Jet streams are fast-moving, narrow-core, meandering air currents in the atmosphere of the Earth. As a part of this, a westerly jet stream with speedy winds in excess of 200 km/h prevails over North India, during the winter season. These are generally observed at higher levels in the atmosphere. Another type of jet stream, popularly called ‘Nocturnal Jet’ appears in the lower levels of atmosphere, and once again over North India during winters. Nocturnal jets are strongest in the early morning hours decrease during the day and are typically seen between 2000’ and 5000’ above the ground level.
The nocturnal jet stream is a result of the pressure gradient. As air temperature near the ground drops after sunset and it goes on through the night, an inversion layer is formed. Normally, the air temperature drops with height but within the inversion layer, the temperature increases with height. If any other air parcel in the vicinity has a pool of cold elevated air, the conditions become ideal for triggering a sufficient pressure gradient. This differential pressure gets manifested as strong winds in the lower layers of the atmosphere. The speed effect trickles down in the lowest layers, close to the ground as well, but gets mitigated due to frictional effects in the boundary layer of the earth.
Similar conditions have been prevailing over many parts of Punjab, Haryana, North Rajasthan, Delhi and neighbouring regions of Uttar Pradesh. The chill factor has risen and feels like temperatures are much lower than the actuals, in the early morning hours. Minimum temperatures of about 5°C or less have been recorded at more than a few places this morning. Pathankot 4.6, Jalandhar 4.5, Firozepur 4.2, Patiala 5.0, Karnal 4.2, Panchkula 5.8, and Delhi 6.9 are some of the stations registering temperatures 3-4°C, below the normal. Just at the cost of reminder, a minimum temperature of 4°C amounts to cold wave conditions in that area.
Strong low-level westerly winds are expected to blow for two more days over the northern plains, including the national capital. There is no let-up likely in the winter chill during early morning hours. Yes, the conditions will ease out as the day progresses due to a fair amount of sunshine. Northern plains and the national capital will not have any rain for the next one week or even longer. Minimum temperatures may dip marginally to hang around 4°C at many places over North India. Cold wave conditions are unlikely to abate soon from some pockets of this region.
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