The above image shows differences from long-term average (1982-1999) of the normalized difference vegetation index (NDVI) for Africa, December 1999. NDVI is simply a measure of vegetation "greenness". This data is used to asses levels of vegetation stress or vigor (productivity). Areas color coded as green show above normal NDVI levels, yellow - near ormal/normal or no change, browns and reds shows below normal NDVI (drought). For December 1999, areas in N.W. Kenya, south and southeastern Ethiopia shows stressed onditions, while parts of Namibia and Zimbabwe show above normal NDVI.