Relevance: GS-1: Important Geophysical phenomena such as earthquakes, Tsunami, Volcanic activity, cyclone etc.

Key phrases: Western Disturbances, Mediterranean Sea, westerlies, extra tropical, low pressure, cyclonic.

Why in News?

• The first of the two intense western disturbances headed for North West India has docked in over Iran with a limb extending into Afghanistan this (Tuesday) morning. Even from this distance, it would be able to influence winter weather over North West India and rule out a cold wave for at least a week.

What is Western disturbance?

• A western disturbance is an extra tropical storm originating in the Mediterranean region that brings sudden winter rain to the northern parts of the Indian subcontinent, which extends as east as up to northern parts of Bangladesh and South eastern Nepal.
• It is a non-monsoonal precipitation pattern driven by the westerlies. The moisture in these storms usually originates over the Mediterranean Sea, the Caspian Sea and the Black Sea.
• Extra tropical storms are a global phenomenon with moisture usually carried in the upper atmosphere, unlike their tropical counterparts where the moisture is carried in the lower atmosphere.
• In the case of the Indian subcontinent, moisture is sometimes shed as rain when the storm system encounters the Himalayas. Western disturbances are more frequent and stronger in the winter season.

Significance of Western disturbances:

• 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 post-monsoon season rainfall across northwest India.
• Precipitation during the winter season has great importance in agriculture, particularly for the rabi crops.
• Wheat among them is one of the most important crops, which helps to meet India's food security. An average of four to five western disturbances form during the winter season.
• The rainfall distribution and amount varies with every western disturbance.
• Western disturbances are usually associated with cloudy sky, higher night temperatures and unusual rain.

Impact of Western disturbances:

• Excessive precipitation due to western disturbances can cause crop damage, landslides, floods and avalanches.
• Dense fog is already being witnessed in the plains of northern India. As the western disturbance will be bringing more moisture, there will be an increase in fog. Low visibility problems will increase in Delhi-NCR, Uttar Pradesh, Punjab and Haryana.
• The strongest western disturbances usually occur in the northern parts of Pakistan, where flooding is reported number of times during the winter season.
• Weak western disturbances are associated with crop failure and water problems across north India.

Effects on monsoon:

• Western disturbances start declining in numbers after winter. During the summer months of April and May, they move across north India.
• The southwest monsoon current generally progresses from east to west in the northern Himalayan region, unlike western disturbances which follow a west to east trend in north India with consequent rise in pressure carrying cold pool of air.
• This helps in the activation of monsoon in certain parts of northwest India. It also cause pre monsoon rainfall especially in northern India.
• The interaction of the monsoon trough with western disturbances may occasionally cause dense clouding and heavy precipitation. The 2013 North India floods, which killed more than 5000 people in a span of 3 days, is said to be a result of one such interaction.

Source: The Hindu BL

Mains Question:

Q. What is western cyclonic disturbance? How it is formed? Are Western Disturbances important for Rabi Crops? Illustrate.