The cold weather commences early in November and comes to an end in the middle of March. The hot weather then sets in and lasts till the middle of June. Soon after this the rainy season commences and continues till the end of September, the beginning of this season occurs when a storm from the Bay of Bengal passes over Bihar. The commencement of monsoon may be as early as the last week of May or as the first or second week of July. The climate in the cold weather is pleasant. The days are bright and warm and the sun is not too hot. As soon as the sun sets the temperature falls and the heat of the day yields place to a sharp bracing cold. The mean temperature in November all over Bihar varies from 19.6oC to 22.2oC. The mean temperature in Gaya and Patna in December is 17oC and 18.2oC respectively. January is the coldest month in Bihar. The mean minimum temperature varies from 7.5oC to 10.5oC though some places like Netarhat record much lower temperatures than 7.5oC. 

        The highest temperature is often registered in May which is the hottest month in the state. Like the rest of the northern India, Bihar also experiences dust-storms, thunder-storms and dust raising winds during the hot season. Dust storms having a velocity of 48-64 Km/Hour are most frequent in May and with second maximum in April and June. The hot winds (loo) of Bihar plains blow during April and May with an average velocity of 8-16 Km/hour. This hot wind greatly affects human comfort during this season. The rainy season begins in June. The rainiest months are July and August. The rains are the gifts of the south west monsoon. There are in Bihar three distinct areas where rainfall exceeds 1800mm. Two of them lie on northern and north-western wings of the state and the third lies in the Netarhat pat. The south-west monsoon normally withdraws from Bihar in the first week of October. An important feature of the retreating monsoon season in Bihar is the invasion of tropical cyclones originating in the Bay of Bengal at about 12oN latitude. Bihar is also influenced by the typhoons originating in the South China Sea. The maximum frequency of the tropical cyclones in Bihar is during September-November especially during the asterism called hathiya. These cyclones are essential for the maturing of paddy, and are required for the moistening of the soil for the cultivation of rabi crops.