All satellite borne sensors measure the intensity of electromagnetic radiation arriving at the satellite. Therefore, for us to understand how a satellite can measure sea surface temperature (SST), we must first define electromagnetic radiation and describe the processes that affect the electromagnetic radiation as it leaves the sea surface and travels toward the satellite.
Electromagnetic radiation refers to all energy that moves with the speed of light in a harmonic wave pattern. The electromagnetic spectrum (Figure 2.01) is a representation of the continuum of wavelengths of electromagnetic radiation, from long radio waves (Fig. 2.01, left) to the short x-rays and gamma rays (Fig. 2.01, right). Note that the portion of the electromagnetic spectrum we can detect with our eyes (the visible portion) in fact constitutes a very small portion of the electromagnetic spectrum. The ability to infer the sea surface temperature from satellites is based on measurements in the thermal infrared portion of the spectrum, wavelengths approximately 10 -5m, slightly longer than visible wavelengths.
All objects with a temperature greater than absolute 0 emit electromagnetic radiation, which is commonly called the objects thermal emission. The temperature of the radiating body determines the intensity and characteristics of the radiation it emits. Two electromagnetic radiation principles describe the relationship between a radiating bodys temperature and the radiation it emits.
1. Stefan-Boltzmanns Law: Hotter objects emit more total energy per unit area than colder objects.
2. Weins Displacement Law: The hotter the radiating body, the shorter the wavelength of maximum radiation.
To visualize this, imagine an iron poker. At room temperature, it is black. After being put into a fire it begins to glow red. This is because as the temperature of the iron increases it emits electromagnetic radiation in the visible part of the spectrum. As its temperature continues to increase, the poker will appear yellow-white. This is because the wavelengths of maximum emissions have moved from the longer red light wavelengths to the shorter yellow light wavelengths (see Fig. 2.01). Once the iron poker is removed from the fire, its color will become red again as its temperature decreases. As it cools further, it will return to its original black color. However, if you touch it now, it will still be warm. Again, as its temperature decreased, the wavelengths of maximum emissions moved from the shorter visible light wavelengths to the longer infrared wavelengths (see Fig. 2.01). At these wavelengths we cant see its emissions.
Now think about the Sun. We see the Sun as yellow because its so hot (6000° K) that it emits most of its energy in the visible portion of the electromagnetic spectrum (around 0.5 x 10-5m, see Fig. 1). Based on Earths temperature (~300° K), most of Earths emitted radiation (from land, the oceans, and clouds) is in the longer wavelength thermal infrared spectrum (around 10-5m, see Fig. 2.01) portion of the electromagnetic spectrum. We cant see this radiation (because our eyes only detect visible light), but satellite sensors can detect and measure the amount of infrared radiation from Earths surface. From the amount of emitted infrared radiation, scientists can then infer the temperature of the land and ocean surfaces, and of the atmosphere.