While not spanning the entire planet like the great duststorms on Mars, terrestrial duststorms generated in Africa do transport material on an intercontinental scale and appear to originate at similar latitudes in the spring or early summer, when high winds and strong vertical mixing occur on both Earth and Mars. As seen in these and other Space Shuttle photos and in environmental satellite images, dust from Africa spans the Atlantic Ocean and is deposited in the Americas almost annually. The dust scatters the light as it rises from the coastline below, blurring the sharp boundary between the sand and the ocean. This change is subtle when small amounts of dust are present, but large amounts of dust, as seen in figure 5, almost completely obscure the coastline. Measurements of terrestrial scenes like these may aid in satellite-based estimations of the amount of dust suspended in the martian atmosphere and lead to better understanding of these global processes on both planets.
Figure 5: This somewhat longer range view of Dakar, Senegal, was taken on May 4, 1993, at 11:19:20 GMT through a huge duststorm originating in the central Sahel. [NASA photograph STS055-86D-082]
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Figure 6: Dakar Peninsula Seen Through Clear Skies. [NASA photograph STS055-75-011]
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Figure 7 shows a 48-km-wide thunderstorm with tops estimated near 14 km photographed by the STS-55 crew over the coast of Nigeria on May 2, 1993, near noon, local time. A MeteoSat (fig. 8) view taken at almost exactly the same time allows us to pinpoint the location of the storm: it appears as a white dot near the middle of the image along the white groundtrack line running from upper left to lower right. For the details of the storm one must look to the Shuttle photograph. This huge thunderstorm was in the early stages of formation, as can be seen by the intense turbulence in the cauliflower shape of the top. Two major updrafts can be seen as the rose-shaped regions in the cloud tops. An easterly wave in the low levels of the atmosphere created a line of instability, which, together with an ample supply of moisture from the warm waters of the Gulf of Guinea and solar heating over the coast throughout the morning, caused this megastorm to occur.
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Figure 7: Thunderstorm Over Nigeria in Space Shuttle View. This 48-km-wide thunderstorm, with tops estimated near 14 km, was photographed by the STS-55 crew over the coast of Nigeria on May 2, 1993, at 11:13:34 GMT (near noon, local time). This huge thunderstorm was in the early stages of formation, as can be seen by the intense turbulence in the cauliflower shape of the top. Looking at the thunderhead the way the crew did, north is toward the right side of the picture. The coastline bisects the photo vertically, with Nigerian land to the right and the waters of the Gulf of Guinea to the left. [NASA photograph STS055-103-084]
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Figure 8: Thunderstorm Over Nigeria in Satellite View. A MeteoSat view taken at almost exactly the same time as the photo in figure 7 allows us to pinpoint the location of the storm seen in figure 8 as a white dot near the middle of the photo along the white groundtrack line from upper left to lower right. [NASA photograph S94-28359]
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