Academic Reading — Test 63

Reading passage

Few features of the atmosphere exert as much influence over the weather of the British Isles as the jet stream, a narrow band of exceptionally fast wind that snakes around the globe at a height of roughly ten kilometres. The particular current that matters most to Britain is the polar jet stream, which forms along the boundary between cold polar air to the north and warmer subtropical air to the south. Because the contrast in temperature between these two air masses is at its sharpest in winter, the jet stream tends to be strongest during the colder months. Wind speeds within its core can exceed two hundred kilometres per hour, although the band itself is rarely more than a few hundred kilometres wide. Despite its modest dimensions, this ribbon of air acts as a kind of conveyor belt, directing the weather systems that pass over the country from west to east. The jet stream owes its existence chiefly to the rotation of the Earth and to the uneven heating of the planet by the Sun. Near the equator the atmosphere receives far more solar energy than it does near the poles, and the resulting temperature gradient sets vast quantities of air in motion. As this air moves, the spin of the Earth deflects it, a phenomenon known as the Coriolis effect, which bends the flow into the broadly west-to-east direction observed over the North Atlantic. The steepest temperature differences occur where polar and tropical air meet, and it is precisely along this front that the wind accelerates into a jet. The position of the front, and therefore of the jet, is not fixed; it migrates north and south as the seasons turn and as the underlying temperature pattern shifts. For Britain, the everyday consequence of this arrangement is that low-pressure systems, or depressions, are carried in from the Atlantic and steered across or around the islands. When the jet stream lies directly over the country, it tends to usher in a procession of these depressions, bringing the wind and rain so often associated with British weather. When the jet shifts to the north, by contrast, Britain may be left under the influence of calmer high-pressure air, which in summer can produce warm, settled spells and in winter can allow cold, frosty conditions to develop. The strength of the jet matters as much as its location: a powerful, fast-moving jet can drive storms across the Atlantic at remarkable speed, while a weaker one allows weather systems to linger. One of the more troublesome behaviours of the jet stream is its tendency to develop large meanders, rather like the bends of a slow-moving river across a flood plain. These waves, known to meteorologists as Rossby waves, can grow so pronounced that the flow buckles and slows. On occasion a meander becomes effectively cut off from the main current, creating a stationary pocket of high or low pressure that may sit over the same region for days or even weeks. Such blocking patterns are responsible for some of Britain's most memorable spells of extreme weather. A blocking high in summer can bring prolonged heat and drought, whereas a block positioned in a different place can funnel cold air down from the Arctic and lock in freezing conditions through the depths of winter. Forecasters monitor the jet stream closely because its configuration offers valuable clues about the days ahead. Modern numerical weather prediction relies on a global network of observations, including data gathered by weather balloons, satellites and aircraft, which are fed into powerful computer models that simulate the movement of the atmosphere. Tracking the jet allows meteorologists to anticipate whether a region will be wet or dry, mild or cold, and to issue warnings when severe weather appears likely. Yet the jet stream remains difficult to predict beyond a week or so, partly because small uncertainties in its position can grow rapidly into large differences in the forecast. This sensitivity is one reason why long-range forecasts are expressed in terms of probabilities rather than certainties. There is also growing scientific interest in how a warming climate might alter the behaviour of the jet stream. Because the Arctic is heating faster than the regions further south, the temperature contrast that drives the polar jet may be weakening. Some researchers suggest that this could make the jet more prone to the wavy, sluggish patterns that produce blocking, and therefore to more frequent episodes of persistent extreme weather. The matter is far from settled, however, and remains an active and contested area of research. What is not in doubt is that this slender river of air, invisible from the ground, will continue to govern much of what the inhabitants of the British Isles experience when they step outdoors.