Altitude dive

A dive in a lake or reservoir above 300 m elevation, where lower surface pressure requires adjusted tables, computer settings, and gauges.

An altitude dive is any dive conducted at an elevation above 300 m (1,000 ft), typically in mountain lakes, reservoirs, and high-altitude sites like Lake Titicaca at 3,800 m or alpine lakes in the Rockies and Alps. The water is the same, but the air above it is not: lower atmospheric pressure at the surface changes the pressure ratios that decompression models are built on.

Because you surface into thinner air, dissolved nitrogen leaves solution more readily, so an identical depth and time carries more decompression stress than at sea level. Traditional dive tables handle this with theoretical depth conversions that treat a dive as deeper than it measures, while modern dive computers adjust automatically or via an altitude setting; verify yours before splashing. Depth gauges calibrated for salt water also read differently in fresh mountain water, and ascent rates should be conservative.

The Altitude specialty covers the adjustments, and there is a related rule in reverse: driving over a mountain pass after diving is a form of ascent, so treat post-dive travel to altitude like flying after diving and allow a proper surface interval. Log the site elevation, it changes the meaning of the whole profile. High lakes are usually cold as well, so plan exposure protection accordingly.