From UPSC perspective, the following things are important :
Prelims level : Western disturbances , Winter rainfall
Mains level : Peculiarities of Indian weather
For the past few days, Chandigarh and its neighbouring states have been experiencing unusually cold days although the night temperatures are normal.
Try this PYQ:
Consider the following statements:
- The winds which blow between 30°N and 60°S latitudes throughout the year are known as westerlies.
- The moist air masses that cause winter rains in the North-Western region of India are part of westerlies.
Which of the statements given above is/are correct?
(a) Only 1
(b) Only 2
(c) Both 1 and 2
(d) Neither 1 nor 2
Temperature anomaly in North
- Meteorological officials have attributed the trend to the cloud cover in the region which was absent until a few days ago.
- It is the result of a western disturbance, which has brought about a spell of precipitation in the northwest Himalaya.
Role of clouds
- During the day, clouds obstruct the heat from the sun from reaching the surface of the earth, reflecting some of it back into space.
- This lowers the temperature. Cold winds blowing down from snow-bound areas in the mountains also contribute to the cooling effect.
- At night, however, clouds act like blankets – they help retain some of the heat energy radiated back by the earth’s surface.
- Overcast weather at night, thus, increases greenhouse warming.
What are western disturbances?
- In northern India, winter rains and clouds are generally caused by moisture-bearing wind systems called western disturbances.
- They originate and gather moisture over the Mediterranean region and flow eastwards towards the Indian subcontinent.
- When some of these winds run into mountains of the northwest Himalaya, they are forced upward.
- At higher altitudes, the temperatures drop and water vapour gets condensed. This leads to cloud formation and eventually rain and snow.
Other factors for severe winters in North
- In north India, the huge temperature difference between summers and winters is due to its continentality (distance from seas and oceans).
- Air from oceans moderates the temperature as it moves onshore, but this effect is missing in continental interiors.
- As a result, north India has greater seasonal differences as compared to peninsular India.
- Temperature also reduces rapidly with altitude, and thus, the Himalayan region is colder still.