Final answer:
The 30 degree latitude line is often called the Horse Latitudes because it's known for calm winds and dry conditions, due to descending air in high-pressure zones that inhibit cloud formation.
Step-by-step explanation:
The 30 degree latitude line in relation to wind is commonly referred to as the Horse Latitudes. This is because these latitudes, particularly at 30 degrees north and 30 degrees south, are characterized by calm winds and little precipitation. The term "Horse Latitudes" is thought to originate from the days when sailing ships depended on trade winds to cross the oceans. Relatively still conditions at these latitudes often stalled ships, and legend has it that horses being transported would sometimes be thrown overboard to conserve water.
The Earth's surface is divided into bands of pressure systems. At about 30 degrees north and south, there is a high-pressure zone where warm air that has risen at the equator descends. As the air descends, it warms and dries, leading to the stable and dry conditions associated with the Horse Latitudes. The descending air inhibits the formation of clouds, resulting in the clear, dry skies typical of these regions.