UncleEbenezer wrote:Similar principle to the warming of water if you have a shallow lake or river with a dark bottom. There's a swim-spot near here where two rivers converge, and quite a big temperature difference - perhaps five or six degrees - between the one with some shallow, wide rock pools upstream and the one without.
The water will absorb heat from the sun and air. The light is absorbed by water (red first, working to blue, last) and the energy converts to heat. Where the water is shallow, or the top layer is not mixing with deeper, cooler water, this can be very noticeable. In some cases you can experience a thermocline; there is a site I dive at where, if not wearing gloves, I can put my hand through the boundary and feel "hot" or "cold" water, depending on which direction I'm traveling.
The thermocline is visible, as the two layers of water, having different temperatures, also have different densities, which creates a refractive interface between the two. Submarines can use the refractive properties to hide behind*, but of course they themselves are blind to what's on the other side without putting a probe through...
In summer, several people drown each year jumping into warm water and piercing the thermocline, entering into cold water quite unexpectedly, causing them to take a sharp, involuntary intake of breath, whilst still under water. You might say it made them go oooh
VRD
*but probably not anything to worry about at the swim-spot!