雅思阅读第078套P3-ALL YOU NEED TO KNOW ABOUT SNOW

ALL YOU NEED TO KNOW ABOUT SNOW

Types of Snowfall

Snow is typically the product of weather conditions in which an extratropical cyclone has formed. Extratropical cyclones bring extremely hazardous weather, such as high winds and heavy rain or snow, and are often referred to as windstorms in Europe. The band of precipitation associated with their warm front is often very extensive. When the warm front and cold front collide, snow can result on the poleward side of the precipitation band; that is, on the northern side in the Northern Hemisphere and on the southern side in the Southern Hemisphere.

Lake-effect snow is another kind of common snowfall. Although the name suggests a particular correspondence of this type of precipitation to lake features, in fact, all narrow bands of water may generate it. Lake-effect snow occurs when the water temperature is considerably higher than the air temperature of a cold front progressing over a large water mass. Warm moist air is then attracted upward at a relatively fast rate, condensing to form vertically oriented clouds. If the temperature difference between the body of water and the air above is significant, say, 13 degrees or more, this can result in heavy and prolonged snowfall.

Mountainous areas are also prone to experiencing heavy snowfall. Accumulations typically occur on the windward side of the mountain as precipitation is ‘squeezed out’ of the warm moist air as it is forced to ascend the slopes; the moisture condenses upon contact with the colder air found at higher altitudes and heavy snowfall can then occur if ground conditions are sufficiently cold.

How Snow Is Formed

Snow crystals, tiny supercooled cloud droplets, form at extremely low temperatures in the atmosphere. Temperatures lower than minus 35 degrees Celsius are required for this supercool moisture to freeze by itself. In warmer clouds, an aerosol particle such as clay or desert dust, or an ice nucleus is needed for the freezing to start.

Once a droplet of water has frozen, it starts to grow in the supersaturated environment of the cloud. Eventually, due to its size, the cloud will not be able to contain the ice crystal anymore. At this point the ice crystal will fall to the ground and, if it is not melted by warmer air at lower altitudes, it will do so as snow. Although the ice crystals that land on the ground are actually transparent, hollow imperfections in them mean that light is scattered and they often appear white in colour owing to diffuse reflection of the whole spectrum of light.

The Snowflake

The shape of a snowflake is determined by the atmospheric conditions present at the time of its formation, specifically temperature and humidity. Between 0 and -3 degrees Celsius, thin flat crystals called planar crystals grow. From -3 to 8, the crystals form needles or prisms with pencil-like shapes. The shape then reverts back to plate-like until after 22 degrees Celsius when column-like structures (needles and prisms, etc.) begin to form again. At temperatures of 22 degrees and below, as well as the column-like structures, more complex growth patterns also form.

Snowfall in the British Isles

Snowfall occurs frequently in the U.K., but the quantities are typically small and it seldom persists for very long. In recent years, a trend towards milder, wetter winters has been developing, though the 2010/11, 2011/12 and 2012/13 winters seem to have bucked the trend and, indeed, represent the longest period of consecutive cold winters for more than 50 years. For its latitude, the British Isles should see far more prolonged cold weather in winter and regular snowfall. However, the Gulf Stream, a mild Atlantic Ocean current, keeps the climate several degrees warmer than regions of similar latitude in other parts of the world. As a consequence, despite the occasional incident of prolonged cold, Britain’s winters are typically not very severe.

Some parts of the isles see little, if any, snow from year to year. The most snow-prone are the Pennines, the Scottish Highlands, the Welsh Hills and the mountains of Northern Ireland. The Scottish Highlands boasts the isles’ highest peaks and also their only winter ski resorts. For years, unreliable snowfall has threatened to close these resorts, though, having had three consecutive bumper seasons, there is now less pressure on the Scottish ski industry, which, not so long ago, was threatened with going out of existence.

Long-term weather forecasts for the British Isles are notoriously hard to get right; however, so far, three months before the official start of the meteorological winter in December, forecasters are predicting another winter of record-breaking low temperatures. They point to sunspot and geothermal activity, and changes in the strength of the Gulf Stream as key indicators of the fact that a cold winter is in prospect. Were their predictions to be realised, then this would point to the isles undergoing a subtle climatic change and a return to more severe winters in general.

SECTION 3: QUESTIONS 27-40

Questions 27-33

Do the following statements agree with the information given in Reading Passage 3?

Write:

YES	if the statement agrees with the views of the writer
NO	if the statement contradicts the views of the writer
NOT GIVEN	if it is impossible to say what the writer thinks about this

27 _________________ When a warm front from an extratropical cyclone meets a cold front, snow is more likely to occur on the poleward side of the weather system.

28 _________________ Lake-effect snow is aptly named, given that it is a weather phenomenon which is only associated with lakes.

29 _________________ Heavy snowfall is more likely to be seen on the side of the mountain that is exposed to high winds.

30 _________________ In the absence of dust or a similar particle to start the freezing process, supercool moisture will not freeze in a cloud whose mean temperature is -34 degrees or more.

31 _________________ The real colour of snow is the same as the colour snow appears to be to the human eye.

32 _________________ Snowflakes shaped like a prism are more likely to form in milder weather than are flakes with more intricate growth patterns.

33 _________________ The thin flat crystals created at temperatures of between zero and minus three degrees Celsius are more voluminous than column-like crystals.

Questions 34-37

Complete the sentences below.

Choose NO MORE THAN THREE WORDS from the passage for each answer.

While snowfall is not an uncommon feature of a United Kingdom winter, it is rare for significant

34 _________________ to accumulate, or for the snow to remain on the ground for any great length of time.

Three consecutive winters have failed to follow the 35 _________________ towards milder, wetter weather.

Despite its latitude, the British Isles does not suffer from 36 _________________ on account of the influence of the Gulf Stream.

Should next winter be another unusually cold one, this may be indicative of the fact that a 37 _________________ is under way.

Questions 38-40

Complete the summary with the list of words A-F below.

Write the correct letter, A-F, in spaces 38-40 below.

The British Winter

Snowfall is not an uncommon sight in Britain during winter, but such weather rarely persists for very long. In fact, up until a few years ago, it looked like British winters were getting milder. However, this perception has changed fairly dramatically over the last three winters, which have been the longest 38 ___________ period of cold winters for more than half a century. It is now feared that the climate of the British Isles is changing and that, should the next few winters be equally severe, this could signal a permanent move to more 39 ___________ winter weather in general. Scientists blame sunspot and geothermal activity as well as the 40 ___________ influence of the Gulf Stream, which, until now, has kept Britain’s climate milder than that of areas of similar latitude throughout the rest of the world.