Shadowtide

Shadowtides are, like tides on Earth, occurrences caused by the gravitational interaction of nearby, influential massive bodies with fluids on the surface of another body, in this case the hydrocarbon smog layer on the planet Impendia and the influence of the nearby moons Umud, Umdu, and Umdeed on its height.

Travelers and colonies on Impendia, especially open-air vessels and colonies, must maintain careful watch over shadowtides and accurately predict the height of the hydrocarbons in order to avoid the suffocating, toxic smog which is deadly for most air-breathing lifeforms. Modulates have evolved to be able to survive shadowtides for their typical durations on Impendia's strateaus, but longer exposure to hydrocarbons can weaken and send modulates into comas.