Academic Reading — Test 11

Reading passage

High above the British Isles, at altitudes of roughly nine to eleven kilometres, a narrow ribbon of fast-moving air winds its way around the planet. Known as the polar jet stream, this current of wind can reach speeds in excess of two hundred kilometres per hour, and its meandering path exerts a powerful influence on the weather experienced at the surface. Although it is invisible to people on the ground, the jet stream is one of the most important factors determining whether Britain enjoys a spell of calm, settled conditions or endures a succession of rain-bearing storms. The jet stream owes its existence to the sharp contrast in temperature between the cold polar regions and the warmer air of the subtropics. Where these two masses of air meet, a steep temperature gradient develops, and the difference in pressure that results drives air to flow rapidly from west to east. Because the temperature contrast is greatest in winter, the jet stream tends to be strongest and most active during the colder months of the year. In summer, when the polar regions warm and the gradient weakens, the current generally becomes slower and drifts further north, often leaving Britain under quieter skies. The position of the jet stream matters a great deal to the British Isles. When the current lies directly over or just to the north of the country, it tends to guide a parade of low-pressure systems, or depressions, eastwards across the Atlantic Ocean towards Europe. These depressions bring cloud, wind and rain, and they can arrive in quick succession, producing the changeable weather for which Britain is famous. When the jet stream shifts southwards, however, it may steer these same storms towards France and the Mediterranean, leaving Britain on the colder, drier side of the flow. The exact track of the jet stream therefore helps to explain why one region may be drenched while another, only a few hundred kilometres away, stays dry. The jet stream rarely follows a straight line. Instead, it develops large waves, known to meteorologists as Rossby waves, which cause it to swing northwards and southwards as it circles the globe. When these waves are shallow, the current flows swiftly and more or less directly from west to east, and weather systems are carried across the country fairly quickly. When the waves grow deep and pronounced, the flow becomes sluggish, and pockets of high or low pressure can become trapped in place. Such stationary patterns, sometimes described as blocking, may persist for days or even weeks. A blocking high in summer can bring a prolonged heatwave and drought, while a block in winter can lock cold air over the country and bring lasting frost and snow. The behaviour of the jet stream also helps to account for some of Britain's most damaging episodes of weather. When the current becomes unusually strong and aligns directly with the Atlantic storm track, it can drive one deep depression after another onto the western coasts, as happened during several notably wet and stormy winters in recent decades. The energy supplied by the jet stream can intensify these systems, deepening their central pressure and strengthening the winds that accompany them. Forecasters therefore pay close attention to the strength and orientation of the current when assessing the risk of flooding and gales. Because the jet stream is so influential, an accurate forecast of its future position is essential to predicting British weather more than a day or two ahead. Modern numerical models, run on powerful computers, simulate the movement of the atmosphere and attempt to anticipate how the current will meander in the days to come. Even so, small uncertainties in the starting conditions can grow rapidly, and the precise track of the jet stream a week ahead often remains difficult to pin down. This sensitivity is one reason why medium-range forecasts for the British Isles, despite great advances in recent years, can still prove unreliable. Understanding the jet stream does not remove the uncertainty entirely, but it does provide meteorologists with a powerful framework for interpreting the restless skies above the islands.