Stem Island's worldly forces are eerily similar to the outer universe's planetoid atmospheres. There are some key differences, however: there is no sun, there are no consistent weather patterns, and there are no seasons.
The sky is pitch black. The ethereal natural lighting in the world fades in and out in intensity. As off-worlders brought in the concept of a clock, it was confirmed to the masses that Stem Island's light goes on a predictable 24 hour cycle. Unlike the moving suns of the outer universe's skies, Stem City's only light source is always coming from directly up.
The temperature on the island has the edges being the hottest and most humid, while the center gets unexplainable chills.
Weather on Stem Island is completely unpredictable. It can last for mere hours or endless weeks. Constricting fog and heavy rainstorms are the only known types of weather. While the various strengths of fog are harmless, the heavy rain usually damages any buildings not unprotected underneath the constant shade of The Grand Shroom's cap or Fungal Waste's forestry.
Years in Stem City are marked by a strange phenomenon. Every 91 days, one of four strange collections of star-like lights hovers high up in the sky for 24 hours before fading away. These signify to the Stem Island people when "one quarter" has passed. A year, or 364 days, is when the last of the four unique light clusters reveals itself.