Satellite remote sensing can provide thermal information in a short time over a wide area. Temperature measurement by remote sensing is based on the principle that any object emits electro-magnetic energy corresponding to the temperature, wavelength and emissivity.

The temperature detected by a thermal sensor is called the "brightness temperature" (see 1.7). Though the brightness temperature coincides with the real temperature if the objects is a black body, the actual object on the earth has a different emissivity e (e < 1) which emits electro-magnetic energy of e.I, where I indicates the radiance of a black body with the same temperature.

Thus the value of e as well as the emitted radiance should be measured in order to compute the exact temperature, as explained in Figure 12.5.1.

However the value of e for sea water is very nearly equal to 1 and also comparatively constant, while the value e for ground surfaces is not homogeneous. Thus sea surface temperature can be estimated more accurately than ground surface temperature.

As the actual brightness temperature includes emitted radiance from the atmosphere, this will cause a temperature error ranging 2-3 degrees Centigrade between the actual sea surface temperature and calculated brightness temperature from satellite data. Thus atmospheric correction (see 9.2) is very important for accurate sea surface temperature measurement.

Figure 12.5.2 shows the sea surface temperature in pseudo color in Northern Japan using NOAA AVHRR data which were atmospheric ally as well as geometrically corrected with overlays of sea coast lines and latitude and longitude grid lines.

Using the most recent technology, the estimated accuracy of sea surface temperature is claimed to be about +-0.5C on a global scale and about +-0.3C on a regional scale.

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