Introduction:
The western disturbance was strengthened by moisture coming in from the Arabian Sea and the Bay of Bengal. It is estimated that India gets close to 5-10% of its total annual rainfall from Western Disturbances. Weak Western Disturbances on the other hand are associated with crop failure and water problems across North India. But strong Western Disturbances can help residents, farmers and governments avoid many of the problems associated with water scarcity.
Storm:
- A storm is any disturbed state of a body especially affecting its surface, and strongly implying a wind force. It may be marked by significant disruptions and lightning, heavy precipitation heavy freezing rain strong winds (tropical cyclone, windstorm), or wind transporting some substance through the atmosphere as in a dust storm, blizzard, sandstorm, etc.
- Storms have the potential to harm lives and property via storm surge, heavy rain or snow causing flooding or road impassibility, lightning, wildfires, and vertical wind shear. Systems with significant rainfall and duration help alleviate drought in places they move through. Heavy snowfall can allow special recreational activities to take place which would not be possible otherwise, such as skiing and snowmobiling.
- Storms are created when a center of low pressure develops with the system of high pressure surrounding it. This combination of opposing forces can create winds and result in the formation of storm clouds such as cumulonimbus. Small localized areas of low pressure can form from hot air rising off hot ground, resulting in smaller disturbances such as dust devils and whirlwinds.
Types:
- Tropical Wave
An inverted trough (an elongated area of relatively low pressure) or cyclonic curvature maximum moving east to west across the tropics. These can lead to the formation of a tropical cyclone. Also known as an easterly wave. - Tropical Disturbance
A tropical weather system with organized convection (generally 100-300 miles in diameter) originating in the tropics or subtropics, having a non-frontal migratory character and maintaining its identity for 24 hours or longer. It may or may not be associated with a detectable perturbation of the wind field. - Tropical Cyclone
A tropical cyclone is a low pressure system (not associated with a front) that develops over tropical and sometimes sub-tropical waters and has organized deep convection with a closed wind circulation about a well-defined center - Extratropical Cyclone
A cyclone (of any intensity) for which the primary energy source is baroclinic (i.e., results from the temperature contrast between warm & cold air masses). - Post-Tropical Cyclone
A cyclone that no longer possesses sufficient tropical characteristics to be considered a tropical cyclone. Post-tropical cyclones can continue to carry heavy rains and high winds. Note: former tropical cyclones that become extratropical and remnant lows are 2 specific classes of post-tropical cyclones - Subtropical Cyclone
A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. Subtropical cyclones originate over tropical or subtropical waters and have a closed circulation about a well-defined center. In comparison to tropical cyclones, the maximum winds occur relatively far from the center (greater than 60 nautical miles) and have a less symmetric wind field and distribution of convection. - Tropical Depression
A tropical depression is a tropical cyclone that has maximum sustained surface winds (one-minute average) of 38 mph (33 knots) or less. - Tropical Storm
A tropical storm is a tropical cyclone that has maximum sustained surface winds ranging from 39-73 mph (34 to 63 knots). - Hurricane
A hurricane is a tropical cyclone that has maximum sustained surface winds of 74 mph or greater (64 knots or greater).
Western Disturbance:
- It is an extratropical storm originating in the Mediterranean region that brings sudden winter rain to the north-western parts of the Indian sub-continent. It is a non-monsoonal precipitation pattern driven by the westerlies.
- Extratropical 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.
Formation:
- Western Disturbance originates in the Mediterranean Sea as extra-tropical cyclones. A high-pressure area over Ukraine and neighbourhood consolidates, causing the intrusion of cold air from Polar Regions towards an area of relatively warmer air with high moisture.
- This generates favourable conditions for cyclogenesis in the upper atmosphere, which promotes the formation of an eastward-moving extratropical depression. They gradually travel across the middle-east from Iran, Afghanistan and Pakistan to enter the Indian sub-continent.
Impact:
- Plays important role in bringing moderate to heavy rain in low-lying areas and heavy snow to mountainous areas of the Indian Subcontinent.
- This disturbance is usually associated with cloudy sky, higher night temperatures and unusual rain. This precipitation 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.
- Excessive precipitation due to this disturbance can cause crop damage, landslides, floods and avalanches. Over the Indo-Gangetic plains, it occasionally bring cold wave conditions and dense fog. These conditions remain stable until disturbed by another western disturbance. When western disturbances move across northwest India before the onset of monsoon, a temporary advancement of monsoon current appears over the region.
