Atmosphere (ATA)

A pressure unit equal to air pressure at sea level; ATA counts total absolute pressure, reaching 2 ATA at 10 m and 4 ATA at 30 m in seawater.

An atmosphere is the pressure of the air over your head at sea level, about 1.013 bar or 14.7 psi. Divers use ATA, atmospheres absolute, to describe total pressure including that surface atmosphere: 1 ATA at the surface, 2 ATA at 10 m of seawater, 3 ATA at 20 m, and 4 ATA at 30 m. The pattern is simple, one extra atmosphere per 10 m of salt water, which makes ATA the friendliest unit for mental dive math.

The absolute in ATA is the important part. A pressure gauge reading at depth might show only the water pressure, called gauge pressure, but physiology and gas physics respond to the total. Working in ATA makes the classic laws practical: multiply the fraction of a gas by depth in ATA and you have its partial pressure, so air at 4 ATA delivers nitrogen at about 3.16 ATA, deep into narcosis territory.

In fresh water the spacing stretches slightly, roughly one atmosphere per 10.3 m, a detail that matters for altitude diving and precise calculations. Most dive tables and computer math quietly run on ATA under the hood.