IELTS Academic Reading Practice Test 99

Modern weather forecasting depends on accurate measurements of the atmosphere far above the ground. Observations made at the surface alone cannot show how temperature, moisture and wind vary with height, yet this information is essential for the computer models that predict the weather and warn of storms. For more than a century, the main instrument for gathering such measurements has been the weather balloon, a relatively inexpensive and robust device that rises through the lower atmosphere carrying small instruments into regions that no aircraft can routinely reach at such low cost.

The deliberate use of balloons to study the atmosphere began in the late nineteenth century. Early attempts employed kites and manned gas balloons, but kites required steady wind and could become tangled on trees, and human aeronauts could not safely remain at great altitudes for long. Meteorologists therefore sought ways to send instruments aloft without carrying people. In the 1890s, weather services in several countries began releasing unmanned balloons fitted with thermometers and barometers, but the readings could only be examined after the balloon had returned to the ground, often many hours or even days later. Despite these early difficulties, interest in upper-air observation grew steadily.

A major advance came in 1902, when the French meteorologist Léon Teisserenc de Bort used repeated instrumented flights to show that air temperature stopped decreasing at a certain point high in the atmosphere. That boundary was later named the tropopause, and the stable layer above it became known as the stratosphere. Balloon soundings had thus revealed the basic vertical structure of the atmosphere for the first time. The discovery encouraged further investment in upper-air observation, although the method still depended on finding fallen instruments, which was extremely difficult in remote or forested country.

The next step was to make the instruments report their readings while the flight was still in progress. During the 1930s, the addition of small radio transmitters to balloon payloads created the radiosonde, a compact package that could send pressure, temperature and humidity data back to a ground station while the balloon continued to climb. This removed the need to recover the instruments and allowed upper-air data to be collected on a regular schedule. Radiosondes were soon produced in large numbers, and national networks began making daily ascents in many parts of the world.

A modern radiosonde is usually lifted by a balloon made from natural latex or synthetic rubber and filled with hydrogen or helium. As the balloon rises, the pressure around it falls steadily, and the balloon expands from about two metres across at launch to the size of a small bus before bursting at an altitude of roughly 30 kilometres. The radiosonde is suspended some distance below the balloon to reduce any influence that the balloon itself might have on the air being sampled. Throughout the ascent, sensors record temperature, humidity and pressure, and many instruments also use satellite navigation signals to calculate wind speed and direction at every level.

Measurements of this kind are made at approximately 900 sites around the world, normally twice a day, at midnight and noon Coordinated Universal Time. Because the balloons are released at the same hours everywhere, the observations form a synchronised global picture of the atmosphere. This global snapshot is essential for the numerical models that produce forecasts ranging from a few hours to more than a week ahead. Under the coordination of the World Meteorological Organization, the data are shared internationally, so that even small countries contribute to and benefit from the observing system.

After a balloon bursts, a small parachute lowers the radiosonde back to the ground. Many radiosondes carry a printed notice asking anyone who finds them to return the instrument to the meteorological service, and a proportion are recovered, checked and prepared for reuse. The vast majority, however, are never found, and the balloons themselves are not reusable. Latex balloons are designed to break down relatively quickly after falling, but in remote areas they can still become visible litter, which has led some operators to look for alternative materials.

Despite its long history, the weather balloon is not a perfect observing tool. A balloon drifts with the wind, so its measurements are not made exactly above the launch site, and the ascent through the atmospheric column can take an hour or more. Balloons also cannot sample the very highest levels of the atmosphere, and each flight gives information along only a narrow path. Satellites provide a much broader view of temperature and moisture over the oceans and other unobserved regions, yet balloon soundings remain valuable because they offer high vertical resolution and because satellite instruments are checked against them.

Some researchers have proposed drones and other aircraft as a potential replacement for weather balloons, and satellites continue to improve. However, no alternative yet matches the balloon's combination of low cost, simplicity and reliability. Their relative cheapness also means that failures are far less damaging than the loss of a specialised aircraft. Gaps in the radiosonde network can noticeably reduce the accuracy of forecasts in the regions affected. For as long as precise upper-air data matter, balloons are likely to remain part of the observing system, carrying out the unglamorous but essential work of telling us what the sky is doing above where we live.

