Physical OceanographyDensity

Density• Measured in mass/volume (g/mL)• Is the major factor in water

circulation and distribution of plankton

• Controlled by temperature and salinity

• Measured with a hydrometer

Why Does Ice Float?• Water molecules push apart during

freezing, increasing the volume.

• Since the mass is unchanged, but the volume is larger the ice has a lower density than the surrounding water and it floats.

• Ex) water: 1.00 g/1.00 mL = 1.00 g/mL densityice: 1.00 g/1.10 mL = 0.90 g/mL density

Liquid Water Frozen Water

• If sea water has a salinity of 35 ‰, then it will freeze at -1.94 °C.

• If ice freezes rapidly, salt pockets are trapped in it – this makes it very brittle and is called ‘first-year ice.’

• Salt pockets migrate downward and out, making ice ‘fresher’ as it gets older; this also makes the ice much stronger.

Generalities about Density

1. Normal salt water has a density between 1.01 and 1.03 g/mL

2. Density increases with depth3. Density increases as temperature

decreases4. Density increases as salinity

increases

5. Water is normally stratified (in layers) based on density

Q: Is it possible to have a layer of warm ocean water below a layer of cold ocean water?

A: Yes. If the warm water has a greater salinity than the cold water, this makes it more dense. Also, if two water masses have the same density but have different temperatures, then one must have a higher salinity than the other.