Ambient Pressure

The total pressure of the water and atmosphere pressing on a diver at any depth, increasing by about 1 bar for every 10 m of seawater.

Ambient pressure is the pressure of everything around you at your current depth: the weight of the atmosphere plus the weight of the water column overhead. At the surface it is 1 bar, and every 10 m of seawater adds roughly another bar, so a diver at 20 m sits under about 3 bar of ambient pressure. Nearly every physical effect a diver feels, from ear squeeze on descent to expanding bubbles on ascent, traces back to this number changing.

The steepest rate of change is near the surface: descending from 0 to 10 m doubles the ambient pressure, while going from 30 to 40 m raises it by only 25 percent. That is why equalization matters most in the first few meters and why shallow ascents demand the same discipline as deep ones. Your regulator second stage automatically delivers gas at ambient pressure, which is what makes breathing feel normal at depth.

Ambient pressure also sets the partial pressures of the gases you breathe, driving nitrogen uptake, narcosis, and oxygen exposure. Dive computers measure it directly and convert it to the depth reading on your wrist.