The monsoon this year has been one of contradictions, alternating between spells of intense rainfall and prolonged dry periods.

The India Meteorological Department uses a Long Period Average, representing the average rainfall received by the country over a 50-year period, to measure current rainfall against. The percentage departure from that is how the Met department defines whether an area has received normal, deficient or excess rainfall.

This series of maps based on a week-by-week percentage departure of rainfall shows just how paradoxical the monsoon has been.

Graphic: Anand Katakam
Graphic: Anand Katakam

One paradox this monsoon is Tamil Nadu, which received much higher than normal rainfall in the week ending July 4, but has remained dry since. Kerala faced intense rain twice before in this season, followed by extended dry spells, as these more detailed maps show.

The scale on the map cannot adequately represent the departures from normal rainfall in a week that has seen eight districts across the three states receive 400% and more rainfall than normal. The heaviest rainfall this week has been concentrated in the Western Ghats region across all three states, even as other parts of the peninsula remained unusually dry.

It is of course important to remember that a high percentage of departure from the normal does not necessarily represent correspondingly high or low rainfall. For instance, Thirunelveli in Tamil Nadu received 2080% more rainfall than normal for the week ending August 15. But in real terms that means it received 100 millimetres of rain against a normal of 4 millimetres. Idukki in Kerala received 438% more than usual rain but with 679 millimetres in the same period.

While variations away from normal might be expected, for Kerala and parts of its neighbouring states, the last two weeks have been truly unprecedented. These are the worst floods in the state since 1924 and might yet surpass those. The normal rainfall for all of Kerala for the week ending August 15 is 1,366.9 millimetres. This year, the state received 4,591 millimetres of rain in the same period. Idukki and Wayanad districts alone received 1,215 millimetres of rain of that.