At wavelengths greater than about 2 cm and less than 10 m, the atmosphere and ionosphere are very transparent to E/M waves. Stewart [1985] says "At the longest wavelengths the atmosphere is clearer than the clearest air for visible light and thermal radiation from the Earth dominates". Microwaves penetrate clouds and since the signal is from thermal emissions, passive microwave measurements can be made in all weather and in daytime or nighttime. This is an important consideration since microwave radiometers don't rely on reflected sunlight, they don't need to be placed in sun-synchronous orbits so they can be placed on almost any platform.
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