Water temperature normally decreases with depth, but in some locations you may notice a sudden and drastic change as you descend. This is called a thermocline, and is common in fresh water lakes and on occasion, the ocean.

During the summer, thermoclines result in warmer surface waters and colder deep waters. Lakes can experience a reverse thermocline during the winter when the upper layer is colder than the deeper layer.

When diving an area known to have a thermocline, it's important to wear the appropriate amount of thermal protection for the depth you plan to dive to. You can get information about local thermoclines from a local dive shop.