Monsoon in India

"Monsoon" is a term used to refer season. The word has originated from the Arabic word “Mausim” which means season. It is most often applied to the seasonal reversals of the wind direction along the shores of the Indian Ocean, especially in the Arabian Sea, that blow from the southwest during one half of the year and from the northeast during the other.

Onset & Advance of Monsoon

The guidelines to be followed for declaring the onset of monsoon over Kerala and its further advance over the country are enlisted below:
Onset over Kerala
a) Rainfall
If after 10th May, 60% of the available 14 stations enlisted*, viz. Minicoy, Amini,Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery,Kannur, Kasargode and Mangalore report rainfall of 2.5 mm or more for two consecutive days, the onset over Kerala be declared on the 2nd day, provided the following criteria are also in concurrence.
b) Wind field
Depth of westerlies should be maintained upto 600 hPa, in the box equator to Lat. 10ºN and Long. 55ºE to 80ºE. The zonal wind speed over the area bounded by Lat. 5-10ºN, Long. 70-80ºE should be of the order of 15 – 20 Kts. at 925 hPa. The source of data can be RSMC wind analysis/satellite derived winds.
c) Outgoing Longwave Radiation (OLR)
INSAT derived OLR value should be below 200 wm-2 in the box confined by Lat. 5-10ºN and Long. 70-75ºE.

Further Advance of Monsoon over the Country
a) Further advance be declared based on the occurrence of rainfall over parts/sectors of the sub-divisions and maintaining the spatial continuity of the northern limit of monsoon, further advance be declared.

The following auxiliary features may also be looked into:
b) Along the west coast, position of maximum cloud zone, as inferred from the satellite imageries may be taken into account.
c) The satellite water vapour imageries may be monitored to assess the extent of moisture incursion.

Northern Limit of Monsoon (NLM)
Southwest monsoon normally sets in over Kerala around 1st June. It advances northwards, usually in surges, and covers the entire country around 15th July. The NLM is the northern most limit of monsoon upto which it has advanced on any given day.

Withdrawal of SW Monsoon
a) Withdrawal from extreme north-western parts of the country should not be attempted before 1st September.
b) After 1st September:
The following major synoptic features should be considered for the first withdrawal from the western parts of NW India.
i) Cessation of rainfall activity over the area for continuous 5 days.
ii) Establishment of anticyclone in the lower troposphere (850 hPa and below)
iii) Considerable reduction in moisture content as inferred from satellite water vapour imageries and tephigrams.

Further Withdrawal from the Country
i) Further withdrawal from the country may be declared, keeping the spatial continuity, reduction in moisture as seen in the water vapour imageries and prevalence of dry weather for 5 days.ii) SW monsoon should be withdrawn from the southern peninsula and hence from the entire country only after 1st October, when the circulation pattern indicates a change over from the southwesterly wind regime.

Commencement of NE monsoon rains
i) Withdrawal of SW monsoon upto Lat. 15ºN.
ii) Onset of persistent surface easterlies over Tamil Nadu coast.
iii) Depth of easterlies upto 850 hPa over Tamil Nadu coast.
iv) Fairly widespread rainfall over coastal Tamil Nadu, south coastal Andhra Pradesh and adjoining areas.

Criteria for Declaring Onset of Northeast Monsoon
For declaring onset of Northeast Monsoon following criteria may be considered:
(1) Withdrawal of south west Monsoon upto 15º N.
(2) Onset of persistent surface easterlies over Tamil Nadu coast.
(3) Depth of easterlies upto 850 hpa over Tamil Nadu coast.
(4) Fairly widespread rainfall over the coastal Tamil Nadu and adjoining areas.
(5) Onset is not to be declared before 10th October even, if the conditions described above exist.

Specifications for activity/ strength of Northeast Monsoon
Weak Monsoon: Rainfall less than half the normal.
Normal Monsoon: Rainfall half to less than one and a half (1½) times the normal.
Active Monsoon: i) Rainfall 1½ to 4 times the normal.
ii) Rainfall in atleast two stations should be 3 cm in Coastal Tamil Nadu and south coastal Andhra Pradesh and 2 cm elsewhere.
iii) Rainfall in that sub-division should be fairly widespread or widespread.
Vigorous Monsoon :
i) Rainfall exceeding 4 times the normal.
ii) Rainfall in atleast two stations should be 5 cm in Coastal Tamil Nadu and south coastal Andhra Pradesh and 3 cm elsewhere.
iii) Rainfall in that sub-division should be fairly widespread or widespread.

Strength of Southwest Monsoon
Descriptive term over the Sea
Wind speed (in knots) reported or inferred to be existing.
Weak monsoon Upto 12 knots
Moderate monsoon 13 to 22 knots
Strong monsoon 23 to 32 knots
Vigorous monsoon 33 knots and above.

Descriptive term over land area
Weak monsoon: Rainfall less than half of the normal.
Normal monsoon: Rainfall half to less than 1½ times the normal (mention of ‘normal’ monsoon may not be necessary in general).
Active/ strong monsoon:
i) Rainfall 1 ½ to 4 times the normal.
ii) The rainfall in at least two stations should be 5 cm, if that sub-division is along the west coast and 3 cm, if it is elsewhere.
iii) Rainfall in that sub-division should be fairly widespread to widespread.
Vigorous monsoon:
i) Rainfall more than 4 times the normal.
ii) The rainfall in at least two stations should be 8 cm if the sub-division is along the west coast and 5 cm if it is
elsewhere.
iii) Rainfall in that sub-division should be fairly widespread or widespread

While describing the activity of the monsoon, i) The normals of stations, when ever available should be used. ii) Till normals for all the stations are available the following procedure should be adopted:

Number of stations in a sub-division with normals a
Normal for these stations b
Average normal for the sub-division b/a
Total number of stations reporting rainfall c
Actual total rainfall reported by these stations d
Therefore, the average rainfall for the sub-division d/c
Compare d/c with b/a and describe the activity of the monsoon accordingly, other conditions being fulfilled.

In the sub-divisions, where the percentage of hill stations is high, the hill stations must be also taken into account for describing the activity of the monsoon. In other sub-divisions, the hill stations will be excluded.

The monsoon activity will be described in all the sub-divisions of northeast India as is done for sub-divisions of other regions.

The monsoon activity need not be described over the Bay Islands and the Arabian Sea Islands.

Subdued Monsoon activity
1. Whenever, a country wide dry spell continues for more than 3 days and is expected to continue for atleast 2 more days, the description of monsoon should be given as ‘weak or subdued monsoon for the country as a whole. RC level bulletins may also include this information.

2. Information on prolonged dry spells/ subdued activity over the met. subdivisions also should be included in the RMC/ MC level bulletins if such a spell is persisting for 2 or more days and is expected to continue.

3. Monsoon activity for a sub-division might be considered subdued if the rainfall realised is less than ½ the normal.

Nearby pages
Monsoonal, Monstrance, Mont Blanc, Mont Cenis, Mont Cervin, Mont de Piete, Mont St Michel