Vegetation indices are used to remotely examine vegetation over a given area. One commonly-used index is the Normalized Difference Vegetation Index (NDVI), which uses the difference between near-infrared (NIR) and red reflectance from the Earth's surface, divided by their sum. In healthy photosynthesizing plants, chlorophyll - the main pigment involved in photosynthesis - strongly reflects NIR light and absorbs most red light. If plants suffer from conditions like drought or disease, however, this ratio shifts, as more NIR light is absorbed and more red light is reflected. Examining the ratio of NIR to red reflectance thus makes it possible to quickly assess the condition of plants, to see if they’re thriving or showing signs of stress. NDVI values range from -1 to 1, with higher values indicating healthier vegetation, and lower values indicating stressed vegetation or barren areas like sand or snow. 

NASA derives NDVI assessments from instruments like the Visible and Infrared Imager/Radiometer Suite (VIIRS), which observes the earth in visible and infrared wavelengths on a daily basis. The VIIRS NDVI products also continue more than 20 years of global NDVI observations from the Moderate Resolution Imaging Spectroradiometer (MODIS), providing a detailed and continuous record of global vegetation change over time. 

NASA NDVI products provide daily, global assessments of vegetation health at high detail. These are useful in conservation and environmental research, drought monitoring and forecasting, and crop assessments, to name just a few examples.