Weather Science E-Book

Brian Clegg

• Herausgeber: Icon Books
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 213

Veröffentlichungsjahr: 2024

Beschreibung

Everyone has an interest in the weather, whether it's to check the prospects for a day out or to know when best to harvest a crop. The Earth's weather systems also provide some of the most dramatic forces of nature, from the vast release of energy in a lightning flash to the devastating impact of tornadoes and hurricanes. For centuries, our only real guide to future weather was folklore, but with the introduction of the first weather forecasts and maps in Victorian times, attempts were made to give some warning of the weather to come. Until relatively recently, these forecasts could be wildly inaccurate - think of Michael Fish's denial that there was a storm on the way the night before the UK's great storm of 1987. This was due to the mathematically chaotic nature of weather systems, first discovered in the 1960s, the understanding of which would transform forecasting from the 1990s and mean that meteorologists became some of the foremost users of supercomputers. From the crystalline perfection of the snowflake to the transfer of energy from the Sun, science lies at the heart of the weather and our understanding of it. In recent years, weather science has moved to the leading edge with advanced modelling, versatile use of satellite data and a better understanding of mathematical chaos. This is a true example of hot science at work.

Leseprobe

For Gillian, Rebecca and Chelsea

 

 

Published in the UK and USA in 2024 by

Icon Books Ltd, Omnibus Business Centre,

39–41 North Road, London N7 9DP

email: [email protected]

www.iconbooks.com

ISBN: 978-183773-153-4

eBook: 978-183773-154-1

Text copyright © 2024 Icon Books

The author has asserted his moral rights.

No part of this book may be reproduced in any form, or by any means, without prior permission in writing from the publisher.

Typeset by SJmagic DESIGN SERVICES, India.

Printed and bound in the UK.

ACKNOWLEDGEMENTS

My thanks as always to the team at Icon Books, notably Connor Stait and Steve Burdett on the editorial side and Rhiannon Morris in publicity. As a lifelong inhabitant of England, the weather has always formed a significant part of my conversation – but researching the science has brought home its importance. Particular thanks to insights gained from the many meteorologists I met while working at the Met Office (as I signed the Official Secrets Act to work with them, if I told you who they were I’d have to shoot you), and to two local Wiltshire farmers who have impressed on me how much the farming life is at the mercy of the weather: Joe Smith and Stan Jones (not pseudonyms, despite the surnames).

CONTENTS

1The Eye of the Storm

2From Folklore to Forecasts

3The Power of the Sun

4Chaos Reigns

5Rain, Snow and Hail

6Storm

7Weather Plus

8Weather vs Climate

9Control

10Perfection and Limits

Further Reading

THE EYE OF THE STORM

1

There is a national stereotype that British people are obsessed with the weather. And there’s an element of truth in it. If all else fails, if conversation is faltering, we will instantly resort to commenting on how cold or wet or hot the weather has been of late (but rarely how good it has been). Yet this fascination we Brits have with the weather should hardly come as a surprise to anyone. After all, weather can all too easily make the difference between plenty and famine, survival and death. This is not a topic any human being can afford to ignore.

According to my dictionary, ‘weather’ refers to the condition of the atmosphere at a particular place and time (as opposed to climate, which has a wider spacetime span). As I write this, for example, the weather in Swindon, England, is sunny and dry, with clear blue skies, but it is also colder than usual for summer here, while southern Europe is suffering an intense heatwave.

Although the official definition of the weather focuses only on the state of the Earth’s atmosphere, which makes it seem limited to the behaviour of a few gases swirling above us, isolated from our everyday existence on the solid surface of the planet, the reality is very different. Weather is as much about what that atmosphere can throw at us on the ground, and about the impact it will have on our bodies, crops and habitat, as it is about the state of the air.

Getting atmospheric

We take the atmosphere for granted. It seems little more than a layer of gas stuck to the outside of the planet by gravity. But this layer forms a vastly complex, ever-changing system – which is why meteorology, the science of predicting the weather, is anything but an easy task.

