Historically, Beijing was situated on the border between sedentary agricultural areas to the south and pastoralist regions beyond the mountains to the north. The Great Wall of China was built across the mountains north of Beijing to guard against nomadic invasions. In modern times, the same mountains that shielded Beijing from the Gobi steppes also form a semi-circular basin which catches the city's air pollution. Severe smog problems develop in the summer as the hot and humid air pressure from monsoons in the south prevent air pollutants from leaving the basin. Smog is less severe in the fall and winter when the direction of the wind currents reverse course as the vast anticyclone high pressure system takes hold and brings cold, dry air from Siberia. In the spring, the northerly winds, pick up dust from desertifying areas of western China and bring occasionally severe dust storms to Beijing. The city's climate is characterized by hot, humid summers and cold, windy, and dry winters.