This pair of images from December 11, 2002, shows the diurnal (daily cycle) fire patterns in central Africa. The top image was captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor on the Terra satellite in the morning, while the bottom image was captured by the MODIS on the Aqua satellite in the afternoon. From left to right, this image spans the countries of Nigeria, Cameroon, Chad, and the Central African Republic. At bottom right, a portion of Democratic Republic of Congo is visible.
As the day progressed, fire activity (indicated by red dots) increased markedly. The increase is due to both human and environmental factors. Many, if not most, of these fires are set by humans for agricultural purposes: clearing farmland, returning nutrients to the soil, regenerating pasture. People become more active over the course of the day, and fire occurrence increases. Fire activity is also influenced by increasing temperatures and decreasing humidity as the morning progresses to afternoon. This increases the potential for planned fires to get out of control or to burn larger areas than intended.
Another interesting difference between the morning and afternoon overpasses is how the relative positions of the sun and the satellite during each overpass changes the appearance of the vegetation. Notice that in the Terra overpass, when the light from the sun would have been coming from the southeast, the vegetation at the right of the image appears dark, and the vegetation in the left half of the image appears bright. During the Aqua overpass, the reverse is true: the sun is coming from the southwest, and the vegetation appears bright in the east and dark in the west. This apparent change in surface observations due to change in the relative position of the sun and the spacecraft is referred to as the bidirectional effect, and scientists must take the effect into consideration when using satellite data to study surface features on Earth.