Usually, the cold weather season begins in mid- November and continues till mid-March. December, January and February are the winter months almost all over India, especially in the northern part.
The chief characteristics of this season are clear skies, pleasant weather, low temperature, low humidity and cool and slow northern winds.
The temperature remains quite low during the winter months. January and February are the coldest months. The days are warm and nights are cold. Slight frost is experienced at places in the hills. The mean daily temperature is below 20 °C over most of the northern part of India. The night temperatures are generally below the freezing point, especially in the hills.
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The southern part of the India has rather warmer conditions and does not experience a well-defined winter season. The coastal areas have moderate temperature almost throughout the year. In general, the temperature goes on decreasing from south to north.
High air pressure over large parts of India is due to the low temperature conditions. However, the air pressure is comparatively lower in the southern parts. The wind starts blowing from the high pressure area of the northwest to the low pressure area of the southeast. The velocity of the wind is low due to low pressure gradient.
During this season the Northeast Trade Winds prevail over a major part of India. These winds blow from land to sea and hence the season is dry.
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The fine weather conditions during the winter season, however, gets disturbed by the inflow of depressions from the west and the northwest. These low pressure disturbances are also called the western disturbances. They originate in West Asia in the region near the Mediterranean Sea. They travel eastwards and enter the Indian Subcontinent after crossing over Iraq, Iran and Afghanistan.
The western disturbances cause light rain in the Northern Plains and snowfall in the Northern Mountains. The jet stream plays an important role in bringing these disturbances to the subcontinent.
The western disturbances are generally active between December and February. After the passage of the western disturbances, widespread fog and cold waves are experienced. They lower the minimum temperature by 5 °C to 10 °C below the normal. During this period, haze is common in the morning and evening.
The retreating Northeast Monsoons blow from land to sea. These winds pick up moisture while crossing the Bay of Bengal and cause widespread rains in Tamil Nadu, south Andhra Pradesh, southeast Karnataka and southeast Kerala.
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The amount of rainfall received from the western disturbances is very small, but it is very useful for the rabi crops, especially wheat and mustard. The amount of precipitation decreases from west to east in the plains and from north to south in the mountains.