The atmosphere is severely dry nine months of the year, and average annual snowfall is only 18 inches (46 cm), due to the rain shadow effect. Western passes receive small amounts of fresh snow each year but remain traversible all year round. Low temperatures are prevalent throughout these western regions, where bleak desolation is unrelieved by any vegetation bigger than a low bush, and where wind sweeps unchecked across vast expanses of arid plain. The Indian monsoon exerts some influence on eastern Tibet. Northern Tibet is subject to high temperatures in the summer and intense cold in the winter.