Illustrated Physics
physics, drawn
Concept

Salinity — where it appears

The mass of dissolved salts in a kilogram of sea water, about 35 grams in the open ocean. With temperature it sets water's density, and it lowers both the freezing point and the temperature of maximum density, which is why fresh water and the sea freeze in different ways.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Where the level goes when a floating ice cube melts. The change in level when a 30 g ice cube floating in a glass 7 cm across has melted completely, against the density of the liquid it floats in (the liquid must be denser than ice, 917 kg/m³, or the cube sinks). In fresh water the level does not move: the meltwater exactly fills the volume the ice displaced. In seawater it rises 0.21 mm and in saturated brine 1.3 mm, because the ice was displacing a denser liquid than it melts into; in castor oil, which is lighter than water and does not mix with it, the level falls 0.32 mm as the meltwater sinks beneath the oil. The change is the meltwater's volume minus the displaced volume, m/ρw − m/ρ, spread over the glass.

The floating ice that still raises the sea

An ice cube melting in a glass of water leaves the level exactly where it was, and the reason is exact: a floating body displaces its own weight, and the meltwater has that weight. The same argument is often used to say that melting sea ice and ice shelves cannot raise the sea. It nearly works. The ice was floating in salt water and melts into fresh, which is lighter, so the meltwater overfills the hole by almost three per cent — and the ice that really fills the sea is the part of a grounded glacier above the thickness at which it would float.

fluids · Buoyancy The whole lake has to be taken to four degrees first. Temperature against depth in a still lake 30 m deep, losing 100 W/m² at the surface from a summer profile, computed layer by layer with any denser water above lighter water mixed down. The warm layer cools and deepens, then the whole column reaches 3.98 °C after 85 days. Only then does the next loss make the surface lighter rather than heavier, so it stays on top, and it runs to freezing in 5 more days. The bottom keeps its 4.1 °C.

The lake that has to cool all the way down first

Water is densest at 3.98 °C, a difference of about one part in eight thousand from its density at freezing. Because of that, the top of a lake cannot go below four degrees until the whole lake has reached four, however deep it is. A cold wind can have the surface at the edge of freezing for months with nothing to show for it. The sea has no such rule, for a reason that comes down to where two curves cross.

fluids · Stratification

Named alongside it

The objects these essays reach for when they reach for this one.

BuoyancyArchimedes principleConvectionDensityFlotationIce shelfLatent heatSea levelStratification

All concepts