Sea surface temperature (SST) is the water temperature at the surface. In practical terms, the exact meaning of "surface" will vary according to the measurement method used. A satellite infra-red radiometer indirectly measures the temperature of a very thin layer (about 10 micrometres thick) or skin of the ocean (leading to the phrase skin temperature) representing the top millimeter; a thermometer attached to a moored or drifting buoy in the ocean would measure the temperature as a specific depth (e.g. the top 1 meter below the sea surface); the measurements routinely made from ships are often from the engine water intakes and may be at various depths in the upper 20 m of the ocean.