squalls and thunderstorms known as kal-baisakhi often arrive from the north or northwest. Monsoons bring rain to the whole state from June to September. West Bengal receives the Bay of Bengal branch of the Indian Ocean monsoon that moves in a northwest direction. Winter (December–January) is mild over the plains with average daily low temperatures of . A cold and dry northern wind blows in the winter, substantially lowering the humidity level. However, the Darjeeling Himalayan Hill region experiences a harsh winter, with occasional snowfall at places. Natural resources West Bengal stands third in India in terms of mineral production by state, contributing about one-fifth of the total mineral production of the country. Coal constitutes 99% of the minerals extracted in West Bengal. West Bengal is the third largest state for coal production, accounting for about half of India's total. Coal is extracted from about 228 mines in the Raniganj and Asansol region of Bardhaman district. High grade bituminous coal is mined at Raniganj, Dishergarh, Santaldih, Kulti, Barakar, Ghushik, and Kajora. Coalfields stretch over an area of about . The coalfields of Raniganj support the Asansol-Durgapur industrial belt by providing fuel to the industries as well as generation of thermal power. Lignite mined in Darjeeling is used to make briquettes. Coal deposits are also found along the Ajoy river in Birbhum district. Fireclay and china clay are also mined. West Bengal ranks next to Bihar and Madhya Pradesh in production of fireclay. Most of this mineral is extracted in the Raniganj region, along with a small quantity extracted from Birbhum and Purulia. In 1993–94, 124,000 metric tons of fireclay were produced in West Bengal. China clay—used in the pottery, paper, textile, rubber, and paint industries—is unearthed at Mohammad Bazar in Birbhum and Mejia in Bankura. The rest of the production comes from Purulia, Bardhaman, Darjeeling, and Jalpaiguri. Limestone, copper, iron ore, wolframite, manganese, and dolomite are also mined in small quantities. Limestone—used in the cement industry—is mined in Bankura, Purulia, Darjeeling, and Jalpaiguri. There are copper mines in Jalpaiguri and Darjeeling. Small quantities of low-quality iron ore are mined in Bardhaman, Purulia, Birbhum, and Darjeeling. There is manganese in the Jhargram region of Paschim Medinipur, Purulia, and Bardhaman. Wolframite is mined at Jhilimili in Bankura. The state's production of dolomite comes from the Dooars region of Jalpaiguri. 38.5 thousand tonnes of dolomite were raised in 1993–94. There are possible deposits