On June 15, 2018, residents of Kathmandu were caught by surprise as dark clouds gathered over the city. The clouds soon led to rain. But when the rain stopped, it left behind a brown slurry on the terraces, gardens and clothes of people who had not managed to take shelter from the downpour.
Nepal’s department of meteorology said that the “muddy rain” contained dust carried by strong winds from the Thar desert in western India, where more than 100 people had died in deadly sandstorms a month earlier. But officials could not explain how this could happen a week into the monsoon when moisture-laden clouds usually move in from the east.
A recent paper from scientists at the Kathmandu Centre for Research and Education, however, has shed light on the strange event that took place in Nepal’s capital two years ago. “Officials did not discuss the atmospheric dynamics regarding the processes responsible for causing that muddy rain,” the authors of the paper wrote. “That’s why we have explained the mechanism behind the event using freely available information from various sources,” said Ashok Pokharel, an atmospheric scientist who led the study.
According to meteorologists, monsoon clouds enter eastern Nepal around June 10. But in 2018, they did so two days ahead of the usual date. This implies that the westerly winds brought dust to Nepal, even after the monsoon had already started. With the desert along the India-Pakistan border superheated due to climate change, a lot more dust and sand swirls into the atmosphere above it, and a strong wind can carry it all the way to Nepal, around 1,500 km away. There is insufficient scientific evidence to conclude that winds blowing into South Asia from the Arabian Sea are getting stronger in non-monsoon months, but scientists say climate change is making the wind patterns more erratic and unpredictable.
So more muddy rain cannot be ruled out. Can scientists get data to study this phenomenon?
“Getting atmospheric data from Nepal was not possible as most of our data comes from ground stations,” said Pokharel. “That’s why we decided to use other freely available data from India and data from various other sources,” he added.
The researchers examined the events leading up to the muddy rain, using various resources such as the Moderate Resolution Imaging Spectroradiometer (MODIS)/Terra, Modern Era Retrospective-Analysis for Research and Applications (MERRA) and the Navy Aerosol Analysis and Prediction System (NAAPS).
The researchers traced the source of the dust to near Shiv Badi in Jodhpur, Rajasthan and the Thar desert of both Rajasthan and Gujarat states of India.
The study found that it all began around June 12, 2018, when big surges of dust triggered by thunderstorms were recorded in the far western parts of India, especially Ahmedabad and its surrounding areas. The storms continued intermittently until June 15, lifting the desert dust into the upper atmosphere. The dust was then carried by prevailing north-westerly winds towards Nepal, where it fell as muddy rain.
“What is interesting to note here is that even during the monsoon season, the wind was strong enough to advect [carry] the dust into Nepal,” said Pokharel. This has not happened in the past, he added.
Nepal’s weather, like other areas in the north of the Indian subcontinent, is dictated by two wind currents, one from the east and the other from the west.
After September/October the south-westerly monsoon, which originates in the Indian Ocean and moves into Nepal from the east, recedes and the westerly winds take over. During the winter months, the westerly winds that come from the Mediterranean gather moisture to form westerly disturbances, which bring rain to western Nepal and other parts of the country. But when the summer sets in, monsoon clouds return and push the westerlies back.
At the same time, the south-western jet stream – massive ribbons of fast winds traversing the earth from west to east – plays a crucial role. During the summer, it is active in regions north of the Himalayas. But during the winter, the jet stream moves near the Himalayas as well as the Mediterranean region. When the summer sets in, the jet moves north and moisture from the Mediterranean does not get to hitch a ride to the eastern areas, including Nepal.
Changing winds in a warming world
Various studies have shown that westerly winds are changing as the world heats up. Most climatic models suggest that in a warmer world, western disturbances will hold more moisture, said Keiran Hunt from the University of Reading in the UK. He said that while various models also suggest that the jet stream will become wider, increasing precipitation on the Tibetan side of the Himalayas, researchers are yet to fully understand how western disturbances interact with the monsoon, and whether western disturbances are taking a longer time to weaken in a warmer world. The Uttarakhand floods in 2013 have also been attributed to the interaction between a late receding western disturbance and an advancing monsoon.
Studies have also indicated that the frequency of sandstorms in western India is likely to increase due to climate change as both westerly winds and monsoon clouds hold more moisture and the deserts heat up due to rising temperatures.
The Intergovernmental Panel on Climate Change (IPCC) Special Report on Climate Change and Land noted, “The frequency and intensity of dust storms have increased over the last few decades due to land use and land cover changes and climate-related factors in many dryland areas resulting in increasing negative impacts on human health, in regions such as the Arabian Peninsula and broader Middle East, Central Asia.”
Meanwhile, a few hours after the muddy rain fell in Kathmandu, Nepal’s meteorology department said that the mud was not harmful and would soon wash away. But Pokharel said that is not the case.
“The mud brought soil from the desert, which might contain various microbes,” he explained. “The microbes foreign to Nepal could have instigated an outbreak in plants, animals or humans. Also, the mud rain would have altered the pH balance of the soil.”
Pokharel said that though Nepal faces a wide range of potential extreme weather events, such as sandstorms and muddy rain, the country’s weather forecasters remain focused on rain and sun. “There’s so much information available for free these days, all one has to do is collate them,” he added.