Finland's climate and soils make growing crops a particular challenge. The country has severe winters and relatively short growing seasons that are sometimes interrupted by frost. However, because the Gulf Stream and the North Atlantic Drift Current moderate the climate, Finland contains half of the world's arable land north of 60° north latitude. Annual precipitation is usually sufficient, but it occurs almost exclusively during the winter months, making summer droughts a constant threat. In response to the climate, farmers have relied on quick-ripening and frost-resistant varieties of crops, and they have cultivated south-facing slopes as well as richer bottomlands to ensure production even in years with summer frosts. Drainage systems are often needed to remove excess water. Finland's agriculture has been efficient and productive—at least when compared with farming in other European countries.[138]