Human beings are not impressive sprinters. A domestic cat, a horse or an antelope can outpace even the fastest person over a short distance, and many animals can leap further and climb better than we can. Yet over long distances in warm conditions, a trained human can outrun almost any other land mammal. Healthy adults regularly complete marathons, and some people run distances of a hundred kilometres or more in a single day. This unusual capacity has led researchers to ask whether humans are, in a physical sense, born to run.

The idea that humans evolved as endurance runners became influential in 2004, when biologists Dennis Bramble and Daniel Lieberman published a paper arguing that the human body carries a remarkable number of features that make sustained running possible. They pointed to long legs with spring-like tendons, an upright posture, a large gluteus maximus muscle, and short toes. The nuchal ligament, which helps stabilise the head, is also more prominent in humans than in apes, and the Achilles tendon stores elastic energy with each stride. None of these features is essential for walking, but together they improve the ability to run steadily without enormous muscular effort. According to this view, endurance running was not a by-product of walking but an adaptation in its own right.

One of the most important advantages humans possess is the ability to keep cool while moving. Unlike many mammals, humans have millions of eccrine sweat glands spread across the skin, and we have very little body hair. This arrangement allows heat to be lost through the evaporation of sweat over most of the body surface. A running antelope must pant to cool itself, and it cannot pant effectively while galloping. A human being, by contrast, can continue to breathe and run at the same time, which means that in hot conditions people can maintain a speed that would quickly overheat a fur-covered animal.

The evolutionary history of these traits has been inferred partly from the fossil record. The skeleton of Homo erectus, which lived from about two million years ago, shows long legs, a narrow waist and other signs of a body built for efficient travel over open ground. Bramble and Lieberman argued that endurance running may have become important with the appearance of Homo erectus, when early humans moved into hot, open environments where food was widely scattered. The ability to travel far on foot would have allowed them to reach distant resources and to return with meat.

Running may also have been used directly to obtain food through a practice known as persistence hunting. Among the San people of southern Africa and a few other groups, hunters have been described chasing an antelope for hours in the midday heat, alternately running and walking. The animal sprints ahead and then must stop to rest and pant, while the human hunter keeps following at a jog. Eventually the animal collapses from heat exhaustion, and the hunter can approach and kill it at close range without needing a powerful bow or a fast spear throw.

Modern endurance events sometimes echo this old interaction between species. Competitors in very long races occasionally finish ahead of horses, especially on warm days, and teams of humans have been recorded running down fleet animals over many hours. Such comparisons are not direct proof of how our ancestors hunted, but they demonstrate that the physiological advantages described by researchers are real and observable today. A fit human can lose heat more efficiently than many quadrupeds, and this capacity becomes decisive as distance increases. Because humans can sweat over much of the body and do not rely on panting, they avoid the rapid breathing that would otherwise limit sustained effort.

Not all scientists accept the endurance-running hypothesis. Some argue that the features identified by Bramble and Lieberman may have evolved for other reasons, such as more efficient walking, carrying food or simply standing upright. The short toes and arched foot, for example, are found in animals that do not run long distances, and the fossil evidence is open to several interpretations. Critics also point out that persistence hunting is rare in the ethnographic record and that living hunter-gatherers use many other methods to obtain meat.

Even among supporters of the hypothesis, there is debate about how much of a role running played compared with walking, carrying and cooperation. It is unlikely that early humans ran marathons in any formal sense, but they may have relied on a combination of walking, jogging and tracking over many hours. The key point is that human bodies are unusually well suited to sustained movement in hot conditions, and this capacity remains with us. It is not an accident that humans are among the few primates that can cover long distances at a steady pace.

The running ape, then, is not a myth. While humans will never match a cheetah over a hundred metres, our sweating skin, long springy legs and ability to breathe while moving give us a rare form of endurance. This physical inheritance, combined with the human habit of setting goals and following them over great distances, may help explain why the marathon and the ultramarathon have become such powerful modern symbols of human persistence. Some researchers also note that humans are among the very few species that voluntarily run long distances for pleasure, a behaviour that may be rooted in this ancient hunting past.

