Monsoon drenches Bengaluru, but why is rain uneven over Karnataka coast?

The southwest monsoon reached Karnataka in early June, yet the rain has been widely uneven, soaking Bengaluru while the coast and the Malnad hills stay parched. Here is the simple science behind the patchy rains and what the IMD expects next.

In Short

• Karnataka rainfall is 35 per cent below normal this June.
• Coastal Karnataka and Malnad are short by half this month.
• Bengaluru stays wet, thanks to its own daily afternoon thunderstorms.

Bengaluru has had its share of grey skies and afternoon showers this fortnight. Yet drive a few hours west, towards the coast and the green hills of the Malnad, and the land is still waiting. The rain simply has not come.

The southwest monsoon, India's main rainy season, reached Karnataka in early June, sweeping in from the Arabian Sea.

But its arrival has been generous in some places and stingy in others. Between June 1 and June 18, the state received just 71 mm of rain against a normal of 109 mm, a shortfall of 35 per cent, according to the Karnataka State Natural Disaster Monitoring Centre.

The gap is widest where it should be smallest. Coastal Karnataka, usually soaked by now, is running 54 per cent below normal.

Malnad, the lush hill country that feeds the state's rivers, is short by half. Bengaluru, curiously, has fared better, sitting close to its modest June average.

So why is the city damp while the hills stay thirsty?

WHAT INFLUENCES KARNATAKA'S MONSOON

Most of Karnataka's monsoon rain is influenced by geography. As moist sea winds rush inland, they slam into the Western Ghats, the long wall of hills lining the coast, and are forced upwards.

As the air climbs, it cools, its moisture condenses, and rain falls in sheets. Scientists call this orographic rainfall, which simply means rain produced when land pushes air upwards.

For this machine to run, it needs a strong, steady stream of wind blowing in from the sea, known to meteorologists as the low-level jet. This year, that wind has been weak and fitful.

The seasonal trough along the west coast, a band of low pressure that usually drags rain ashore, has stayed largely asleep. Without a firm shove of sea air, the Ghats cannot wring out their usual downpour, and the coast and Malnad are left dry.

WHY BENGALURU IS RECEIVING RAIN

Bengaluru sits inland, in the rain shadow of those same hills, so it never leaned much on that sea-driven deluge.

Its showers come instead from convection.

The daytime Sun heats the ground, warm air rises and swells into towering clouds, and by late afternoon, they burst into a thundershower.

These storms are local and unpredictable, but they have fired often enough to keep the city wet.

THE BIGGER WORRY

The India Meteorological Department expects this monsoon to be below normal overall, at 90 per cent of the long-term average.

For now, weak La Nina conditions in the Pacific are fading towards neutral. But forecasters expect El Nino, a warming of the Pacific Ocean that tends to choke India's monsoon, to develop later in the season.

There is some relief in sight. The IMD expects rain to revive along the coast in the coming days, and the hills may yet drink their fill.

For a state that banks on these months to fill its reservoirs and water its crops, every quiet week counts.

- Ends