I just commented on my facebook status that I'm at a meeting about sea surface temperature. That part was safe. Rest of the comment was to observe that I'm now back to wondering whether the sea has a surface, where it is if it does, and if it does, whether it has a temperature. That prompted a friend to comment 'Great ... this is going to bug me now.' So for him, here's a longer version.
This sort of question is very common to science. Of course my musing for facebook is overstated. But there is usually a real question about what exactly it is you've observed when you take an observation. When you have very different observing methods, they may well observe things that are different from each other. There are, let's say 4, different ways of observing the sea surface's temperature. For a diagram, see the wikipedia article on sea surface temperature
The standard method, and reference for others, is calibrated buoys that carry a thermometer at a known depth, typically 1 meter. A major drawback to this method (all methods of observing have drawbacks!) is that you need a buoy. They're not cheap, and it would take several million of them to give us a high resolution data set for global sea surface temperature (acronymed SST).
The longest-used satellite method for obtaining SST is to observe the earth in infrared wavelengths. Infrared doesn't propagate well through water, so the satellite sees a 'skin' temperature averages over 1 wavelength -- about 10 millionths of a meter (10 microns). A drawback to this method is that it can't see through clouds. Clouds also emit infrared, so the satellite tells you the temperature of the clouds, rather than the sea surface. Weather forecasters use this to improve their forecasts. But it means there's a lot of the earth these satellites can't see on any given day.
A recent addition to our satellite observing of sea surface temperature is to look at microwave wavelengths. This usually can see through clouds as the wavelength is carefully chosen to be one that cloud drops don't emit or absorb much. This gives us a temperature at about 1 mm depth. The drawback for these is that the satellite averages over a large area -- 25-50 km diameter, as opposed to the 1-4 km for the infrared satellites.
The fourth class is the 'everything else' group: ship 'bucket' temperatures, hull contact temperatures, water intake temperatures, temperature sensors on chains (often from buoys) extending well below the surface (or 1 m depth), or the ARGO floats -- which observe temperatures from 2000 m depth up to close to the surface. All of these observe temperatures at depths greater, sometimes much greater, than 1 meter below the surface.
If the 10 micron, 1 millimeter, 1 meter, 5-20 meter temperatures reported by the different methods were the same (within observing error), then it'd be a concern for specialists alone. The reality, however, is that the different temperatures can be substantially different. Most of the time, over most of the globe, they're close. But once you have calm winds (less than 5 m/s, 10 mph, roughly) and strong sunlight, you can accumulate skin (that 10 micron temperature the infrared satellites see) heating enough to warm temperatures. If the winds are very calm, under 2 m/s, it can be by a few degrees -- but only in the skin. The 1 millimeter ('subskin') warms, but not by as much. 1 meter down the temperature may change only a little. And at 5-20 meters, almost entirely unchanged. So ... if you want the 'sea surface temperature', which of the 4 do you want? And is that 5, or 20 meters for the 'unchanged'? How close to entirely unchanged is close enough?
Hence my question: Where is the sea surface?
As mentioned by someone else -- under hurricane conditions, are you even sure that there is a sea surface?
