Source : INDIA TODAY NEWS
Something is happening in the upper atmosphere above northwest India right now.
A massive weather system is moving in from the Mediterranean Sea, thousands of kilometres away, and it is about to collide spectacularly with one of the hottest stretches of land on Earth.
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The result, according to the India Meteorological Department (IMD), will be dust storms, thundershowers, and spells of rain across Rajasthan, Punjab, Haryana, Delhi, and western Uttar Pradesh from Sunday through Tuesday.
What is this system? Why is it doing this? And why, every time it arrives, do you get a wall of dust before any rain falls?
WHAT IS A WESTERN DISTURBANCE?
A Western Disturbance is a weather system that originates over the Mediterranean Sea.
It is an extratropical cyclone, which simply means it is born in the mid-latitudes, the temperate band that sits between the tropics and the poles, rather than in the hot tropical belt where monsoon systems form.
Once born, it rides the subtropical westerly jet stream, which is a powerful band of high-altitude winds that flows like a river across the sky at roughly nine to 12 kilometres above the Earth’s surface.
These winds carry the system eastward at speeds of up to 43 kilometres per hour, across West Asia, Iran, Afghanistan, and Pakistan, before it enters northwest India.
As it travels, it carries moisture into the upper atmosphere, the cooler layers of the sky, high above the surface. This is different from the monsoon, which carries moisture in the lower, denser layers of the atmosphere closer to the ground.
According to the IMD’s extended range forecast for June 18 to July 1, a fresh Western Disturbance is expected to affect the western Himalayan region and adjoining northwest plains next week.
THE SCIENCE OF THE WALL OF DUST
There is science behind why a dust storm almost always arrives before the rain does.
Northwest India is extraordinarily hot in June. The land heats rapidly through the day, and by afternoon, the surface temperatures in Rajasthan can exceed 40 degrees Celsius.
This intense heat causes the air near the ground to become lighter than the cooler air above it, creating a deeply unstable atmosphere.
The atmosphere has a technical term for the energy stored in this instability: Convective Available Potential Energy, or CAPE. Think of it as the latent violence in a pressure cooker waiting to release.
When the Western Disturbance pushes cooler, denser air into this setup from above, the contrast becomes explosive.
Thunderstorms develop rapidly. As rain forms inside these storm clouds and begins to fall, it passes through extremely dry, hot air beneath the clouds. Much of the rain evaporates before it ever reaches the ground.
This evaporation chills the air around the falling droplets, creating pockets of rapidly cooling, sinking air called downdrafts.
These cold air masses plunge toward the surface at great speed.
When they hit the ground, they spread outward in all directions, like water from a tap hitting a flat plate. The outward-rushing winds can gust anywhere from 60 to 80 kilometres per hour.
In Rajasthan’s Thar Desert, the soil is dry, loose, and fine. Those winds lift the soil straight into the sky, creating the characteristic wall of dust, called an aandhi, that locals know is the sign of a storm approaching.
WHY DO THUNDERSTORMS MOSTLY OCCUR DURING EVENINGS AND NIGHTS?
Thunderstorms during this season follow a remarkably consistent daily rhythm, and the science behind it is elegant.
Solar radiation heats the surface maximally in the afternoon, which is when the atmosphere reaches its highest CAPE.
As the evening arrives, that stored energy discharges through convection, which is the rapid upward movement of warm, moist air that triggers thunderstorm cells.
Storm clusters that form over the Aravallis and the western Himalayas during the evening then organise themselves into larger systems and move eastward during the night, reaching Delhi, Haryana, Punjab, and western Uttar Pradesh in the late evening or early morning hours.
This is why IMD forecasts for Delhi show the possibility of thunderstorms gusting to 60 kilometres per hour arriving towards the afternoon and evening on June 21, with maximum temperatures in the 38-to-40-degree-Celsius range.
RAJASTHAN GETS IT WORSE. HERE IS WHY
The IMD forecast for Rajasthan includes thundersqualls with wind speeds reaching 70 to 80 kilometres per hour, significantly stronger than the activity expected over Delhi. This is not random.
Rajasthan sits directly in the path of the incoming disturbance and is also the hottest, driest part of northwest India. Desert heating creates the strongest surface instability.
The Thar Desert’s loose soils supply the most dust. And the absence of significant vegetation means there is nothing to absorb the wind energy.
According to IMD data, Rajasthan also experienced some of the most extreme high temperatures earlier this week, with departures well above normal. That accumulated heat is the fuel this incoming system will ignite.
For most of the northwest plains, the IMD’s extended-range forecast confirms that rainfall will remain below normal for the country as a whole during the coming week, with total seasonal rainfall already running 46 per cent below average.
This disturbance will bring relief, but not rescue.
– Ends
SOURCE :- TIMES OF INDIA
