Thermocline

A sharp boundary between warmer water above and colder water below, often felt as a sudden temperature drop and seen as a shimmering layer.

A thermocline is a distinct boundary layer where water temperature drops abruptly with depth, separating a warm, sun-mixed surface layer from colder water underneath. Crossing one can feel like swimming through an open refrigerator door: a drop of 5 degrees C or more within a meter or two is common. The boundary often looks like shimmering, oily water because the density difference bends light.

Thermoclines matter for exposure protection and dive planning. A site that reads 26 degrees C at the surface may sit at 18 degrees C below 20 m, which changes whether a 3 mm wetsuit is enough or a 7 mm or drysuit is warranted. Cold also increases decompression stress and gas density effects, and many fish species stack up right at the boundary where nutrients concentrate.

Thermocline depth moves with season, tide, and wind, so it is worth noting in your dive log along with the temperatures above and below. Most dive computers record a temperature profile automatically, which makes the layer easy to spot when you review the dive. Sites near upwelling coasts can shift their thermocline dramatically within a single week, so recent reports beat seasonal averages.