Thiruvananthapuram, Nov. 16
COME winter, weather events over northwest and north India come to be influenced by low-pressure systems originating in the Mediterranean Sea or the west Atlantic, which slide past Iran and western Pakistan to make their way in.
These are aptly called western disturbances (WDs), and they join the monsoons, cyclones and severe thunderstorms in modulating the weather over the mainland. Orography, vegetation cover and land-sea contrasts play important roles in this process.
These extra-tropical systems travel in the form of waves in the middle and upper tropospheric westerlies with an average speed of 10 deg. longitudes per day, says Dr Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting (NCMRWF). Their magnitude is of the order of 20-30 deg latitudes.
Over the Indian region, their frequency peaks during winter. About two to three are known to traverse northwest India during a winter month. But their number varies from year to year.
They quite often interact with monsoonal flows giving rise to widespread rainfall over northwest India during monsoon. During summer, they help moderate the weather in the plains. Heat waves generally occur when WDs are fewer in number.
These disturbances are known to induce `secondaries' in the Persian Gulf. They pull moisture from the Arabian Sea to produce reasonably good precipitation. Winter rains brought about by these systems are crucial for rabi crops in northwest India.
As a rule, WDs are more difficult to predict than monsoon. They travel eastwards in higher latitudes of 30 deg N to 60 deg N and barge into the western Himalayas. This topography helps extract the moisture and make the belt within 34 deg N to 36 deg N receive maximum rainfall.
WDs approach the Indian subcontinent from the west in the form of a trough in the upper and middle tropospheric westerlies.
They occasionally deepen after entering the Indo-Pak area, particularly over Rajasthan and Punjab. The intensification is the combined effect of incursion of moist air from the Arabian Sea (or sometimes from the Bay of Bengal) and the orography of the region.
Typically, the typical day-to-day changes in weather in northwest India when a WD passes in winter start with the build-up of low and medium clouds making for partly cloudy skies.
Slight warming of the air can be experienced during the nights and early mornings. There will be a slight fall in day temperatures and the prevailing winds will be southwesterlies.
Days two and three will be marked by the arrival of multi-layer clouds and a nearly overcast sky. Rains commence, but will be mostly light in nature. Day temperatures fall further while night temperatures remain steady.
Prevailing winds will range from being light southeasterlies to easterlies.
On day four, the clouds move away leaving the skies clear. Day temperatures rise but night temperatures fall appreciably. The winds pick up in strength and will blow from the northwest.