In lakes, mixing occurs for short periods of time in the spring and fall. During the summer, increased solar radiation and resulting higher air temperatures warm the surface layer. Wind stirs this upper layer and a “rolling barrel” effect is created with the layer below the thermocline. The thermocline ensures that a minimal amount of mixing occurs between the lower barrel and the upper barrel. Solar radiation levels and air temperatures decline in the fall, but water temperatures do not begin to decline until late fall. (Remember the lag time effect described above.) Thus, in late fall, the surface water temperature decreases and the temperatures of the two barrels approach each other. As this happens, the thermocline breaks down. This allows mixing of the upper and lower water systems.