Thermal Infrared Radiometers
Thermal infrared radiometers on satellites measure the amount of radiation
leaving the earth's surface in the thermal infrared wavelengths. The intensity
of infrared radiation reaching the sensor is often referred to as the brightness
temperature of the surface below.
Thermal infrared radiometers are often used to detect fires on the ground.
In these cases, a temperature is chosen as a cutoff point: pixels that are emitting
more energy at a relevant wavelength than the cutoff temperature are assumed
to contain a live fire.
Thermal infrared wavelengths are useful in the detection of dust. Dust particles
are sufficiently large to interact with thermal infrared radiation leaving the
earth's surface. Instruments that include thermal infrared channels can detect
a decrease of thermal infrared reaching the satellite when dust is present.
The retrieval of dust is more efficient over "bright" landscapes.
In deserts, because humidity is low, and surface temperatures are high, the
contrast in upwelling thermal infrared between dusty and non-dusty landscapes
is good. For example, reductions in brightness temperature up to 25°C have
been observed during the day in the 10.5 to 12.5 µm channel of Meteosat for
major dust events. The contrast is lower during the night, because the dust
layer has an insulating effect.