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How Monsoon Trough Affects Monsoon Over India

You must know why sometimes there are excess spells of rainfall during monsoon and sometimes there is no rain at all.

Here we are going to explain the most important semi permanent feature of Monsoon which is responsible for active spells and dry spells during Monsoon period.

Monsoon Trough

The one of the factor which affects monsoon activities is a Monsoon Trough.

What is a trough? A trough is a belt of low pressure extending to large area. This trough seen during monsoon period, hence  known as Monsoon trough.

Features of Monsoon Trough:

  • This trough is located in east west direction from Northwest Rajasthan till the Bay of Bengal.
  • The height of the trough extends upto 5.8 kms upward in the atmosphere and
  • It tilts Southward with height due to temperature difference of 2 deg C with height.

 

POSITION OF TROUGH, FORMATION OF LOW IN BAY AND IT’S EFFECT ON RAINFALL:

Refer to the picture below for more understanding.

Whenever a low is formed in Bay of Bengal, on east coast it enters in the Northwest direction. Then it follows the path of the monsoon trough line giving very good rainfall in the southwest sectors.

The positions of the monsoon trough and the intensity of the low affects the rainfall activity over various parts of India.

See below trough positions:

1) Normal Position: (See Fig. 1)

When we say that the trough is in normal position, that means it passes through Ganganagar, Allahabad, Calcutta and Head Bay. This gives good rain to Central India. As a effect of low formation and it’s passage along the trough, the strong wind currents form over the west coast. These currents are blocked by Sahyadri mountains resulting in formation of offshore trough. Hence due to this offshore trough west of coast of India receives good rain (as shown in red shaded box).

 

Normal Position of Monsoon Trough
(Fig. 1)

 

2) South of the Normal Position: (See Fig. 2)

When the trough shifts to the south of it’s normal position, than the monsoon conditions are active giving excessive rainfall in central India and over the west coast of India due to offshore trough.  This position is best for monsoon condition giving rain to all over Peninsular India.

Southern Position of Monsoon Trough
(Fig. 2)

 

3) North of the Normal Position: (See Fig. 3)

When the trough shifts to north of normal position it means that it shift to foothills of Himalayas giving good rain to Uttarakhand, Himachal Pradesh, Jammu and to all Northeast states of India. This condition is known as “Break Monsoon” where rainfall activity over rest of India ceases.

Northern Position of Monsoon Trough
(Fig. 3)

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