form of air mass classification. While air mass identification was originally used in weather forecasting during the 1950s, climatologists began to establish synoptic climatologies based on this idea in 1973. A monsoon is a seasonal prevailing wind which lasts for several months, ushering in a region's rainy season. A variety of Germanic and major Slavic tribes crossed through Moravia during the Migration Period before Slavs established themselves in the 6th century AD. At the end of the 8th century, the Moravian Principality came into being in present-day south-eastern Moravia, Záhorie in south-western Slovakia and parts of Lower Austria. In 833 AD, this became the state of Great Moravia with the conquest of the Principality of Nitra (present-day Slovakia). Their first king was Mojmír I (ruled 830–846). Louis the