The most accurate assessment of drought and climatic conditions can be made by taking measurements of the climatic conditions where the climate is happening — on the ground or in the atmosphere. These are called in situ observations. There are several valuable in situ observational networks around the world that provide crucial data to support weather forecasters, industries and economies, and government and private sector decision makers and policy makers. To get the best information, networks with many stations spaced close together (high spatial density) are needed, but this kind of dense spatial coverage can be expensive. In their high earth orbits, satellites can supplement in situ data by providing consistent observations at high spatial density with global coverage.
Satellites measure energy intensities (radiances) at several wavelengths of the electromagnetic spectrum. This information is useful because everything — the ground, the oceans, the atmosphere, clouds, rain, vegetation, cities, people, etc. — absorbs energy at certain wavelengths and emits energy at other wavelengths. You may be familiar with some of these: visible satellite imagery of clouds showing the movement and strength of storms and fronts; infrared imagery which measures the temperature of clouds and weather systems; water vapor maps generated from a spectroscopic analysis of satellite data.
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