Condensation clouds

Condensation clouds seems like a strange name for an interesting phenomenon - after all, aren't all clouds formed by condensation? Well, it turns out that condensation clouds, also known as Wilson clouds, are actually a somewhat different phenomenon than your run-of-the-mill precipitation source.

They appear in shock wave situations, like super-sonic aircraft and large explosions, but only in particularly humid air (or in some underwater nuclear tests - vaporize enough water and you end up with really humid underwater bubbles). The obvious part of any shock wave is the high pressure at the 'positive phase' of the shock wave - this is what causes buildings to implode near a large explosion (the remnants are then blasted with winds of several hundred miles per hour in the shock's wake). But there's more to a shock wave than that. There's also a negative phase, in which the air is much lower-pressured. As the shock front expands, the positive and negative phases experience very little air flow with the rest of the world. As a result, the positive phase's pressure decreases as it expands, so its destructive power diminishes with distance. The negative phase of the shock wave similarly experiences a decrease in pressure as the shock radiates outward, and because air and heat are not being readily exchanged between the negative phase and the rest of the world, this manifests as a decrease in temperature. In sufficiently humid air, that decrease in temperature is enough to cause water vapor in the negative phase of the shock wave to condense. When large explosions are involved, the result is a roughly spherical-looking cloud around the explosion that has little or nothing to do with the debris and fireball inside.

Similar effects can be observed when airplanes fly really fast in humid conditions or do high-acceleration maneuvers - the curves of the wings and the body of the plane cause local areas of low and high pressure, and the low-pressure areas often experience condensation. That is why you often see a conical cloud behind planes in pictures of supersonic flight; the plane is moving pretty darn fast, so there's a lot of opportunity for low-pressure areas.

As for the eventual fate of the condensation cloud, when the extenuating circumstances that created it wear off, the water re-evaporates and the cloud seems to disappear. In some cases, parts of the condensation cloud can stabilize, which is apparently the source of those rings you sometimes see around the stems of mushroom clouds.