The mix of gasses that makes up the atmosphere took a significant time to understand. Oxygen, that familiar gas essential for animal life, is around 21 per cent of atmospheric content, though there is far more nitrogen, at 78 per cent. Add to this a range of other lesser gases (notably argon and the greenhouse villain carbon dioxide), water both in the form of vapour (transparent gas) and tiny droplets, and a host of particles including plenty of bacteria, and you have the near-invisible but essential blanket that gives the Earth the potential for life. However, it does not have a uniform structure.

The atmosphere is conventionally divided into five layers, the stratosphere being the only one with a name that is frequently used. This sits above the layer that we live in, which has the less familiar designation of the troposphere. We stay in the troposphere for around the first 12 kilometres (7.5 miles) at sea level, though it can be less above mountains.

The Five Layers of the Earth’s Atmosphere.

The troposphere is the primary weather layer. The atmosphere is only held in place by gravity, which gets weaker as you move away from the Earth’s surface. This results in a thinning out of the gasses in the upper layers: around 75 per cent of atmospheric material lies in the troposphere, making up 90 per cent of the atmosphere’s mass. As you head up through this deep layer, temperatures fall to around -80°C (-112°F). But at the top of the troposphere, a boundary known as the tropopause, temperature stops dropping with height and starts to rise. This change means that we have reached the stratosphere.

The stratosphere contains most of our atmosphere’s ozone, a relatively uncommon variant of oxygen that has three atoms per molecule instead of the usual two (its chemical formula is O3). Ozone is present in the metallic tang produced by an electrical discharge and is highly poisonous,* so it’s just as well that it is tucked away up in the stratosphere.

The O3 gas is good at absorbing the energetic ultraviolet rays of the Sun, which is why the hole in the ozone layer caused such concern, as any extra ultraviolet getting through would significantly increase the incidence of skin cancer. This absorption of incoming energy from the Sun results in the stratosphere being able to retain its temperature, despite there being less of a blanket of air above to keep it warm than there is above the troposphere.

At the top of the stratosphere, through the boundary called the stratopause, we reach the mesosphere. With little ozone here to latch on to energetic light, the temperature starts to fall away with height again, in a layer that reaches as far as 80 kilometres (50 miles) above the Earth’s surface. This is the coldest layer of the atmosphere, as it lacks the ability to hold warmth in.

The atmosphere is now getting very thin as we reach the thermosphere. With unprotected exposure to sunlight, temperatures begin to soar at this point, reaching as high as high as 2,500°C (4,500 °F), around half the temperature of the surface of the Sun. And then comes the final extremity, the exosphere, where the remaining components get thinner and thinner, mostly composed of light gases like hydrogen on their way out into space.

There is no clear point at which the atmosphere ends. You can’t specify where the edge of it is – the pull of gravity goes on effectively for ever, shown by the fact that it holds the Moon in place around 380,000 kilometres (236,000 miles) away. But that pull drops off with the square of the distance. Get far enough out and you need something as massive as a moon to get enough mutual attraction. A few molecules of gas don’t stand a chance. The limit of the exosphere is disputed, but it is often put at around 10,000 kilometres (6,200 miles) up.

Note that the conventional point at which space begins, the Karman line, is just 100 kilometres (62 miles up), but this is well within the exosphere. It is an arbitrary limit, arguably set so that so-called astronauts who are strictly no more than high-fliers (such as those on the International Space Station at 400 kilometres – 250 miles – up) can claim to be space travellers.

Non-trivial weather

The Earth’s weather is certainly gentle if we compare it with some of our neighbours in the Solar System. On Venus, for example, vast electrical storms rend the sulphuric-acid clouds over thousands of square miles, while Jupiter has downpours of glowing pink neon rain and what for a long time was considered to be a cyclone bigger than the entire planet Earth forming the Great Red Spot. If this were a giant storm, it would have been raging for over 200 years – which seems highly unlikely. With a modern understanding of chaotic systems (see Chapter 4), it is now thought to be a self-organised region of calm that has more in common with long-term phenomena such as the Gulf Stream and the Jet Stream than a hurricane.

But though our weather on Earth is much milder than these planetary extremes, we are totally dependent on weather systems for our survival. And it would be unfair to call the weather dull – whether we’re thinking of the everyday basics of rain and sun, wind and cloud, or the many dramatic moments the weather brings.

