How one weather system buried Rajasthan in dust, then brought rain

At midday on Saturday, May 30, the sky over Churu turned orange. A wall of dust, driven by winds of about 56 kmph off the Thar Desert, rolled across northern Rajasthan, cutting visibility to near zero and forcing drivers to switch on their headlights in broad daylight. A day later, the dust still hung over the region. The European Union's Copernicus Atmosphere Monitoring Service put the dust over Rajasthan at about 1.7 around noon on Sunday in its forecast, a level normally seen during severe dust storms. Read Full Story The rain that chased it came from the same weather system. An active western disturbance, paired with a cyclonic circulation over north-west India, lifted desert dust on its leading winds even as it pulled in the moisture that broke days of punishing heat, according to the India Meteorological Department. The system that choked Rajasthan's air on Saturday brought showers, and relief, to Punjab, Haryana and the plains beyond. But the desert did not let go: the Copernicus forecast keeps dust over Rajasthan elevated through the week, thickening again on Thursday, June 4. WHAT SATELLITES SEE Aerosol optical depth is a unitless gauge of how much airborne dust and haze dim sunlight. Clean air sits near 0.1, a hazy day is around 0.3 to 0.5, and a severe dust storm climbs past 1.0. Over Rajasthan, the storm and the dark patch on the map are the same event. THE DRIVER A western disturbance is an eastward-moving storm system that forms over the Mediterranean and carries rain to north-west India and Pakistan, mostly in winter, but also before the monsoon. In late May, colliding with the region's heat, it turned unstable. Gusty winds lifted Thar dust into a storm, while the same system's moisture fell as rain. The dust did not stay put. Over the days that followed, the Copernicus forecast tracked the plume drifting east and south across north-west India before the rain began to wipe it from the sky. THE COMPLICATION Relief and hazard arrived together. The rain that cooled the plains is what cleared the air, because showers wash dust out of the sky. That is why the bloom fades in the satellite record in the days after a storm. But the dust itself is a health load. It lifts coarse particulate matter known as PM10, fine enough to irritate the eyes and lungs, and dense enough on Saturday to halt traffic. The same weather that brought comfort to one state brought near-darkness to another. THE STAKES Pre-monsoon dust storms are a recurring hazard across north-west India. Researchers have associated more frequent and intense dust events with drier conditions and shifting wind patterns in a warming climate, a trend NASA and Copernicus have documented for desert dust reaching Europe. As deserts dry and heat builds, the winds that loft this dust are the same ones India will see each summer. In the days that followed, the rain scrubbed dust from the air over Punjab, and the forecast showed the bloom thinning. The orange sky over Churu has cleared — until the next hot wind off the Thar. Ends