4.1.10 (HL)—Water temperature and density

In a commonly stratified water body, warmer, less-dense water lies above colder, denser water and the density contrast restricts mixing. Freshwater adds an important exception because it is densest near 4°C.

Ice is less dense and floats, slowing heat loss. This creates a surface-down freeze and leaves liquid water below for life.

A lake surface cools from 8°C to 4°C and sinks; cooling to 1°C afterward leaves that water nearer the surface rather than driving it below 4°C water.

Ice floats and insulates; maximum density occurs at 4°C, not 0°C.

Do not apply seawater density behaviour to freshwater without checking salinity.