Lower a thermometer from a ship in the tropics and the reading does not fall smoothly. It follows three stages:

1. The mixed layer. Wind and waves stir the top tens of metres, so temperature and salinity are almost uniform. It is typically about 20 to 100 m thick, thicker in stormy places and in winter.
2. The thermocline. Below the mixed layer the temperature drops quickly with depth. In the tropics and mid-latitudes it runs from roughly 100 to 200 m down to about 1,000 m.
3. The deep ocean. Below the thermocline the water is uniformly cold, mostly between about 1 and 4 °C, all the way to the bottom.