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4.6: The Thermal Structure of Real Lakes

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  • 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).

    Fig. 4-10.png
    Figure 4-10. Time series of temperature profiles during cooling of your hot bath from the surface.
    Fig. 4-11.png
    Figure 4-11. Time series of temperature profiles during warming of your cold bath from the surface.

    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).

    Fig. 4-12.png

    Fig. 4-12-1.png
    Figure 4-12. Temperature profiles in a temperate-region lake.