North India is experiencing the early onset of the cold wave conditions. Cold wave condition generally sets in by mid-December and last until January. Along with cold nights its experiencing warm days.

Several factors play role in early cold wave condition.

• Western disturbances impacted the western Himalayan region and adjoining plains 
• A strong subtropical westerly jet stream with winds up to 278 km/h has prevailed over northwest India
• Extended dry weather over the last two months has further contributed. As reduced soil moisture, intensifies the cooling effect during clear nights.
• Early and heavy snowfall in the hills has also add to cold conditions.

LEARNING FROM HOME/ WITHOUT CLASSES/ BASICS

The India Meteorological Department (IMD) definition of  a Cold Wave

For  plains

the minimum temperature being equal to or less than 10°C at a weather station or 4.5°C to 6.4°C below the normal temperature for that period. Or When the minimum temperature is less than or equal to 4°C

For hilly regions

When the minimum temperature is less than or equal to 0°C.

Severe Cold wave

when the minimum temperature falls below 2°C. Or temperature recorded as more than 6.5°C below normal for that period, for plains.

WESTERN DISTURBANCE

Western Disturbances (WD) are low-pressure areas, extratropical storm originating,  embedded in the Westerlies-the planetary winds that blow from west to east between 30 and 60° latitude. The disturbance travels from the “western” to the eastern direction. Disturbance means an area of “disturbed” or reduced air pressure. Equilibrium exists in nature due to which the air in a region tries to normalize its pressure. In the term “extra-tropical storm”, a storm refers to low pressure. “Extra-tropical” means outside the tropics. As the WD originates outside the tropical region, the word “extra-tropical” has been associated with them.

They originate in the Mediterranean region, travel westward, and enter India loaded with moisture, where the Himalayas obstruct them, causing rain and snow over northern India. The moisture in these storms usually originates over the Mediterranean Sea and Atlantic Ocean. WDs are important to the development of the Rabi crop in the northern subcontinent They gradually travel across the middle east from Iran, Afghanistan, and Pakistan to enter the Indian sub-continent. Western Disturbances are low-pressure systems, embedded in western winds (westerlies) that flow from west to east.

 Moreover, Western disturbances, specifically the ones in winter, bring moderate to heavy rain in low-lying areas and heavy snow to mountainous areas of the Indian Subcontinent. They are the cause of most winter and pre-monsoon season rainfall across northwest India. Precipitation during the winter season has great importance in agriculture, particularly for the rabi crops.

JET STREAMS

Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere, typically occurring around 30,000 feet (9,100 meters) in elevation.

Within jet streams, the winds blow from west to east, but the band often shifts north and south because jet streams follow the boundaries between hot and cold air.