In the past, when summer nights grew cold, the lake released the day’s accumulated heat, causing surface water to become denser and sink. This phenomenon drives the shallow mixing that occurs in summer. As nights have warmed, however, this process has weakened, and mixing has slowed.

Counterintuitively, as the layer of surface water has become warmer, it has also become thinner. “In the summer, there is half as much warm water floating on the surface now, on average, than there was in 1971,” Girdner said. This creates a sharper density difference with the cold water below, which in turn increases the amount of wind energy required to break through and mix the layers.

“I think about it like a vinaigrette,” said Kevin Rose, a freshwater ecologist at Rensselaer Polytechnic Institute in New York who collaborates with Girdner and Chandra. “There’s resistance to mixing.”

So what does all of this have to do with the fact that the lake is getting clearer? That’s where biology comes in. In Crater Lake’s warm surface water lives a community of phytoplankton. A thinner warm surface layer means less habitat, so there are fewer phytoplankton, which means fewer particles in the water to scatter light. This boosts the water’s clarity overall and the depth to which light can penetrate.

Crater Lake’s winter processes, which mix the lake all the way to the bottom, are undergoing their own profound changes. These transformations involve the weakening of a phenomenon called reverse stratification, in which a layer of very cold water, cooled by frigid winter air, forms on top of a slightly warmer layer that is around 4 degrees Celsius, the temperature at which water is heaviest. (At temperatures below that, water molecules begin to organize into lighter ice crystals.) When strong wind pushes the extra-cold surface water horizontally, as it approaches the lake’s edge some of it is forced down. If it is pushed down far enough, the increased pressure causes it to become denser than the 4-degree water layer. It then sinks to the bottom in a matter of hours, creating a mixing effect.