Satellites can detect differences between rock, forests, and grasslands because these surfaces will emit energy differently back into space. In measuring these differences and observing patterns of vegetation, or lack of growth, NOAA scientists can monitor how droughts are changing across the world.
The global drought risk weekly composite is derived from the Normalized Difference Vegetation Index datasets developed by NOAA and the National Weather Service from measurements of the AVHRR sensor onboard the POES satellite. Of note is that the drought imagery is based solely on analysis of vegetation health and stress, not the soil moisture conditions. The dataset relies on removing areas that are determined to be covered in snow, by using the same data that is displayed in the Real-Time Snow & Ice Cover.
However, sometimes there are small differences in areas where snow is melting quickly or forming. This is because the drought data is generated weekly, where the snow and ice data is generated daily. This index serves as a very reliable proxy measurement for drought the world wide. Yellow areas indicate areas under moderate drought conditions; red indicates the areas experiencing extreme drought conditions.
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