• The monsoon’s arrival over Kerala has been delayed to June 3, according to an update by the India Meteorological Department (IMD).
• Private weather forecast agency, Skymet, however, said that the monsoon had arrived.
• This was because two of the three criteria — as defined by the IMD — had been met.
• Currently, IMD’s own data indicated that except for the OLR, the other criteria were met. Thus, there is an element of subjectivity in arrival.

What are those criterias defined by IMD?

1. Rain-bearing westerlies being at a minimum depth and speed;
2. At least 60% of the available 14 stations in Kerala and coastal Karnataka, reporting rainfall of 2.5 mm or more for two consecutive days after May 10;
3. A certain degree of clouding, indicated by a parameter called ‘outgoing long wave radiation’ (OLR), being below 200 W/square meter.

What is meant by ‘Outgoing Long Wave Radiation’ (OLR)?

• Outgoing Long-wave Radiation (OLR) is electromagnetic radiation of wavelengths from 3–100 μm emitted from Earth and its atmosphere out to space in the form of thermal radiation.
• It is also referred to as up-welling long-wave radiation and terrestrial long-wave flux, among others.
• The flux of energy transported by outgoing long-wave radiation is measured in W/m.
• In the Earth’s climate system, long-wave radiation involves processes of absorption, scattering, and emissions from atmospheric gases, aerosols, clouds and the surface.
• Over 99% of outgoing long-wave radiation has wavelengths between 4 μm and 100 μm, in the thermal infrared part of the electromagnetic spectrum.

Factors responsible for south-west monsoon formation:

1. Intense heating of Tibetan plateau during summer months.
2. Permanent high pressure cell in the South Indian Ocean (east to north-east of Madagascar in summer).

Factors that influence the onset of south-west monsoons:

1. Above points +
2. Subtropical Jet Stream (STJ).
3. Tropical Easterly Jet (African Easterly Jet).
4. Inter Tropical Convergence Zone.

Factors that influence the intensity of south-west monsoons:

1. Strengths of Low pressure over Tibet and high pressure over southern Indian Ocean.
2. Somali Jet (Findlater Jet).
3. Somali Current (Findlater Current).
4. Indian Ocean branch of Walker Cell.
5. Indian Ocean Dipole.

Factors responsible for north-east monsoon formation:

1. Formation and strengthening of high pressure cells over Tibetan plateau and Siberian Plateau in winter.
2. Westward migration and subsequent weakening of high pressure cell in the Southern Indian Ocean.
3. Migration of ITCZ to the south of India.

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