pressure conditions over the northern plain. In south Figure . : India : Mean Monthly Temperatures of the Day in January India, the air pressure is slightly lower. The isobars of mb and mb pass through northwest India and far south, respectively (Figure .
). As a result, winds start blowing from northwestern high pressure zone to the low air pressure zone over the Indian Ocean in the south. Due to low pressure gradient, the light winds with a low velocity of about - km per hour begin to blow outwards. By and large, the topography of the region influences the wind direction.
They are westerly or northwesterly down the Ganga Valley. They become northerly in the Ganga-Brahmaputra delta. Free from the influence of topography, they are clearly northeasterly over the Bay of Bengal. During the winters, the weather in India is pleasant.
The pleasant weather conditions, however, at intervals, get disturbed by shallow cyclonic depressions originating over the east Mediterranean Sea and travelling eastwards across West Asia, Iran, Afghanistan and Pakistan before they reach the northwestern parts of India. On their way, the moisture content gets augmented from the Caspian Sea in the north and the Persian Gulf in the south. What is the role of Westerly Jet Streams in steering these depressions in India? Rainfall : Winter monsoons do not cause rainfall as they move from land to the sea.
It is because firstly, they have little humidity; and secondly, due to anti cyclonic circulation on land, the possibility of rainfall from them reduces. So, most parts of India do not have rainfall in the winter season. However, there are some exceptions to it: In northwestern India, some weak temperate cyclones from the Mediterranean sea cause rainfall in Punjab, Haryana, Delhi and western Uttar Pradesh. Although the amount is meagre, it is highly beneficial for rabi crops.
The precipitation is in the fo rm of snowfal l i n th e l ow e r Himalayas. It is this snow that sustains the flow of water in the Himalayan rivers during the summer months. The