The Clouds – Part 2_ Why clouds are mostly white?

Recalling my trip to New York by plane back in 2019, as I gazed out the window into the whiteness, I found it hard to reconcile a cloud as being fundamentally liquid. The individual droplets that make up a cloud are of course too small to see, but they are also transparent. So why are clouds white?

Well, while light from the sun passes straight through many of the droplets in a cloud, sooner or later it will hit a droplet and be reflected, just as the sun is reflected off the surface of a lake. This bounces the light off in another direction, so it hits another droplet and is reflected again. This continues, and the ray of light is bounced around like a pinball until it leaves the cloud. When it finally reaches your eyes, you see a pinprick of light originating from the last droplet of water that the light bounced off. The same happens to all the other rays of light that hit the cloud, so that what your eye sees is billions of pinpricks of light originating from all over the cloud. Some will have taken longer routes and lost their brightness, and so that part of the cloud will appear darker. Your brain tries to make sense of all these pinpricks of light. It is used to interpreting shades of light and dark in reference to a 3-dimensional object, which has material characteristics that correlate with what you are seeing. This is why clouds appear to be objects, sometimes fluffy as if made of wool, and sometimes denser, as if they might be a floating mountain. Of course, another bit of your brain denies all this, and points out to your subconscious that these are not objects at all, but tricks of the light. Still, even knowing this, it’s hard to see clouds as just an agglomeration of water droplets.

Much of the beauty of the sky is due to clouds and their water content. It affects the light we perceive in myriad ways and is one of the main reasons why different places in the world are so sublimely different in terms of light. But as the tiny droplets that make up a cloud become more dense, it becomes harder and harder for light to bounce its way through from top to bottom, and the cloud appears dark gray. We all know what this means, especially in Britain—it’s going to rain. The tiny spherical water drops that are floating in the cloud start to get bigger, and gravity begins to exert a greater force on them. When the droplets are just the size of tiny dust particles, buoyancy and air convection currents exert a far greater force on them than gravity does, so they just float around like dust. But as they get bigger, gravity starts to dominate, pulling them down toward Earth and turning them into rain. If we’re lucky, that is; otherwise they may form a storm cloud, the very storm clouds that kill hundreds of people every year.

Storm clouds are made under a very particular set of circumstances. As droplets experience cold air, water vapor changes from a gas back into a liquid. It’s the opposite of what happens when your wet clothes dry on a clothesline. In doing so, it gives off energy in the form of heat—we call this latent heat. Latent heat is emitted from H2O molecules while they’re still inside the cloud, meaning the air in the cloud gets warmer. As we know, warm air rises, so the cloud bulges out at the top. That’s how puffy cumulus clouds are made. But if all that happens while a lot of warm, humid air is rising up from the ground—as might happen on a summer’s day—then the convection currents pushing the cloud droplets upward might be strong enough to reverse the rain and send it upward too; the droplets will go miles into the sky, until the air carrying them finally cools enough to stop rising. That high up in the atmosphere, the rain droplets freeze, become ice particles, and then fall again, but depending on the climatic conditions, they might be pushed upward again by still more warm air. Meanwhile the cloud is getting bigger and taller, and darker and darker, a cumulus cloud being transformed into a cumulonimbus cloud —a storm cloud. The convection currents pushing the droplets up increase to speeds of 100km/h, and the cloud becomes a complex swirl of activity, with ice particles falling through an updraft of air that’s carrying still more droplets, all of which are colliding violently over several miles.

The scientific community still isn’t sure how the conditions inside a cumulonimbus cloud lead to the buildup of electric charge. But we do know that, as it does on the ground, the electricity arises due to the movement of charged particles, which originate from atoms. All atoms share a common structure: a central nucleus containing positively charged particles called protons, surrounded by negatively charged particles called electrons. Occasionally some of the electrons break free and start moving around; this is the basis of electricity. When you rub a balloon on a wool sweater, you create charged particles on the balloon. Then, if you hold that balloon up to your head, your hair will move in response to the charges on the balloon, attracting the opposite charges on your hair. Negative charge ultimately wants to be reunited with positive charge, and it stretches your hair toward the balloon in order to accomplish this, causing your hair to stand on end. If the amount of charge were higher, there would be enough energy for the charged particles to jump through the air, creating a spark.

In a cloud, instead of gently rubbing balloons you have water droplets and ice particles, all turbulently crashing into one another with tons of energy, giving some of the ice particles a positive charge as they’re carried to the top of the cloud, and some of the raindrops a negative charge as they fall to the bottom. This separation of positive and negative charges over many miles of cloud is driven by the energy of the winds inside the cloud. But the attractive force between the positive and negative is still there—they want to get back together, which is to say there is a voltage building up inside the cloud. It can get so large, reaching hundreds of millions of volts, that it strips electrons away from the molecules in the air itself. When this occurs, it happens very quickly, triggering the release of an electric charge that flows between the cloud and Earth, or between the top and the bottom of the cloud, depending on the conditions. The discharge is so big that it glows white-hot -it’s lightning. And thunder is the sonic boom of the surrounding air rapidly expanding as it’s heated to tens of thousands of degrees in temperature.

The energy of lightning is so huge that it can and does vaporize people, hence the high death toll. Electricity always flows down the path of least resistance—it’s like a liquid in that respect. But while liquids flow down gravitational fields, electricity flows down electric fields, and since air doesn’t conduct electricity very well, it has high resistance to the flow of electricity. Humans, on the other hand, are composed mostly of water, which does conduct electricity well. So if you’re a lightning bolt emanating from a thundercloud, trying to find the path of least resistance to Earth, a person is often your best vehicle. While lightning might prefer to go through a tree because it’s taller and longer, and thus more of the conductive path can go through its watery branches, if a person is sheltering under that tree, then the lightning might, and often does, jump over to the person on the last part of its journey to Earth.

