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1、Unit 1 The world of our sensesSection 2: Background informationabout fog and sharkI. The London fogLondon used to be famous for its heavy fog. This was because people in London burned coal which gave out heavy smoke, a polluted form that actually is called smog. In 1873, a coal-smoke smog, thicker a
2、nd more persistent than natural fog, covered the city for days and caused 268 deaths. Another fog in 1879 lasted from November to March and caused four long months of sunshineless darkness. In 1952, a four-day fog killed about 4,000 Londoners. It was not until then that some measures were taken to s
3、top the heavy fog. In 1956, Parliament enacted the Clean Air Act, Which effectively reduced the burning of coal. It was the beginning of a series of air-pollution reforms in England. Now the air in London is quite cleanthough the city still sometimes is covered in a clean, ordinary fog.II. Understan
4、ding fog and cloudsFog is a cloud based on the ground, rather than in the atmosphere. Fog forms when there is a lot of moisture near the ground, or when the air near the ground cools to the dew point. The temperature to which air must be cooled in order for the water vapor in the air to condense int
5、o liquid water is known as the dew point. If air near the ground cools to this temperature, water vapor from the air will become visible as dew on the ground or fog in the air.Clouds are usually the most obvious feature of the sky. They both reflect weather patterns and play a role in what the weath
6、er does. The links below take you to a great deal of information about clouds.Locations of cloudsLow-level clouds: (generally found below 6,500 feet) Low-level clouds are usually composed of liquid water droplets, but they can have snow and ice crystals in cold weather.Mid-level clouds: (generally f
7、ound between 6,500 and 23,000 feet) Most mid-level clouds are composed of liquid water droplets during summer and a liquid droplet-ice crystal mix during winter. Mid-level cloud names are preceded by an alto prefix.High-level clouds: (generally found above 20,000 feet) High-level clouds are composed
8、 of ice crystals and tend to be very thin and wispy. High-level cloud names are preceded by a cirro prefix.Different kinds of fog, where fog formsAdvection fog forms when humid air flows over cold ground or water.Radiation fog forms on generally clear, cool nights.Steam fog forms over water, often i
9、n the fall.Overnight rain can enhance fog that forms in the morning.Precipitation fog forms when rain or snow falls. As precipitation falls into drier air below the cloud, the liquid drops or ice crystals evaporate or sublimate directly into water vapor. The water vapor increases the moisture conten
10、t of the air while cooling the air. This often saturates the air below the cloud and allows fog to form.Upslope fog is very common along large hills and mountains. It forms when winds blow up the side of a hill or mountain, which cools the air.Valley fog forms in mountain valleys during winter and c
11、an be more than 1,500 feet thick. Often, the winter sun is not strong enough to evaporate the fog during the day. When the air cools again the following night, the fog often becomes thicker, which makes it even harder for the sun to burn it off the following day. These fogs can last for several days
12、 until strong winds blow the moist air out of the valley. The tendency for cool, dense air to pool at the bottom of valleys also enhances valley fog.III. The introduction of sharksSharks are amazing fish that have been around since long before the dinosaurs existed. They live in waters all over the
13、world, in every ocean, and even in some rivers and lakes. Unlike bony fish, sharks have no bones; their skeleton is made of cartilage, which is a tough, fibrous substance, not nearly as hard as bone. Sharks also have no swim bladder (unlike bony fish).Size There are many different species of sharks
14、that range in size from the size of a persons hand to bigger than a bus. Fully-grown sharks range in size from 7 inches (18cm) long (the Spined Pygmy shark), up to 50 feet (15m) long (the Whale shark). Most sharks are intermediate in size, and are about the same size as people, 5-7 feet (1.5-2.1m) l
15、ong. Half of the 368 shark species are under 39 inches (1m) long.Body Sharks have a variety of body shapes. Most sharks have streamlined, torpedo-shaped bodies that glide easily through the water. Some bottom-dwelling sharks have flattened bodies that allow them to hide in the sand of the ocean bed.
16、 Some sharks have an elongated body shape. Varieties of sharks There are about 368 different species of sharks, which are divided into 30 families. These different families of sharks are very different in the way they look, live, and eat. They have different shapes, sizes, color, fins, teeth, habitat, diet, personality, method of reproduction, and other attributes. Some types of shark are very rare (like the great white shark and t
