Western Disturbance: Effects on Indian Subcontinent

The days have been unusually warm for winter this December, this has mostly been on account of fewer western disturbances affecting weather this year, according to IMD scientists.

About Western Disturbance (WD)

They are extra-tropical storms that originate in the Mediterranean region and are commonly known as western disturbances.
A high pressure is exhibited over the areas like Ukraine and neighboring countries causing the intrusion of cold air from Polar Regions towards an area of relatively warmer air with high moisture. This change in pressure from cold air to warm air generates favorable conditions for cyclogenesis in the upper layer of the atmosphere, which promotes the formation of an eastward moving extratropical depression in the sea. Then these gradually travel across the middle-east from Iran, Afghanistan and Pakistan to finally enter the Indian sub-continent.
The term “extra-tropical storm”, refers to low pressure. As the Western Disturbance originates outside the tropical region, the word “extra-tropical” has been associated with them. Then it travels from the “western” to the “eastern” direction.
A WD is associated with rainfall, snowfall, and fog in northern India.

Note:

About the Tropical Storms

They are formed in the regions between the Tropics of Cancer and the Tropics of Capricorn.

It occurs due to the low pressure over the warm tropical oceans.

These storms represent an intermediate stage between loosely organized tropical depressions and more intense tropical cyclones, which are also called hurricanes or typhoons in different parts of the globe.

The Coriolis force, which occurred due to Earth’s rotation, generates a closed and symmetric circulation pattern.

Appearance of a Western Disturbance in the Weather Model

A perfect west-to-east flow or a zonal flow of the winds (jet streams) at an altitude of 30,000 feet depicts stable weather (lack of low-pressure regions) in that region. A jet stream is a river-like current of air circulating across the globe at upper levels of the troposphere (near the altitude of 30,000 feet).
However, when a broad low-pressure region (referred to as trough) is present, this current dips southward, creating a kind of “U shape”. This reflects the presence of a WD.
In the northern hemisphere, such an anti-clockwise flow of air indicates a low-pressure region, which is associated with WD.
The moist winds from the Arabian Sea normalize the reduced air pressure. As a result, cloudy weather and rains are witnessed in regions such as central and northern India.

Significance of Western Disturbance

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.
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 forms during the winter season.
The rainfall distribution and amount vary with every western disturbance.

Impact of Western Disturbance

Snow and Rain

WD is a regular feature in winter, they bring rain and snow over the hills, and more moisture to the plains of northern India (5-10 percent of India’s total annual rainfall comes from WD)
In monsoon and summer, they have more tropical characteristics and bring rain and thunderstorms.
The cloud cover results in higher minimum temperatures at night and lower day-time or maximum temperatures.

Causes Fog

Dense fog is also 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 also increase.
In the areas of Indo-Gangetic plains, it occasionally brings cold wave conditions and dense fog.

Causes Disaster

The excessive precipitation formed due to this disturbance can cause crop damage, landslides, floods, and avalanches. For example, 2010 Cloudburst in Leh, the Kashmir floods in 2014.
It helps in prolonging the life of tropical cyclones and maneuvers at the interior of the continent. Recently, Cyclone Tauktae arising from the Arabian Sea causes heavy rainfall in the interior states including Rajasthan, Uttarakhand, Himachal Pradesh, Uttar Pradesh and Delhi.

Impact on Agriculture

The precipitation formed due to WD has a great impact on agriculture, as it is highly beneficial for Rabi crops (wheat, barley, peas, gram, mustard, etc).