Monsoon at Kerala's door, but knock not strong enough: What's missing?

Kerala is already witnessing heavy rain, but the IMD is still waiting for a few crucial atmospheric boxes to be ticked before officially declaring the monsoon's arrival. Here is what's required.

In Short

• Southwest monsoon expected to arrive over Kerala on June 4
• IMD issues orange alert for heavy rain in parts of Kerala
• Monsoon not declared yet due to incomplete atmospheric conditions

Despite heavy rains and wind sweeping Kerala and other regions, the Indian Meteorological Department (IMD) has not yet declared the arrival of monsoon, as of 10:15 am on June 4.

The southwest monsoon is expected to arrive over Kerala today, on June 4, marking the beginning of India's crucial rainy season after a brief delay from its usual onset date of June 1.

The IMD said conditions have become favourable for the monsoon to advance into parts of Kerala, Tamil Nadu and adjoining areas of the Arabian Sea and Bay of Bengal.

On Thursday morning, heavy rain continued to batter several parts of the state, with the IMD issuing an orange alert for Alappuzha, Kottayam and Ernakulam districts for three hours on Thursday morning, warning of thunderstorms, heavy rainfall and surface winds that could reach speeds of up to 40 kmph.

Then, why has the weather department not announced the arrival of the southwest monsoon?

WHY HAS IMD NOT DECLARED THE MONSOON?

Despite widespread rain across Kerala over the past two days, the IMD does not declare the onset of the monsoon based on rainfall alone.

The weather department follows a set of guidelines adopted in 2016 that require three separate conditions to be met before the monsoon can be officially declared.

According to reports, the rainfall requirement appears to have been satisfied. However, forecasters were still assessing whether the remaining atmospheric conditions, having to do with the monsoon winds and cloud cover, had fully reached the required thresholds.

These checks help the IMD confirm that the rains are part of a sustained seasonal shift rather than a temporary spell of wet weather.

As a result, even though Kerala is already experiencing monsoon-like conditions, the official declaration is still awaited.

That then brings the focus on what these thresholds are.

WHAT ARE IMD'S CONDITIONS TO DECLARE MONSOON?

The first requirement is widespread rainfall, requiring at least 60% of 14 selected weather stations across Kerala, Lakshadweep and coastal Karnataka to record 2.5 mm or more rainfall for two consecutive days.

The 14 selected weather stations include, Minicoy, Thiruvananthapuram, Punalur, Amini, Kollam, Kottayam, Kochi, Alappuzha, Thrissur, Thalassery, Kannur, Kozhikode, Kasaragod and Mangaluru.

Then comes the second requirement, which relates to the monsoon winds.

These moisture-filled winds blow in from the Arabian Sea and bring rain towards India. The IMD checks whether the winds are strong enough and spread high enough into the atmosphere to support a sustained monsoon advance.

More specifically, the winds must be strong and deep enough to reach up to 600-hectopascal level in the atmosphere, which is roughly about 4 kilometres above the Earth's surface.

The third and the final requirement has to do with cloud cover.

Using satellites, the IMD monitors the extent of thick rain-bearing clouds over the region, as a well-developed cloud pattern is one of the key signs that the monsoon system has become established.

The weather agency monitors the cloud cover using Outgoing Longwave Radiation (OLR), a metric used to measure the amount of heat escaping from the Earth into space.

In simple terms, lower OLR values indicate that the skies are covered by dense clouds capable of producing widespread rain.

The monsoon onset criteria demand OLR to be below 200 watts per square metre.

RAIN IS HERE, ALMOST

These criteria are what stands between the ongoing rain showers in Kerala being deemed as the arrival of monsoon.

Once the three conditions align, the IMD will declare the onset over Kerala, and will continue to monitor the monsoon as it travels across the country.

After arriving over Kerala, the southwest monsoon typically advances northward in stages over the next four to six weeks. It usually reaches coastal Karnataka, Goa and parts of northeastern India within the first week of onset, before spreading across Maharashtra, Telangana, Andhra Pradesh, Chhattisgarh and eastern India through June.

The monsoon generally arrives in Delhi around June 27 and covers most of northwest India by the end of June or early July.

Under normal conditions, it reaches the farthest parts of Rajasthan and Punjab by around July 8, completing its journey across the country.

- Ends