Why Are Your Feet Cold in a Warm Lake? A Boundary You Can Swim Through

Warm water is less dense than cold water, so heat absorbed at the surface stays near the surface, and wind mixes it downwards only to the depth its energy can reach. Below that the water remains at the temperature it had before, which in a deep lake or ocean is close to its coldest. Between the warm mixed layer above and the cold water below lies a zone where temperature drops steeply with depth, sometimes by several degrees in a metre, and that zone is the subject here. Mixing across it requires lifting dense water against gravity, which takes far more energy than mixing within either layer, so it persists.

Cutting off exchange has several consequences that follow directly:

• •Oxygen from the air cannot reach the water below
• •Decomposition on the bottom consumes what oxygen is there
• •Deep water can run out of oxygen entirely by late summer
• •Nutrients released on the bottom cannot reach the surface
• •Surface plants therefore run short of nutrients in summer
• •Fish are confined to the layer that has both oxygen and tolerable heat

In a temperate lake the whole arrangement collapses once a year, and the event is dramatic in its effects even though nothing is visible. As autumn cools the surface, the warm layer loses its buoyancy until the water column is nearly uniform in temperature, at which point a moderate wind can mix the entire lake from top to bottom. That single event carries oxygen down to the bottom and nutrients up to the surface, resetting the system for the following year. Lakes too deep or too sheltered to overturn fully retain permanently oxygen-free bottom water, which preserves anything that sinks into it remarkably well and is why some lake beds hold intact ancient material.

The ocean has a permanent version as well as a seasonal one, and distinguishing them matters. Across most of the world between roughly the tropics and the middle latitudes, a permanent layer sits at a depth of several hundred metres, separating warm surface water from the cold deep water that fills the great majority of the ocean volume, and it persists year round. Above it a shallower seasonal one forms each summer and disappears each winter, exactly as in a lake. Near the poles neither exists in the same form, since the surface is cold already, and there the density difference that matters comes from salinity rather than temperature.

Sound in water bends towards colder water because it travels more slowly there, which turns the layer into an acoustic feature with military consequences. A submarine sitting below it is substantially harder to detect from above, because sound from a surface ship bends away downwards at the boundary and creates a shadow region where little energy arrives, and submarines have used that deliberately since the Second World War. The same bending creates channels at certain depths in which sound travels enormous distances with little loss, and hydrophone networks exploiting one such channel can detect events across an entire ocean basin. Surface vessels counter by lowering a sonar transducer through the layer on a cable.

Water warmed at the surface floats on cold water below, and mixing across the sharp boundary between them requires lifting dense water, so it rarely happens. The lower layer is therefore cut off from the air and can lose its oxygen entirely by late summer. Autumn cooling lets a temperate lake overturn completely in one event. Sound bends at the boundary, which hides submarines below it.