The extratropical storm that originates in the Mediterranean region which brings sudden winter rain to the north-western parts of the Indian sub-continent is known as Western Disturbance. It is a non-monsoonal precipitation pattern which is driven by the westerlies. This may happen during any season, not necessarily in monsoon. These extratropical storms are a global phenomenon. This phenomenon usually carries moisture in the upper layer of the atmosphere, unlike their tropical counterparts where the moisture is carried in the lower layer of atmosphere. In the case of the Indian subcontinent, moisture is sometimes shed as rain when the storm system encounters the Himalayas or Himalayan region.
Formation of western disturbances
Western Disturbance has its origin in the Mediterranean Sea as extra-tropical cyclones. A high-pressure is exhibited area over the areas like Ukraine and neighbourhood countries causes 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 favourable conditions for cyclogenesis in the upper layer of the atmosphere, that 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.
Impact of Western disturbances
The western disturbances create the following impacts in the atmosphere
- It plays a significant role in bringing moderate to heavy rain in low-lying areas and huge quantity of snow to mountainous areas of the Indian Subcontinent.
- This disturbance usually occurs with a cloudy sky, higher night temperatures and unusual rain.
- The precipitation formed has great impact in agriculture, particularly for the Rabi crops.
- The excessive precipitation formed due to this disturbance can cause crop damage, landslides, floods and avalanches.
- In the areas of Indo-Gangetic plains, it occasionally bring cold wave conditions and dense fog.
- These climatic conditions remain stable until disturbed by another western disturbance.
- When these western disturbances travel across northwest India earlier to the onset of monsoon, a set of temporary advancement of monsoon current appears over the affected region.