DISCUSSION: During the course of a given calendar year, there are substantial differences which occur with respect to the transition of primary vegetation cover across parts of northern and central Africa. This is primarily a result of the progression of atmospheric features such as the Hadley Cell and Walker Circulation which act to modulate the general magnitude and location of both rising and sinking motion around the world at any given point in time. Thus, at different times of the year, there is a variable amount of rising and sinking motion present over various parts of the Sahel (i.e., the more tropical and equator-oriented regions of central and northern Africa).
In looking at the animated graphic which displays a typical year may look like from space in terms of the vegetative cover across northern and central Africa, it is critical to understand how this graphic is created by way of the Normalized Difference Vegetation Index (NDVI). Attached below are some exact excerpts from the NASA Earth Observatory article which explains the process by which this graphic is generated.
"To determine the density of green on a patch of land, researchers must observe the distinct colors (wavelengths) of visible and near-infrared sunlight reflected by the plants. As can be seen through a prism, many different wavelengths make up the spectrum of sunlight. When sunlight strikes objects, certain wavelengths of this spectrum are absorbed and other wavelengths are reflected. The pigment in plant leaves, chlorophyll, strongly absorbs visible light (from 0.4 to 0.7 µm) for use in photosynthesis. The cell structure of the leaves, on the other hand, strongly reflects near-infrared light (from 0.7 to 1.1 µm). The more leaves a plant has, the more these wavelengths of light are affected, respectively.
NDVI is calculated from the visible and near-infrared light reflected by vegetation. Healthy vegetation (left) absorbs most of the visible light that hits it, and reflects a large portion of the near-infrared light. Unhealthy or sparse vegetation (right) reflects more visible light and less near-infrared light. Nearly all satellite Vegetation Indices employ this difference formula to quantify the density of plant growth on the Earth — near-infrared radiation minus visible radiation divided by near-infrared radiation plus visible radiation."
To learn more about the inside scoop on the process by which such information is collected, click on the following link.
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© 2018 Meteorologist Jordan Rabinowitz