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| Himalayas-Image by David Mark from Pixabay |
1. A Western Disturbance (WD) is an extra-tropical storm originating in the Mediterranean region that brings sudden winter rain mostly to the western parts of Nepal.
2. It’s a non-monsoonal precipitation pattern driven by the westerlies.
3. Moisture in WD usually originates over the Mediterranean Sea, the Caspian Sea and the Black Sea.
4. Extra-tropical storms are global phenomena with moisture usually carried in the upper atmosphere, unlike their tropical counterparts where moisture is carried in the lower atmosphere.
5. In Nepal’s case, moisture is sometimes shed as rain/snow when the storm system encounters the Himalayas.
6. WD are more frequent and strong in the winter season that brings moderate to heavy rain in low-lying areas (Terai) and heavy snow to mountainous areas of Nepal.
7. WDs are the cause of most winter and pre-monsoon season rainfall across Nepal.
8. Precipitation during winter season has great importance in agriculture, particularly for the winter crops like wheat, barley and potatoes.
9. An average of four to five WDs forms during the winter season with distribution and variation in rainfall.
Book Reference: Ways of the Weather,
P A Menon
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