According to India Meteorological Department (IMD), this winter season’s most powerful Western Disturbance (WD) is all set to hit north India.
Already, six to seven WDs have affected north India in February 2019, while a few more western disturbances are expected by March 1.
WD or the areas of “disturbed air pressure” traveling to India from the west are a regular feature through the winter season.
These are basically “extra-tropical storms” or low pressure systems originating outside the tropical regions.
Unlike a tropical cyclone which is formed in the lower atmosphere, a temperate cyclone (like WDs) is usually formed in the upper atmosphere. Therefore, they can travel greater distances and can hence affect large areas of land (sometimes can travel eastwards along the sub-Himalayan belt and reach up to Arunachal Pradesh).
The factors that determine the strength of WDs are the location and intensity of the jet stream and the amount of moisture being carried by the low-pressure system.
The westerly jet streams (massive fast winds traversing the earth from west to east) carry them.
The rain and snow they bring in their wake to north India and neighbouring regions is formed from the moisture they carry from the Atlantic Ocean or the Mediterranean Sea.
They help sustain Himalayan glaciers and the local ecology and provide nourishment to winter crops (Rabi Crops- wheat, barley, mustard, gram, lentil, etc.).
They are also associated with cloudy skies and an increase in night-time temperatures in parts of north India. They can also cause strong winds that help disperse suspended pollutants in the smog-filled cities of the region, including New Delhi.