DR. CHANDRA SHEKHAR BALACHANDRAN

The North East monsoon is here and the south-eastern part of India is enjoying the rain. But how and where did it all begin?

If you are around the Tropic of Cancer, in the northern hemisphere of Earth, like India, this is the time to begin thinking about warm clothing. There begins a noticeable chill in the air in the early mornings and late evenings.

In the rural areas, there is an increasing need for fuel — not just for cooking but for warmth also. In high-elevation areas, where snow and frost are possible, people prepare for winter.

On September 22, we passed the second of the two annual equinoxes (equi – equal, nox – night; equal day and night). The Sun’s position in relation to the Earth is now shifting gradually towards the Tropic of Capricorn (in the southern hemisphere). This marks the period called dakshinaayana (southward movement).

High and low

When the sun is directly overhead, the heat is intense. That heat has now started moving away from us towards the Tropic of Capricorn. Wherever the land or water surface is warm, the air in contact with that area gets heated. This leads to a lower pressure. This means, air from nearby areas of higher pressure, rush into the areas of lower pressure.

Because of this, the winds that sweep over India from now through December, come increasingly from the North East, from beyond the Himalayas. These winds are part of the northeast monsoon. (“Monsoon” comes from Arabic mawsin meaning season.)

Those winds are cold because they are blowing over cold land. They dump a lot of snow on the Himalayas and are dry when they cross over the mountains and come to the rest of India.

In the southeastern part of India (Tamil Nadu, Andhra Pradesh, and to an extent in Karnataka) we see some amount of rainfall from these monsoons. This is because the Bay of Bengal is still warm and is giving up moisture to the air. This brings rains.

Despite advanced computer technology and data processing abilities, it is still difficult to predict the movement of the monsoons.

The North East monsoons is important for winter agriculture also. The “rabi” crops are closely tied to the North East monsoon.

Things you can do

Log on to http://tinyurl.com/y6gqb8 and watch the changes related to the earth’s revolution.

To see the movement of a low-pressure belt (Inter-Tropical Convergence Zone – ITCZ), changes in pressure and rainfall patterns log on to http://tinyurl.com/3v7osk

Discuss with your Geography teacher how these two correlate.

Ask your physics teacher about how solids (land), liquids (water), and gases (atmosphere) behave in relation to temperature.