The summer months are a period of exceedingly rising temperature and declining air pressure in the northern half of the country. Towards the end of May a lengthened low pressure area develops. It is called the monsoon low pressure trough. It stretches from the Thar Desert in the north-west to Patna and Chotanagpur plateau in the east-southeast. Circulation of air starts to set in round this trough.
In the centre of the low pressure trough in the north-west, dry and hot winds blow during the afternoon and frequently they remain even up till midnight. These hot and dry day winds are locally known as 'loo'. Direct vulnerability to these singeing winds may prove fatal in some cases. Dust storms in the evening are very frequent during May in Punjab, Haryana, eastern Rajasthan and Uttar Pradesh. Though momentary, they harbinger a welcome breather from the crushing heat, because they bring along light rains and pleasing chilled breeze.
At times, the moisture-laden winds are drawn towards the fringe of the trough. An abrupt contact between dry and moist air-masses gives rise to local storms of immense strength. These local storms are connected with brutal winds, uncontrolled downpour and even hail storms.
Towards the closing of summer, pre-monsoon showers are a frequent occurrence in Kerala and coastal areas of Karnataka. Locally they are known as 'mango' showers, because they aid in the early ripening of mangoes. Entry of pre-monsoon showers and early advancement of monsoons further north, is arrested by a belt of comparatively high air pressure, lying over the Deccan plateau.
The horrendous north-westerly and northerly winds in Bengal and Assam also cause very precipitate downpours. They are fundamentally evening thunder storms. Their unfavoured nature can be understood from the local terminology of 'Kalbaisakhi'-'calamity of the month of Baisakh'.
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