Now you can apply your knowledge of the thermal structure of bathtubs to real lakes. Think about a lake in a temperate region, with good contrast between summer warmth and winter cold, as in New England. Also, assume that the lake isn’t ridiculously shallow. Start with a hypothetical all-warm lake (but see the end of this section, after on full annual cycle).
Late fall: The surface waters are cool, with a uniform temperature profile and free vertical mixing (Figure 4-12A).
Early winter: The picture above holds until the surface water is about 4°C. Cooling past that point produces less dense water. Now stable stratification develops, because colder water remains at the surface. Vertical convection turns off (Figure 4-12B).
Middle winter: The lake reaches 0°C at the surface and becomes covered with ice. There’s gradual cooling downward by conduction. There’s no mixing, because of the stable stratification. The bottom water stays at nearly 4°C, unless the lake is shallow and/or the winter is very cold (Figure 4-12C).
Early Spring: The ice melts, the surface warms, and convective instability develops, because below 4°C the warmer water is more dense than the colder water (Figure 4- 12D).
Middle Spring: The surface water reaches 4°C, and the whole lake mixes, because 4°C water at the surface is denser than the water at any level and convection operates through the entire depth. This is called the spring overturning (Figure 4-12E).
Late Spring to Early Summer: The surface warm mixed layer lies above the thermocline. The cold water below the thermocline is gradually warmed by conduction (Figure 4-12F).
Early Fall: There’s cooling at the surface, and mixing downward to the level at which the temperature equals the surface temperature (Figure 4-12G).
Late Fall: The surface develops the same temperature as the bottom, now greater than 4°C by conduction. Mixing is complete. This is called the fall overturning (Figure 4-12H).