Throughout a lot of the world, the tallest structures are often buildings, and in the West, for a long time, the tallest building in any town or city was a church. Many early church spires were made of wood, and they would burst into flames when lightning hit them. Fortunately, in 1749, Benjamin Franklin realized that if you just placed a metal electrical conductor on top of buildings and connected that to the ground with a piece of conducting wire, you’d give the lightning an easier path down and thus avoid a lot of the destruction caused by lightning strikes. These conducting wires are still used today and continue to save hundreds of thousands of tall buildings from being damaged by lightning. The same principle explains why being inside a car protects you from lightning: if the lightning strikes the car, it will be conducted around the outside of the metal bodywork, a path less resistant than going through the passengers.Which brings us to aircraft and the dangers of lightning. When an aircraft is flying through a storm cloud, the turbulent air causes the aircraft to shake and roll, to drop or rise suddenly as the pressure changes. If, in the midst of this, there’s lightning in the clouds, the plane will most likely become a part of that lightning’s conductive path.

As we know, many older aircraft are built from an aluminum alloy fuselage, and, as it would in a car, the metal protects the passengers from the lightning’s charge. But the carbon fiber composites that modern passenger aircraft are made of don’t conduct electricity very well (the epoxy glue holding the carbon fibers together is an electrical insulator), so, to compensate for this, aircraft-grade carbon fiber has conductive metal fibers built into its composite structure, ensuring that when lightning strikes, it travels around the skin of the aircraft and doesn’t harm the passengers. So while aircraft get struck fairly frequently—once a year on average—there haven’t been any recorded accidents on planes as a result of lightning strikes in over 50 years. In other words, it’s more dangerous to be on the ground, under a tree, during a lightning storm than in an airplane. They don’t mention this in the preflight safety briefing, even though it makes flying much safer. But—as already discussed—the preflight briefing is not really about safety.

By now my plane was getting pretty close to the ground, relatively speaking. As we continued to reduce altitude on our approach to JFK International Airport, the low clouds kept us from seeing much out the window. The New York Area is sometimes prone to fog. Fog, like clouds, is a liquid dispersion of water droplets in air: it’s essentially a cloud at ground level. Fog seems harmless enough if you’re looking out at it from a snug house, warmed by a log fire, sipping a glass of brandy—it gives the city an air of romanticism, a sense that mysterious new things are possible. But if you’re walking on a moor or driving on a highway or skiing down a mountain or descending at a rate of 9m/s in an airplane, fog means just one thing—potential death.

The history of sea fogs and ships dashed upon rocks as a result of the crew’s not being able to see these hazards is still a very real and frightening part of seafaring life. Fog can close airports and can cause airplanes, unless they are fitted with modern fly-by-wire systems, to abort landings. Fog is scary, fog is dangerous—and perhaps that’s why celebrations of the dead, like Halloween, are often held at times of the year when fog and mist are prevalent.

Fog forms at ground level for the same reason that clouds form in the sky. Damp, humid air cools, and as a result the H2O in the air liquefies into fine droplets of water. Just like at high altitudes, the formation of droplets requires a site of nucleation, and traditionally, in cities, that was provided by smoke from fires that were used for cooking or for keeping houses warm. But in modern times the nucleation sites usually stem from the exhaust from factory chimneys and cars. When there’s a chronic excess of that kind of pollution, a thick fog called smog will form, often hovering for days on end, capturing the pollution and keeping it above the city.

In London, the recorded history of smog goes back to 1306, when King Edward I banned coal fires for a period of time to try to combat the problem. The smog got so bad, at times you couldn’t see your hand in front of your face. But despite Edward’s efforts, smog continued to plague London for centuries, until the Great Smog of 1952, which was so lethal that it killed four thousand people in just four days, provoking the government to pass the country’s first clean-air laws. New York often experiences dense fog. This is due to a combination of conditions that bring the warm, damp air of the Atlantic Ocean over the city, where it then cools and condenses into fog as a result of car exhaust emissions.We were descending into just such a fog now, and despite knowing that those at the controls of the aircraft and on the ground at the airport are accustomed to these conditions and know how to make a safe landing in them, I felt myself becoming increasingly anxious as we continued to descend toward the ground while outside there was nothing to see but white spookiness.

Then, bong!! went the intercom:

“Flight attendants, please prepare for landing.”

The safety-critical moment had arrived—we were going in. The cabin fell silent, except for the drone of engines and the blast of the air conditioning. Everyone seemed to be tuning in to the same anxiety. Occasionally the fog would clear enough for me to catch sight of some feature on the ground, a tree or a car, but then the whiteness would reassert itself, and the aircraft would wobble or drop as the engines warbled away into my fearful ear. As we got lower and lower, I felt more and more tense. I know, rationally, that flying is the safest form of long-distance travel, but I’m always worried about being the exception. The deadly fog was outside. We were all strapped in, including the cabin staff, who were looking out at us impassively. They did this several times a week. How did they cope, I wondered, with this last part of the flight, this part when it’s clear that our lives are in the hands of the pilots’ ability to cope with something unseen and unexpected?

When I 1st began to travel by plane many years ago I was worried all the time, but during the years I’ve got used and learnt to stay supercool and at least to look unaffected; During our descent to JKF Airport I was looking out of the window serenely, clearly completely confident that our imminent collision with the ground would be a success.

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