low pressure in the Bay of Bengal region often leads to the formation of storms in the coastal areas. Winter (December–January) is mild over the plains with average minimum temperatures of . A cold and dry northern wind blows in the winter, substantially lowering the humidity level. The Darjeeling Himalayan Hill region experiences a harsh winter, with occasional snowfall. Flora and fauna The "India State of Forest Report 2017", recorded forest area in the state is , while in 2013, forest area was , which was 18.93% of the state's geographical area, compared to the then national average of 21.23%. Reserves and protected and unclassed forests constitute 59.4%, 31.8% and 8.9%, respectively, of forested areas, as of 2009. Part of the world's largest mangrove forest, the Sundarbans in southern West Bengal. From a phytogeographic viewpoint, the southern part of West Bengal can be divided into two regions: the Gangetic plain and the littoral mangrove forests of the Sundarbans. The alluvial soil of the Gangetic plain, combined with favourable rainfall, makes this region especially fertile. Much of the vegetation of the western part of the state has similar species composition with the plants of the Chota Nagpur plateau in the adjoining state of Jharkhand. The predominant commercial tree species is Shorea robusta, commonly known as the sal tree. The coastal region of Purba Medinipur exhibits coastal vegetation; the predominant tree is the Casuarina. A notable tree from the Sundarbans is the ubiquitous sundari (Heritiera fomes), from which the forest gets its name. The distribution of vegetation in northern West Bengal is dictated by elevation and precipitation. For example, the foothills of the Himalayas, the Dooars, are densely wooded with sal and other tropical evergreen trees. Above an elevation of , the forest becomes predominantly subtropical. In Darjeeling, which is above , temperate forest trees like oaks, conifers and rhododendrons predominate. 3.26% of the geographical area of West Bengal is protected land, comprising fifteen wildlife sanctuaries and five national parks—Sundarbans National Park, Buxa Tiger Reserve, Gorumara National Park, Neora Valley National Park and Singalila National Park. Extant wildlife includes Indian rhinoceros, Indian elephant, deer, leopard, gaur, tiger and crocodiles, as well as many bird species. Migratory birds come to the state during the winter. The high-altitude forests of Singalila National Park shelter barking deer, red panda, chinkara, takin, serow, pangolin, minivet and kalij pheasants. The Sundarbans are noted for a reserve project