The Rising Rain

The toxic seas are not still. Most surface activity is them rising and falling, small bubbling and roiling effects, and the occasional dramatic upswell. Every so often though millions of tiny pockets of fog ranging in size from a small marble to a human fist come up from the depths all at once. This is the rising rain.

The Behavior
At the bottom of fog strata boundaries there are often huge thermal currents. These currents warm the fog unevenly, and at cross currents they can cause isolated pockets of warm fog trapped just beneath the strata boundary. These pockets then rise, eventually breaking through in tiny streams that move towards the surface. As these streams rise they expand and are separated by denser air falling into them, until they finally form droplets.

Surface Rain
Some of this rain will actually reach the surface above the fog. For the last few hundred feet of rising larger droplets tend to expand very rapidly, reaching the surface as jets that then fall in cloud form. For individual islands this is usually a once a century occurrence, although it does vary by location. From above this looks like rising raindrops that suddenly vanish.

Foglift
It is more common for the rains to disperse entirely in the top strata of the fog. The sudden expansion of gas lifts the fog slightly, causing a faint swelling on the surface that then spreads out as waves. Most of the time these waves are tiny by the time they reach shore, but every so often the swelling is big enough or close enough to cause a high wave of fog. These are known to decimate low lying villages and completely destroy hidden fog cities.

Rainrise
The first phase of the rising rains (and usually the only phase) is the rainrise itself, where looking into the fog makes it appear that it is raining there, but where the surface is unaffected. It's almost like watching rain fall onto glass above you, but the entire process is going down to up and the glass isn't a clean surface but a thick layer where the rains stop.

Deep Rains
Only some of the rains are known about by surface dwellers. The rising rain can also happen between deeper strata far from the surface or even the upper fog, with the rain thicker and hotter but otherwise much the same. It's possible that some few sages or travelers have discovered this, but it's not common knowledge.

Original Posts
There is a phenomenon known as the rising rains. Deep strata fog sometimes coalesces into pockets of higher temperature and lower density gas, and eventually these gases start rapidly rising upwards, eventually dissipating higher in the fog. On the surface looking down they look like rising rain. It is not uncommon for this dissipation to be close enough to the surface to cause a high wave of fog that decimates low lying villages; the effect on the hidden fog cities is even worse. Cycles like this happen between multiple strata, but it is only the first fog sea to thickening fog rainrise that gets noticed, and even then only where it rises out of the true fog.