At any point, there are an estimated 12 trillion tonnes of water gathered up in the sky.

A single cloud can be so dense that one large cumulus (the fluffy, cotton-candy-like ones) can weigh 1,000 tonnes.

The shapes of these formations — and where in the atmosphere they end up (high above or low-hanging, which can be the difference between 2,000 and 12,000 metres from the surface) — determine whether the land beneath them heats up or cools down, because at different altitudes they can either reflect or absorb the sun’s heat.

“Clouds are a beautiful natural phenomenon, but the physics of how they work is incredibly complicated,” says Madhavan Nair Rajeevan, a weather scientist who worked with the India Meteorological Department and National Atmospheric Research Laboratory before retiring as a secretary with the union ministry of earth sciences.

The 63-year-old has spent almost four decades studying the intricacies of the Indian monsoon. Clouds, he says, with a shake of his head, remain an enduring mystery.

Stratus check

What are the key questions he would like answered?

Top of his list, he says, is: What exactly makes a cloud rain? What makes the droplets of moisture suddenly coalesce?

We also don’t know how clouds grow, or what determines their size, shape and height.

We don’t know why they behave so differently at different latitudes.

“We get very heavy rain along both the Western Ghats and the coast of Myanmar, but despite looking similar, they’re very different cloud formations and phenomena,” Rajeevan says. “The Burma coast is all tall, convective formations, but over the Western Ghats the formations are shallow. Shallow cover usually leads to light or no precipitation. So how do we suddenly get so much heavy rain over the Western Ghats? We still don’t understand it.”

Blow hot, blow cold

How is it that we’re learning about Mars and the moons of Jupiter, and still know so little about clouds?

Part of the issue is that they are constantly shifting, and are hard to access.

To gather data, one must fly research aircraft or drones into them, an act that carries an inherent element of risk and failure (hail, lightning, extreme winds and ice accumulation are among the potential hazards).

Satellites can only offer a top-down view. They cannot see what’s going on inside. Combine that lack of data with the chaos of cloud physics and the fact that they’re affected by everything from sunlight to humidity levels to surface features, and it begins to become clear why weather models struggle to accurately simulate how these formations will move, where they will end up or how they will behave.

This is a problem scientists around the world are confronting, with a growing sense of urgency, amid the climate crisis.

If we cannot predict cloud behaviour, we cannot have accurate global climate models — because any accurate model must factor in the cloud feedback process.

And this, Rajeevan says, remains the “most uncertain parameter” of climate prediction. Put simply, the cloud feedback process refers to how rising temperatures affect the type and intensity of cloud formation, and how the type and intensity of these formations in turn affects how warming is amplified or slowed down.

Low-hanging clouds tend to block out and reflect incoming sunlight, and have a cooling effect. Higher clouds absorb more solar energy than they reflect, leading to more warming. As Earth warms, models are indicating that we will have decreased low cloud cover while high formations will rise even higher in the atmosphere.

We don’t know how accurate these predictions are; we just know that they are far from definitive, Rajeevan says.

And that is a problem. “Because even with the same amount of greenhouse gas emissions, predictions could be off by as much as 0.8 degrees over 50 years, because of the unaccounted-for impact of the cloud feedback process alone.”

Silver linings

There have been some advances, lately.

The European Space Agency’s Earth Cloud Aerosol and Radiation Explorer (or EarthCARE) satellite, launched last year, is using refined remote-sensing techniques in an attempt to capture more accurate observational data about the interactions between clouds, aerosols and solar and infrared radiation. Similar missions such as NASA’s PACE (Plankton, Aerosol, Cloud, Ocean Ecosystem) satellite are adding to the pool of information, and will hopefully lead to more robust datasets.

Artificial intelligence can then be deployed to analyse such datasets faster.

Here at home, the Indian Institute of Tropical Meteorology in Pune is building a “convective cloud chamber” to simulate and study monsoon formations, with the hope that this will help us understand these phenomena better, especially in the light of planned interventions such as cloud-seeding.

As we study them, however, the skies themselves are changing. Decades-long efforts to improve air quality, combined with sustained emissions, may be altering how clouds are formed, by altering the volumes of aerosols and other particulate matter in the atmosphere.

What this means, Rajeevan says, is that we really don’t know what Earth’s cloud cover could look or behave like in the future.

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CLOUDY… WITH A CHANCE OF STRANGE

How much drama is really unfolding up there? Well…

* Thunderstorm clouds can sometimes turn the sky green because of the interaction between sunlight and large water droplets and hail.

* The average cloud moves at 50 kmph, but ones at higher altitudes can move at up to 160 kmph — another reason they are so hard to study.

* How high can they go? Noctilucent or “night-shining” clouds are rare phenomena that occur about 80 km above sea level (not far from the Karman line that separates Earth from space, at 100 km). Only visible at twilight, they appear as delicate blue or silver streaks across the sky.