layer. The amount of large irregular particles is insignificant. Particles formed from detonations above, or in, the ocean, will contain short-lived radioactive sodium isotopes, and salts from the sea water. Molten silica is a very good solvent for metal oxides and scavenges small particles easily; explosions above silica-containing soils will produce particles with isotopes mixed through their volume. In contrast, coral debris, based on calcium carbonate, tends to adsorb radioactive particles on its surface.