Syllabus Section- Physical Geography

Why in news

For the past few days, Chandigarh and its neighbouring states have been experiencing unusually cold days although the night temperatures are normal. The maximum or day temperatures in the region are several notches below the long period average or normal value, while the minimum or night temperatures are more or less normal.

Note- In Haryana, cold day conditions have been observed at isolated places – a cold day is declared when the maximum daytime temperature of a weather station in the plains goes below 16°C. In Himachal Pradesh, the maximum temperatures are five to six degrees below normal.

Why is this so?

• 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.

• 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. An overcast weather at night, thus, increases the greenhouse warming.

What happens when the cloud cover goes away?

• The reverse. After Thursday, dry conditions and a clear sky are expected to prevail. This will increase the day temperatures, but nights will become cooler.

How did the clouds get here?

• In northern India, winter rains and clouds are generally caused by moisture-bearing wind systems called western disturbances, which 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.

What causes cold winters in North India?

• Billions of years ago, a big heavenly body smashed into the then young earth and caused a tilt in the axis of our planet around which it rotates. This tilted axis remains in the same direction as the earth revolves around the sun.

• So when the earth’s north pole is facing the sun, solar rays hit the northern hemisphere directly and lead to rise in temperatures and the summer season. After half a revolution or six months, the same pole faces away from the sun and the northern hemisphere receives the sun’s rays obliquely.

• As a result, the rays spread over a greater surface area and are low in intensity, leading to the reduced temperatures and the winter season.

• The climate of an area depends on a number of other geographical factors. In north India, the huge temperature difference between summers and winters is due to its continentality. 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.


Source- Indian Express

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