The Hindu Business Line : Fog alert on as induced `low' impacts west Rajasthan

THE fog alert for plains of northwest India and west Uttar Pradesh from Tuesday onwards has been retained as the causative western disturbance lay in wait over north Pakistan and adjoining Jammu and Kashmir.

The associated but induced low-pressure area extended to west Rajasthan and neighbourhood, the National Centre for Medium Range Weather Forecasting (NCMRWF) said in its update on Sunday.

Under its influence, scattered to fairly widespread snowfall/rain is likely over the hilly region of Jammu and Kashmir, Himachal Pradesh and Uttaranchal. It will be scattered over the plains of Punjab, Haryana (including Chandigarh and Delhi) and north Rajasthan during the next two days, the NCMRWF forecast said.

Model predictions suggest a rise in night temperatures by about two deg C over many parts of northwest India during the next 24 hours and fall in day temperature by about two-three deg C over the next two days. After the passage of western disturbance, night temperatures are expected to fall at many places in the region by three to four deg C. This will create ambient conditions for the welling up of fog that will bring the plains under an extensive wrap.

The western disturbances are associated with eastward moving upper air troughs in the sub-tropical westerly winds, often extending down to the lower troposphere of the north Indian latitudes during winter months. These troughs approach the Indian subcontinent from the west and give rise to formation of closed cyclonic circulations in the lower tropospheric level to create typical weather conditions, said Dr Akhilesh Gupta of the NCMRWF.

It is generally believed that these systems originate in the Mediterranean or the West Atlantic region, with `secondaries' developing over the Persian Gulf en route either directly or as a result of the arrival of low-pressure systems from southwest Arabia.

These systems intensify once they enter northwest India largely from the combined effect of incursion of moist air from the Arabian Sea to the south and southwest (or sometimes even from the Bay of Bengal to the southeast) and the orography of the western Himalayas.

The resulting snowfall/blowing snow in the Himalayan region and biting cold winds over the plains of northwest India can wreak havoc over large geographical areas.

Over the Indian region, the frequency of occurrence is highest during the winter season, and may vary from year to year.

These systems quite often interact with monsoonal flows, giving rise to widespread rainfall over northwest India during monsoon, said Dr Gupta. Though the rainfall amounts and frequency are less compared to those during monsoon, the spells of light to moderate precipitation are of immense value for rabi crops.

Precipitation over northwest India and Himalayas realised under the influence of western disturbances is also vital for replenishment of water resources.

High reaches of the Himalayan region receives more frequent and intense snowfall events compared to places at the lower heights.

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