Gravity Planes and Levels

The reason Air Envelopes follow their sources is because of gravity. Bodies weighing less than 1 ton only generate enough gravity to pull and maintain their air pockets. Bodies weighing greater than 1 ton begin to generate a Gravity Plane -- a plane through their center of gravity that is parallel to their two longest perpendicular cross-sections that extends to the edges of their Air Envelope. Uniquely this does mean that a body falling off the side of a spelljammer will fall through the Gravity Plane and oscillate down and other down until stopped. Bodies weighing at least 37,500,000,000,000,000 (37.5 thousand-million-million) tons become spherical and begin to generate radial gravity perpendicular to their surface. Bodies outside the envelope do not experience any recognizable effect of gravity, but bodies within the bubble will accelerate toward the surface, or "fall".

For the sake of gameplay, we will simplify most cases into the following possibilities:

Zero Gravity. Creatures without flying speeds on a surface that does not weight enough to generate a Gravity Plane, or while freely floating in zero gravity, all melee attacks (except for piercing damage attacks) are made at disadvantage. Additionally, creatures without flying speeds can only move by collecting themselves on a surface and then high jumping away from that surface. A creature moving in this manner cannot change direction or stop until they reach a new surface; in the meantime, each turn the creature moves in a straight line a distance equal to their high jump height. There is no fall damage.

Low Gravity. On a surface with low gravity, all terrain is considered difficult, but all jump heights and distances are doubled, creatures can fall twice as far before suffering fall damage, and fall damage is halved. Ranged weapon attacks have their ranges doubled.

Standard Gravity. This is standard gravity. Creatures engage with their movements and fall damage normally.

Heavy Gravity. All terrain on a surface with heavy gravity is considered difficult and creatures cannot jump or fly. Fall damage is doubled and creatures can only fall half the normal distance before they suffer it. Ranged weapon attacks have their ranges halved and melee attacks are made with disadvantage. Bludgeoning and slashing damage is doubled.

Extreme Gravity. Extreme gravity is deadly. While on a surface with extreme gravity, your speed is equal to your Strength Score and you cannot jump or fly. Fall damage is triggered at a quarter the distance and multiplied by 4. Every turn a creature spends on a surface with extreme gravity grants them a point of exhaustion.

The Gravity Planes of two bodies remain distinct until the two bodies come into direct contact with each other, such as during a collision, at which point the gravity plane of the heavier body dominates and overwrites the other. When spelljamming ships are concerned the "effective weight" is used to decide which gravity plane dominates: the weight of the ship multiplied by the percent of Hull HP the ship has remaining. For bodies that generate radial Gravity Fields, the more massive object's gravity dominates as soon as their Air Envelopes interact. When the two bodies separate their individual Gravity Planes will reestablish themselves. When a spelljammer lands on the surface of a planet or docks with a heavier station, precautions will need to be taken for any ventral decks beneath the ship's Gravity Plane and their cargo.