Answer to Question 1
Answer: Some areas located on the equatorward edge of the subtropical highs are dry for most of the year, except briefly when the system shifts poleward during the summer. This condition exists in the Sahel of Africa, the region bordering the southern margin of the Sahara Desert. Unlike the Sahara, which is dry all year, the Sahel normally experiences a brief rainy period each summer as the ITCZ enters the region. During the rest of the year, the descending air of the Hadley cell leads to dry conditions.
Answer to Question 2
Answer: Ridges and troughs give rise to wavelike flow in the upper atmosphere of the middle latitudes. The largest of these are called long waves, or Rossby waves. Usually, there are anywhere from three to seven Rossby waves circling the globe. Like other waves, each has a particular wavelength (the distance separating successive ridges or troughs) and amplitude (its northsouth extent). Though Rossby waves often remain in fixed positions, they also migrate west to east, or on rare occasions from east to west. Rossby waves exert a tremendous impact on day-to-day weather, especially when they have large amplitudes. They are capable of transporting warm air from subtropical regions to high latitudes or cold polar air to low latitudes. Because upper-level air tends to change temperature only slowly in the absence of strong vertical motions, Rossby waves can bring anomalous temperatures to just about any place within the middle to high latitudes.
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Desertification in the Sahel
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