Advanced 8 minutes
Altitude diving: when the surface is no longer one atmosphere
What changes when you dive in a mountain lake, and why the trip home matters too
Every table and every dive computer starts from a silent assumption: that when you leave the water, you'll surface into one atmosphere of pressure. In a lake at two thousand meters, that assumption is false, and the whole calculation shifts.
In this article you'll see why altitude changes decompression, what equivalent depth is and how dive computers handle it, what else changes in a mountain lake (buoyancy, cold, visibility, logistics) and why altitude also matters after a sea-level dive.
It's a situation many divers run into without looking for it: an inland lake on a trip, a mountain pass on the way to the airport, a flight the day after your last dive. This article is educational material and doesn't replace a specific altitude diving course.
Why decompression changes
What governs decompression isn't depth in meters but the ratio between the pressure you were under and the pressure you're surfacing to. At sea level, that surfacing pressure is one atmosphere. At altitude it's lower: at two thousand meters it's around 80% of sea-level pressure, and it keeps dropping with elevation.
The consequence is twofold, and both parts point the same way. First, lower surface pressure means the difference between the gas dissolved in your tissues and the ambient pressure at the end of the ascent is greater: in other words, the supersaturation described in the decompression sickness article is more pronounced. Second, a lake's water column exerts pressure starting from a lower baseline, so every meter of depth represents a larger relative change.
The practical result is that a thirty-meter dive in a mountain lake is, decompression-wise, considerably more demanding than a thirty-meter dive at sea. No-decompression limits shorten, and stops are made at different depths.
The classic way to handle this is altitude tables and the concept of equivalent depth: you convert the lake's actual depth into the sea-level depth that would produce the same pressure ratio, and plan using that figure. Stop depths are also corrected, coming out shallower than they would at sea.
Today most dive computers make this adjustment automatically, but it's worth knowing how: some detect atmospheric pressure and adapt on their own, while others require you to select the altitude range manually. Assuming the computer has already figured it out is a mistake worth checking before you get in the water.
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