These maps compare the current satellite-derived greenness with the average greenness for the same 10-day period (e.g., 1 to 10 January) in previous years, providing insight into how current conditions relate to historical ones. Four map products are available, comparing the current 10-day greenness with (1) the long-term average, (2) the previous 4-year average, (3) the same period one year ago, and (4) the previous 10-day period. On the maps, red indicates areas where current greenness is below the long-term average, green indicates better-than-average conditions, and yellow represents normal conditions. The greenness index, also called the Normalized Difference Vegetation Index (NDVI), is calculated by comparing the difference between near-infrared (which vegetation reflects strongly) and red light (which vegetation absorbs) and is measured by sensors onboard satellites.