<P> In the open ocean, the thermocline is characterized by a negative sound speed gradient, making the thermocline important in submarine warfare because it can reflect active sonar and other acoustic signals . This stems from a discontinuity in the acoustic impedance of water created by the sudden change in density . </P> <P> When scuba diving, a thermocline where water drops in temperature by a few degrees Celsius quite suddenly can sometimes be observed between two bodies of water, for example where colder upwelling water runs into a surface layer of warmer water . It gives the water an appearance of wrinkled glass that is often used to obscure bathroom windows and is caused by the altered refractive index of the cold or warm water column . These same schlieren can be observed when hot air rises off the tarmac at airports or desert roads and is the cause of mirages . </P> <P> Thermoclines can also be observed in lakes . In colder climates, this leads to a phenomenon called stratification . During the summer, warm water, which is less dense, will sit on top of colder, denser, deeper water with a thermocline separating them . The warm layer is called the epilimnion and the cold layer is called the hypolimnion . Because the warm water is exposed to the sun during the day, a stable system exists and very little mixing of warm water and cold water occurs, particularly in calm weather . </P> <P> One result of this stability is that as the summer wears on, there is less and less oxygen below the thermocline as the water below the thermocline never circulates to the surface and organisms in the water deplete the available oxygen . As winter approaches, the temperature of the surface water will drop as nighttime cooling dominates heat transfer . A point is reached where the density of the cooling surface water becomes greater than the density of the deep water and overturning begins as the dense surface water moves down under the influence of gravity . This process is aided by wind or any other process (currents for example) that agitates the water . This effect also occurs in Arctic and Antarctic waters, bringing water to the surface which, although low in oxygen, is higher in nutrients than the original surface water . This enriching of surface nutrients may produce blooms of phytoplankton, making these areas productive . </P>

What is the term for the colder denser water below the thermocline in a fresh water lake