Saturday 10 December 2022
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EducationPhysicsErratic weather conditions, failing rainfall forecasts explained

Erratic weather conditions, failing rainfall forecasts explained

The monsoon is holding a lot of unpleasant surprises this year. While it is below average in some parts of the country, the rainfall has surpassed previous records in some other parts. What is worrisome, like in the 1970s and 80s, the Indian Meteorological Department (IMD) is getting the forecasts wrong. This, despite the advanced equipment the weather offices in India have now.

It is disconcerting because torrential annual rains are the lifeblood of Indian farming, which remains the most significant source of jobs in the country since the hyped Green Revolution that introduced farming even in areas that are situated far away from rivers where ancient and mediaeval Indians used to practise farming. The intensity and timing of the summer monsoons, which were never particularly regular, are becoming even more erratic. It’s also a problem for bankers who will be flooded with loan applications from undeserving farmers followed by populist politicians' act of waiving loan repayments.

What are the factors determining the weather, especially rainfall patterns?

The factors are varied, depending on the topography. For example, in southern India right now, strong Westerly winds are flowing in from the Arabian Sea, bringing a lot of moisture along.

The second factor is the presence of an east-west shear zone located 10° north over the southern peninsula. This zone allows active winds of high speeds to interact. It also allows monsoon winds to remain active, thus causing intense rainfall.

But in the north, say NCR, the rainfall has been relatively low, one may say "poor". And it's erratic. However, it's relative to the south. In absolute terms, Delhi has broken its record. The normal rainfall level in Delhi is 258 mm, out of which 242 mm has already happened.

The factor in north India is the absence of any low-pressure systems or western disturbances close to the capital. Rainfall is caused in N India also due to something called the "monsoon trough". In the language of physics, a trough is the lower part of a wave and crest the upper part. In the context of weather, it is the minimum sea level pressure. If the three seas surrounding the Indian peninsula experience this, the impact is felt across the country. But here too, the eastern parts experience it the most, followed by southern parts and then western and northern parts.

How much is excess rainfall?

If the rainfall received in an area is higher than the average it received over the past 50 years, it may be said to be excess or surplus rainfall.

How is rainfall collected?

The standard instrument for the measurement of rainfall is the 203 mm rain gauge. This is essentially a circular funnel with a diameter of 203 mm which collects the rain into a graduated and calibrated cylinder. The measuring cylinder can record up to 25 mm of precipitation. Any excess precipitation is captured in the outer metal cylinder. The top of the rain gauge is 0.3 m above the ground.

What predictions for the coming 1-2 months in what is left of the monsoon?

It will be average in August and September. Technically, "average" or "normal" rainfall means 96%-104% of a 50-year average, which is 89 cm.

In spite of heavy rainfall in Assam and regions around it that resulted in floods, still, most parts of the northeast received below-normal rainfall in June and July. Why has the monsoon been unpredictable this around?

We had earlier explained the Assam flood situation and look at the irony! We're now saying Assam has not received enough rainfall. It is actually not uniform across the Northeast. Usually, the North East receives about 20-25% of its annual rainfall in the pre-monsoon period. But this year, it was 62% above average — the highest in the last 10 years, according to India Meteorological Department.

Why has the IMD forecast been off the mark?

The IMD will take exception to this statement. As Sirf News discussed in the explainer on the heat wave in northern India, the department now has very advanced equipment to forecast weather.

The error margin for monsoon forecasts has declined, from 7.94% between 1995 and 2006 to 5.95% between 2007 and 2018. Even so, over the last 11 years, the actual rainfall matched the forecast only half the time.

Actually, no natural phenomenon can be predicted accurately; it’s even more difficult when change causes aberrations. There are statistical and dynamic models that are prone to errors because the variables they depend on fluctuate.

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