Air envelope

An air envelope was the ellipsoidal bubble of air that surrounded a spelljammer or another large object in wildspace and permitted those within that object's gravity well to breathe.

Technically, an air envelope was "attached" to every body that had gravity. Since everything had some level of gravity, even a human-sized creature leaving the atmosphere of a world would carry with it a small amount of air, but the level of breathable air would only last on average less than 100 seconds, and then the creature would suffocate in the vacuum of wildspace.

In general, an air envelope tended to extend from the surface of an object by about that object's thickness, so a perfectly spherical object with a diameter of ten feet would have an envelope extending all around it ten feet from its surface. In other words, the air envelope encompassing the sphere would have a total diameter of 30 feet. Non-spherical objects had ellipsoidal envelopes.

Arguing against this rule for the size of air envelopes is the fact that many exceptions existed among the celestial bodies of a crystal sphere. Some large worlds had no atmospheres at all, while some tiny worlds had thick atmospheres. Some worlds had air envelopes but filled with poisonous air. Crystal shells themselves had no gravity and thus no air of their own.

Large vessels, such as spelljammers, had strong enough gravity planes and enough size and mass to carry air envelopes large enough to sustain air-breathing life for extended periods of time, typically on the order of months. In general, each ton of mass of a spelljammer occupied a volume of 100 cubic yards, the combination of which resulted in an air envelope large enough for a human crew member to breathe for four to eight months. Because the large distances between worlds and crystal spheres necessitated vast amounts of air, spelljammers were rarely built with keel lengths of less than 100 feet.

As the inhabitants within an air envelope breathed in fresh air, they exhaled carbon dioxide, which fouled the air. As the air within the envelope became fouled, it became harder for creatures to carry out tasks. When an air envelope was fully depleted, the air was literally deadly to most air-breathing life. (Undead and creatures such as golems had no problems functioning in fouled or deadly air, of course.) To refresh a ship's air, it simply needed to overlap its air envelope with another, and the atmospheres were exchanged to an equilibrium. Practically speaking, this meant that a spelljammer could enter the atmosphere of a world and leave again with fresh air.

A bizarre magic of the phlogiston turned the bodies of creatures within air envelopes of deadly air into stone. They could survive in suspended animation until their preserved bodies came into contact again with fresh air. Their flesh would soften and they would be revived, no matter how much time had passed.