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 winds 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