Anyone who has spent hours making decisions, from a hospital manager scheduling staff to a parent planning a week of meals, knows the feeling of mental exhaustion that can set in. The experience is so common that it is often treated as simple common sense. Psychologists call this state decision fatigue, the idea that making choices gradually depletes a limited mental resource, leaving a person less able to make good decisions later in the day. The concept has become widely accepted and has been used to explain many behaviours. The evidence, however, is more complicated than the familiar story suggests.

The idea has its roots in a body of work by the social psychologist Roy Baumeister, who began studying what he called ego depletion in the late 1990s. Baumeister and his colleagues proposed that self-control depends on a shared resource, rather like a muscle that becomes tired after use. The muscle metaphor was intended to capture the feeling that self-control tires with use, just as a physical muscle does. In a typical early experiment, participants who were asked to resist eating freshly baked biscuits and instead eat radishes later gave up more quickly on a difficult puzzle than people who had not had to exercise such self-control.

Baumeister went on to argue that the resource behind willpower is glucose, the sugar that fuels the brain. He noted that the brain uses a disproportionate amount of the body's energy, and that glucose levels fall during demanding mental tasks. Although the brain represents only about two per cent of body mass, it consumes roughly twenty per cent of daily energy intake. A series of studies appeared to support this view: in one often-cited experiment, people who drank a sugary lemonade performed better on a later self-control task than those who drank an artificially sweetened drink, although critics later argued that the sugar could not have reached the brain quickly enough to explain the result.

The theory soon moved out of the laboratory. A widely discussed study of Israeli judges, published in 2011, examined decisions about whether to grant parole. The researchers found that the proportion of favourable rulings dropped steadily over the course of a morning session, from roughly two-thirds at the start to nearly zero just before a mid-morning break, and then rose again after food. Many commentators took this as evidence that judges' decisions were shaped by mental depletion and falling blood sugar. The paper became one of the most frequently cited examples of decision fatigue outside psychology.

By the middle of the 2010s, however, the scientific foundations of ego depletion began to look shaky. A large multi-laboratory replication project published in 2016 found little to no support for the effect, and several earlier studies failed to reproduce the glucose findings. Some researchers argued that the original studies had been too small and that publication bias had inflated the evidence. Still, the debate has not been fully resolved, and some researchers maintain that the effect appears under certain conditions. Baumeister and others defended the research, but the simple story of a limited mental fuel tank is no longer accepted as confidently as it once was.

So what is going on when people seem to make poorer decisions after a long day? One alternative explanation is motivational. People may not run out of willpower; they may simply become less willing to spend effort when the rewards seem small or remote. A tired person may also shift from careful deliberation to simpler rules of thumb. Experiments by researchers such as Michael Inzlicht and his colleagues have suggested that changes in attention and priorities, rather than the exhaustion of a fuel supply, can account for many effects attributed to decision fatigue.

The judge study itself has also been re-examined. Later analyses pointed out that the fall in favourable rulings might reflect the scheduling of breaks and the order of prisoners' appearances rather than decision fatigue. For example, lawyers may arrange less hopeful cases for particular times, or the pattern may be partly statistical. Nevertheless, the case remains a favourite illustration in popular books. While the findings remain suggestive, they are not the clean demonstration of mental depletion that they are often presented as being.

My own view is that decision fatigue, as a description of a familiar experience, should not be dismissed entirely. Making choices is often genuinely tiring, and difficult decisions are best tackled when a person is rested, fed and free from pressure. The psychologist Carol Dweck and others have shown that people who believe willpower is a limited resource are more likely to experience its depletion, which suggests that expectations and beliefs are part of the phenomenon. But the evidence does not support the idea that a sugary drink can reliably reset judgement. Hunger may still matter, but its effect is probably mediated by expectations and mood.

The practical conclusion is not that decision fatigue is imaginary, but that the cause is more psychological and contextual than physiological. Organisations should avoid scheduling complex decisions when people are hungry or after long stretches of work, but they should not imagine that a soft drink will solve the problem. The metaphor of the mind as a fuel tank has been useful, yet it has outlived the evidence. A better account is that choice is difficult because it carries responsibility, and our willingness to engage in it depends on what we expect to gain. That is why the best advice is to structure the day so that important choices come before minor ones.