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Zones
While there is no ground, there is an absolute up and down in Aeron and a center "plane." Along the plane the air temperature is a comfortable 20C to 25C. As one travels upward the temperature drops by about 1C per kilometer. It increases by the same rate as one travels downward. By taking the temperature one can determine one's "altitude."

Eventually the temperature rises or falls enough that life cannot exist. There are iceberg seas well above the habitable zone and molten worlds well below it.

The different temperatures drive an atmospheric circulation system that results in winds and occasional storms (some of which can be very severe). Clouds are common.

Day and Night
There is no sun, but there is a day-night cycle. The background light grows brighter and dimmer over a 24-hour period, going from pitch black (midnight) to bright (noon). At night one can actually see stars above and below. No one knows what these stars are or even where the sunlight comes from.

Gravity
In general there is no gravity in Aeron.  Objects float freely through the air.  However, large-enough objects do create a limited gravitational attraction.

A 1G gravity field is generated by every object vehicle-sized or larger.  The gravity is only experienced directly over the object (it does not extend either to the sides or below).  Gravity appears to be distinctly quantized.  The gravity field of a vehicle (one displacement ton or greater) extends only about 10 meters straight up.  The gravity field of a relatively small island (less than 100 kilometers in diameter) only extends about 100 meters above the "ground" while the gravity field of larger islands extend a kilometer above the ground.  The practical upshot is that vessels can moor to the edges of an island, and can fly over it if it stays above the limits of the field.

Aerodynes (fixed-wing aircraft, gliders, and rotary-wing aircraft) are often used to transport people and cargo from an island surface to waiting aerial vessels lingering outside the limits of the gravity field.

Collisions between islands and other islands and seas are not uncommon.  If the two are on the same plane it can be bad enough as hundreds of millions of tonnes of material bump into one another.  If the two meet on different planes and the one above enters the gravity field of another it can be disastrous.  Sometimes one will just "brush" the field of another and the result will be a rain of water or dirt onto the lower island.  However, if a large enough part of one island enters the field of another it will begin "falling" toward the latter with catastrophic results for anyone caught between.  The largest of the islands probably formed through such accretion.

Islands
Islands exist at all altitudes and come in sizes ranging from a few meters across to many thousands of kilometers in diameter.  Most are roughly disk-shaped, and because of the nature of gravity typically are only inhabited along their upper sides (one really can fall off the bottom of a world in Aeron).  Also, because of the nature of the "atmosphere," large islands in the middle of the habitable zone can have regions stretching into the frigid zones and down into the hot zones.  Advanced civilizations in Aeron often use this temperature differential to generate power.

Because of the nature of Aeron, islands are constantly moving (though some much more slowly than others).  Because of this accurate maps are not possible.