If the surface layers of a body of water are undisturbed by wind or tides, layers of water with different temperatures can lie on top of each other without mixing because differing densities. When the upper water layers warm in the summer months, they become separated from deep water by a transition zone known as a thermocline. In a thermocline, the temperature decreases rapidly with small increases in depth. This phenomenon linking temperature change with depth is called temperature stratification. Temperature stratification occurs in most bodies of water from small ponds to oceans, but thermoclines are primarily found in the larger bodies of water.