At its best, weather is something to celebrate: the perfect blue skies of a summer holiday; the moment of awe when you pull back the curtains and everything is covered in a pristine blanket of snow; or the coming of life-giving rains after a drought. We usually take the weather for granted, moaning about another day of rain, or half-noticing a pretty cloud formation without really taking it in. But sometimes – whether it be a heatwave like that hitting Europe as I write, or a major storm – the weather becomes headline news. Never more so than in 2005 when Hurricane Katrina blasted into New Orleans.

Katrina

By 5am on Saturday 27 August 2005, Katrina had reached Category 3, with wind speeds around 185 kph (115 mph). The hurricane scale makes no bones about it – this is problematic. ‘Devastating damage. Structural damage to small residences. Flooding near coast destroys small structures.’ At this point, the storm was about 560 kilometres (350 miles) from the US coastline. There was concern that the storm would head directly towards the city of New Orleans. And it was still growing in strength.

In some cases, local governments and business got busy. Several parishes near to New Orleans issued mandatory evacuation orders. The local nuclear power station shut down and prepared for the worst. Despite repeated flooding and storm damage in the past, though, New Orleans itself did as it always had – hoped for the best from its defensive levees and flood walls. Over the previous decades, the city had also lost one of its biggest natural defences, a large area of wetlands that should have acted as a buffer against excessive storm surges. It was generally accepted that the city’s defences could cope reasonably with a Category 3 hurricane, but anything more would result in carnage.

According to the hurricane scale, a Category 5 storm, with winds over 250 kph (156 mph), will result in catastrophic damage. It is predicted to cause: ‘Complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or swept away. Major damage to lower floors of all structures located less than fifteen feet above sea level and within 500 yards of the shoreline. Massive evacuation of residential areas on low ground within five to ten miles of the shoreline may be required.’

Pretty much all of New Orleans was on low ground. Not only would the often-flimsy buildings suffer, but the impact on the population would be catastrophic. In many parts of the US where weather can have a devastating impact, there are storm shelters. But New Orleans had none – there was nowhere high enough to put them to escape the potential impact of a vast storm surge. New Orleans did have a disaster plan that involved evacuating 72 hours before a major storm impacted, giving the 100,000 or so residents without cars the chance to get out – but it was not put into action for Katrina. The city’s mayor was concerned that a mandatory evacuation would leave him in danger of legal action from businesses that were forced to close.

Hurricane Katrina on 28 August 2005.

NASA, Public Domain, via Wikimedia Commons

As individuals and groups slowly started to take action through the day, closing shops and encouraging friends and relatives to evacuate, the storm got closer and stronger. By the early morning of Sunday 28 August, Katrina had reached Category 5, with wind speeds of up to 320 kph (200 mph) measured. It would lose some energy before it made landfall – but that was too little, too late. On Monday morning, Katrina struck.

Canals breached their walls; roofs were ripped off buildings, including a section of the roof of the Superdome stadium where thousands were sheltering. Rain poured down. In the end, around 80 per cent of the city was under water, and would remain so for weeks. As many as 1.5 million people in the states of Alabama, Mississippi and Louisiana had to leave their homes, of whom around 40 per cent were unable to return and had to resettle elsewhere, in some cases more than 800 miles away. Tens of thousands were trapped with limited resources, food and water. The cost was estimated at $125 billion. Over 1,500 people are thought to have died in Louisiana, at least half of them in New Orleans.

This was weather at its most raw. Many of the deaths would have been avoidable, had the authorities acted sooner and more decisively. But whatever the residents could have done, the damage to property would have been huge – very few human-built structures can stand up to the impact of a massive hurricane. The weather should rightly be a subject of awe.

Recently, we have also started to take note of the weather’s big brother: climate. Considering climate moves us away from a particular locale in time and space, instead giving us the bigger picture over years or decades, perhaps for a continent or for the whole world. The topic of climate change – what is happening, what is driving it, whether it has a significant manmade component – is as much a concern for politics as it is a part of science. Yet it’s impossible to look at weather without also thinking about climate change and its implications. The climate is covered in more depth in the companion Hot Science title Hothouse Earth, but is also discussed in Chapter 8.

Because weather has such an impact on our lives, whether in deciding when to plant and harvest crops or simply if it’s a good day for a picnic, we have long had an interest in the possibilities of predicting what the weather will be like in the coming days and weeks. As we will see, weather and forecasting are natural companions.

