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When liquids are heated they expand. Knowing that water expands when heated helps scientists to model the movement of ocean currents around the world.
Click here for a link to CSIRO remote sensing images of water temperatures around Australia
This same principle is used in the every day mercury thermometer. Mercury is used within thermometers as it is liquid at -39oC and expands rapidly when heated, as well as that it's convex meniscus allows for accurate temperature readings.
Reference: Brady, J. E. & Holum, J. R. (1993). Chemistry. The Study of Matter and Its Changes.
John Wiley & Sons, New York