The planet Cinder is less geologically stable than most Class M worlds. Earthquakes are the source of nearly as many of the many ruins that dot the landscape as all other causes combined. Viewed from space many sections of the planet glow, as lava runs in rivers or pools into seas. The Midrealms of Cinder, with its meager scattering of volcanoes, enjoy a relatively lava-free existence. Life in the Midrealms seems positively pastoral compared to that lived by the poor, pitiful bastards in Volcanistan, who are harshly oppressed by both the terrible environment and the cruel Lava Lords.
But the Midrealms are still a dangerous place. Those smoking mountains do erupt from time to time. And the Smoldering Plains are dangerous to cross without a guide. Vents in the earth occasionally release clouds of poisonous gas. An old traveler's tip says run perpendicular to the wind when a hazy cloud is blowing in, but that doesn't work with Poison Wraiths. And only crazy people dare enter the Fire Swamp.
Most of the above listed hazards can be avoided by simply staying the hell away from the most active regions of the Midrealms. Hey, if you build your house on the slope of Mount Burningdeath, that's pretty much your own dang problem. But the Glooms can affect all the peaceful inhabitants of the Midrealms. No one understands the underground forces that lead to the Glooms. All that is known is that a couple times a year the volcanoes and the ground level vents of the region simultaneously release a large quantity of smoke and ash into the atmosphere. Usually this ejecta hovers in the sky for three or four days before blowing away or coming down to earth as "black snow". During this time the sun is dimmed or even blotted out completely and monsters roam the lands in great numbers. Orcs basically consider a Gloom to be a loot-and-pillage holiday.
Those are the "good" Glooms. Bad ones can last for weeks instead of days. A long Gloom in the spring can kill an entire crop. And three generations later people still speak in whispered tones about the Year With No Sun. In some ways the ground Glooms are worse. The ash and smoke stay low near the ground, reducing visibility to near zero and filling the lungs with slow poison. Bad times. Whole towns can die during a ground Gloom.