* The supposedly health-enhancing odour of the seaside, often described as ozone, is actually the smell of dimethyl sulfide and bromophenols, produced by bacteria, plankton and algae. No one is quite sure why it was attributed to ozone – and these chemicals are not particularly good for you either in high concentrations, but they are not dangerous at seaside levels.

FROM FOLKLORE TO FORECASTS

2

As physicist Niels Bohr said (and it wasn’t original then), ‘Prediction is very difficult, especially about the future.’ The difficulty of foreseeing what is to come is not always a problem for science. We can, for example, predict exactly when Halley’s Comet will return and sweep around the Sun on its 75-year orbit (the next time it will be sighted from the ground will be in 2061), but we are incapable of forecasting the weather more than a few days ahead.

Knowing how the weather is going to develop is incredibly important, but how can we know the future when dealing with something as intangible as this? In principle it seems it should be feasible. Ever since Newton gave us a mechanical picture of the universe, it has seemed possible that predictions of the future could become near-perfect. The eighteenth-century French academic Pierre-Simon Marquis de Laplace noted that because of the apparently clockwork-like nature of everything, if a hyper-intelligent being could have access to all the data in existence ‘nothing would be uncertain and future, as the past, would be present to its eyes’.

First attempts

Until recent times, even the vaguest possibility of Laplace’s vision has been inconceivable for anything other than a deity. Yet forecasting how the weather will change is a challenge that has been faced for as long as human beings have thought about nature and how it works. Before a scientific approach was possible, predicting the weather was largely left to the priest or the shaman. At first sight, what they did appears to be not so much a prediction as an attempt at weather control. The idea was to intercede with the gods who were thought to inflict the weather on the people of the Earth as a reward or punishment for their actions.

In practice, though, a savvy priest could indeed be capable of spotting signs that helped predict the weather. If he or she had a fair idea of what was coming, they could then claim to be asking the gods to bring bad weather as a punishment or good weather as a reward. If they got the prediction right, they would appear to wield enormous power, keeping the priesthood safe in their position of esoteric knowledge.

This supernatural view of the weather as something caused by gods, spirits or magic would last well into the medieval period, when witches were sometimes blamed for bringing on a patch of bad weather that damaged an opponent’s crops (something that sadly can still happen in parts of the world). As with many topics, one of the first to think scientifically about the weather was the Greek philosopher Aristotle, who lived between 384 and 322 BCE. We now know that Aristotle was often wrong about science – but in a sense that doesn’t matter.

The problem with the proto-science of Aristotle’s time is that it worked more like the law does today than a true science. Instead of relying on experiment and careful observation to decide the most likely scientific explanation for a phenomenon like the weather, different theories would be debated, and whichever idea came out best in the debate would be accepted as the truth, even if it bore no resemblance to reality. Infamously, Aristotle opined that women had fewer teeth than men. He never bothered to check and people took his word for it.

Aristotle’s scientific legacy was not so much in the accuracy of his theories, as his ability to set the agenda. He covered a vast range of subjects that science would come back to again and again. Some of the topics were abstract, such as the nature of infinity, but others were very practical, with significance for the everyday lives of everyone. And one such topic was forecasting the weather.

Aristotle discussed one of the fundamental components of the weather system: the flow of water through the atmosphere and around the world. He described how water evaporates from bodies of water like oceans and lakes, is carried through the air by the wind and then falls elsewhere as rain. His concept might have been vague, but it did present a basic picture of a key aspect of weather. Aristotle made this the subject of a book called Meteorologica,the word from which we get the clumsy name ‘meteorology’ for the study and forecasting of the weather. It sounds like it should be about meteors, but it originally just meant the study of things that are raised up and lofty – in essence what goes on in the air (though in Aristotle’s original version it did indeed include those meteors).

The ancient Greeks were largely interested in describing nature, exploring its qualities rather than applying any kind of quantitative measurements. So, for instance, like many early cultures, they used wind vanes to see how the wind varied in direction, but they didn’t think about making a numerical measurement of wind speed. Probably the best-known surviving piece of Greek weather technology is the Tower of the Winds, a 12-metre (40-foot) high structure in Athens, devised by the astronomer Andonikos around 50 BCE and originally topped with an ornate weathervane.

