The Western part of Rajasthan is a desert because the temperature remains very high in summer and the Aravallis lie parallel to the path of the Arabian Sea Branch of the southwest monsoons , there by causing no orographic rainfall. When there is an excess moisture in cloud that cannot hold it further and sudden upliftment of cloud with same moisture. As cloud moves suddenly upwards it looses it moisture holding capacity. This happens in windward side of mountain. These Aravalli range are not blocking winds which carry moisture. They are parallel to the direction of cloud movement, so there is no upliftment of clouds carring moisture, so there are very little rains in Rajasthan. Thus the deserts are formed.