Density
Water density (mass per unit volume; kg/m3) plays an important role in lake dynamics. The density of water is largely influenced by temperature and salinity. Pure water is at its densest (1000 kg/m3) at 4 °C (39 °F). Water expands (becomes less dense) as the water temperature is increased or decreased from 4 °C (See Figure 1). Increasing amounts of dissolved substances (increasing salinity, which is represented here as Specific Conductivity) causes the water to become denser (See figure 2).
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Figure 1. Water density as a function of temperature
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Water in the form of ice at 0 °C (32 °F) is at its minimum density (917 kg/m3). In contrast, liquid water at 0 °C has a density of 999.8 kg/m3. This density difference allows ice to float upon the water in which it forms. Similarly, water of lower density will float upon water of higher density. For example water at 25 °C will float upon water at a lower temperature. Salinity’s effect on density is important in some of our systems (primarily Onondaga Lake, Onondaga Creek and Seneca River downstream of the Onondaga Lake outlet). From Figure 2, the effect of salinity on density is quite profound were water at 15 °C and specific conductivity of 2200 µS/cm has same density as pure water at 4 °C (its maximum density).
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Figure 2. Water density as a function of Specific Conductivity at selected temperatures
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Discussion and near-real-time plots of density stratification
Discussion of density currents, including and near-real-time plots
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