Apart from minor local developments, it wasn’t until the seventeenth century that Renaissance thinkers began to look for ways to make direct measurements, pinning down the specifics of weather. How much rain had fallen? How fast was the wind blowing? How did temperature vary throughout the day? Was there any relationship between the air pressure and what was happening to the weather? Were there any regular patterns, making it possible to predict how the weather would change in the future from its current state?

As basic science began to explain the relationship between temperature, pressure and volume of a gas like the air – and at the same time instrument makers added thermometers and barometers to the armoury of would-be weather forecasters – it became possible for the first time to get a feeling for what the weather was actually doing at the moment, and from this to make a stab at predicting how it would change in the future.

The Tower of the Winds.

George E. Koronaios, CC BY-SA 2.0

Reading the sky

Attempts had, of course, been made before. Folk weather forecasting made use of (and still does) cloud patterns, plants, seaweed and the behaviour of animals and birds to predict what was likely to come. Much of this lacked a scientific basis, but occasionally they got it right. Many people are familiar with the rhyme ‘Red sky at night, shepherd’s delight; red sky at morning, shepherd’s warning’, suggesting that good weather followed a red sky around dusk (producing happy shepherds), but a red sky at dawn was likely to presage a storm.

Variants of this weather saying have been around for a long time. For example, in the Gospel of Matthew in the Bible, dating back to the last part of the first century CE, Jesus is asked to show a sign from heaven. He replies, ‘When evening comes, you say, “It will be fair weather, for the sky is red,” and in the morning, “Today it will be stormy, for the sky is red and overcast.” You know how to interpret the appearance of the sky, but you cannot interpret the signs of the times.’

The durability of the red sky rule of thumb reflects an element of scientific basis. Red skies appear for the same reason that the setting sun appears red, while the sky unencumbered with clouds generally appears blue – because of the scattering of sunlight. The Sun’s light is white – a mix of the colours of the rainbow. On the whole, the molecules of the air (and dust particles) scatter more of the blue light in the mix than red, bouncing light away from its usual straight-line progress. The result is a blue sky because the blue light from the Sun is less likely to come directly to your eye and more likely to take a roundabout trip across the heavens.

With the bluer aspects of its white light extracted and sent off on an excursion through the atmosphere, the Sun appears yellow in the day and red in the evening, when the light has to go through more air because of the shallower angle of its rays, thus more light is scattered. Red skies most frequently accompany high atmospheric pressure, which has a tendency to trap more scattering particles in the air. High pressure tends to turn up at night if it is moving into a region in the northern hemisphere, where winds from the west prevail, while high pressure in the morning is more often on its way out. As high pressure over an area tends to make for better weather, this results in a degree of success for this folk prediction.

There are also certain types of cloud that have been used to forecast what’s on the way, with some good cause. Of course, sighting a huge black thunderhead moving in isn’t exactly rocket science, nor is the link between darkness in the clouds and rain, but there are more subtle indicators. Altocumulus clouds in a pattern rather like lines of fish scales (sometimes called a mackerel sky) and comma-shaped high cirrus clouds, known as mares’ tails, both indicate a good chance of high winds on the way, as these clouds often form in advance of a storm.

Nature and legend

Relying on nature for short-lived local forecasts also has some merit. It is still said today that if cows are lying down during the day, it is likely to rain, because they prefer not to lie on wet grass so get settled before the rainfall sets in. They do often seem to read the signs well, though there are plenty of other reasons a herd of cattle could decide to settle down other than an impending downpour. Similarly, swallows are said to presage stormy weather if they fly low. As storms are often accompanied by strong winds at the higher altitudes where swallows usually fly, these winds can force the birds to dip lower, acting as natural wind-speed detectors.

Similarly, the traditional use of seaweed and pinecones as weather-forecasting instruments has a degree of logic. When the humidity – the amount of water vapour in the air – is high, some seaweeds have a noticeably different feel. They are more floppy and less dry, while pinecones open up to reveal their seeds. The assumption is (and it can sometimes prove to be the case) that this increase in humidity could be a warning that rain is on the way.