clouds can grow vertically through the atmosphere very quickly. Sometimes a layer forms in the lower atmosphere through which the temperature increases with height, and this tends to slow down the upward growth of any cloud, including the cloud from an atomic bomb. Such a layer is called an "inversion" and shallow inversions of variable nature are found in the lower layers of the atmosphere. However, if we go sufficiently high (35,000-40,000 feet in U.K., 50,000-55,000 feet at the Equator) we find an inversion which is permanent and sufficiently pronounced that no ordinary cloud can penetrate it. This inversion therefore forms an upper limit to our weather, rain 'clouds, thunderstorms and so on, and is called the "Tropopause". The layer beneath it, down to earth's surface, is called the "Troposphere". Above the tropopause is the "Stratosphere" in which the temperature is almost constant.
      The mushroom cloud for a small atomic bomb would probably not reach the tropopause. The cloud from a medium size bomb would almost certainly reach the tropopause but would be unable to penetrate it, and thus would spread out beneath the tropopause like the head of a very flat mushroom. With a really large bomb, the cloud would penetrate the tropopause and ascend into the stratosphere, finally coming to rest at some great height, about 100,000 feet. It would probably look like the figure below: