More about Sea Surface
Temperature
"A general definition of sea surface temperature
(SST) is that it is the water temperature at 1 meter below the sea surface.
However, there are a variety of techniques for measuring this parameter that
can potentially yield different results because different things are actually
being measured. The earliest technique for measuring SST was to dip a thermometer
into a bucket of water manually drawn from the sea surface. The first automated
technique for determining SST was accomplished by measuring the temperature
of water in the intake port of large ships. This measurement is not always consistent,
however, as the depth of the water intake as well as exactly where the temperature
is taken can vary from vessel to vessel. Probably the most exact and repeatable
measurements come from fixed buoys where the depth of water temperature measurement
is always approximately 1 meter, and very robust electrical temperature probes
are used. These measurements are usually beamed to satellites for automated
and immediate data distribution. A large network of coastal buoys in U.S. waters
is maintained by the National Data Buoy Center (NDBC).
Since the 1980s satellites have been increasingly utilized to measure SST and
have provided an enormous leap in our ability to view the spatial and temporal
variation in SST. The satellite measurement is made by sensing the ocean radiation
in two or more wavelengths in the infrared part of the electro-magnetic spectrum
which can then be empirically related to SST. These wavelengths are chosen because
they are (1) within the peak of the blackbody radiation expected from the earth
and (2) transmit well through the atmosphere.
The satellite measured SST provides both a synoptic view of the ocean and a
high frequency of repeat views, allowing an examination of basin-wide upper
ocean dynamics that is not possible with ships or buoys. For example, a ship
traveling at 10 knots would require 10 years to cover the same area a satellite
covers in two minutes. However, there are several difficulties with satellite-based
absolute SST measurements. First, because all radiation emanates from the top
"skin" of the ocean, approximately the top 0.1 millimeter or less,
it may not represent the bulk temperature of the upper meter of ocean. The "skin"
temperature may be either warmer or cooler than the bulk temperature due to
the effects of solar surface heating, back radiation, sensible heat loss, and
surface evaporation.
This discrepancy can make it difficult to compare satellite measurements to
measurements from buoys or shipboard methods, complicating ground truth efforts.
In addition, the satellite cannot look through clouds, creating a "fair
weather" bias in the long-term trends of SST. Nonetheless, these difficulties
are small compared to the benefits in understanding gained from satellite SST
estimates."
(From, Florida Bay Atlas)