what are the Western disturbances ?how are the Rains received from them useful in India
Answers
The disturbance travels from the “western” to the eastern direction. Disturbance means an area of “disturbed” or reduced air pressure. Equilibrium exists in nature due to which the air in a region tries to normalise its pressure.
In the term “extra-tropical storm”, storm refers to low pressure. “Extra-tropical" means outside the tropics. As the WD originates outside the tropical region, the word “extra-tropical” has been associated with them.
A WD is associated with rainfall, snowfall and fog in northern India. Upon its arrival in Pakistan and northern India, clouds along with rain and snow also arrive. The moisture which WDs carry with them comes from the Mediterranean Sea and/or from the Atlantic Ocean.
Appearance of a western disturbance in weather model
A perfect west to east flow (technically referred to as 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). Click here to read about how jet streams influence the weather. However, when a broad low pressure region (referred to as trough) is present, this current dips southward, creating a kind of “U shape”
What you see above are the winds circulating at an altitude of 38,000 feet. This map shows the condition on Saturday, December 13. Arrows indicate the direction in which the winds (the jet streams) are blowing. Now check the region of Pakistan and bordering northwest India. You can see how the winds are dipping to create a “U” figure. This reflects the presence of a WD.
Now let’s go down a little in the troposphere to an altitude of 5,000 feet above the ground. Seen above is a weather map showing the winds and relative humidity at 5,000 feet above the ground as on Saturday, December 13. Green patch indicates moist air. If you look towards Rajasthan, you can see that winds are blowing anti-clockwise. In the northern hemisphere, such an anti-clockwise flow of air indicates a low pressure region. This low pressure is associated with the WD mentioned above. To normalise the reduced air pressure over Rajasthan, moist winds from the Arabian Sea seem to be blowing towards Rajasthan. As a result, cloudy weather and rains will be witnessed in the green-shaded areas of central and northern India.
A Western Disturbance is an extratropical storm originating in the Mediterranean region that brings sudden winter rain to the northwestern parts of the Indian subcontinent.[1][2] It is a non-monsoonal precipitation pattern driven by the westerlies. The moisture in these storms usually originates over the Mediterranean Sea and the Atlantic Ocean.[3] Extratropical storms are a global phenomena 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.