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  • 大气中的天气和水

    Lesson Objectives
    ::经验教训目标

    • Explain what causes weather.
      ::解释是什么导致天气。
    • Describe humidity and its role in weather.
      ::描述湿度及其在天气中的作用。
    • Explain how clouds are classified.
      ::解释云层的分类方式
    • Identify types of precipitation and how they form. 
      ::确定降水类型及其形成方式。

    Lesson Vocabulary
    ::词汇表课程

    • cirrus cloud
      ::紫云云
    • cumulus cloud
      ::积积云云
    • dew point
      ::下点
    • fog
      ::雾雾
    • freezing rain
      ::冻冻雨雨
    • hail
      ::冰冰冰
    • heat index
      ::热热指数
    • humidity
      ::湿湿
    • relative humidity
      ::相对湿度
    • sleet
      ::潜水
    • stratus cloud
      ::stratus 星云云
    • weather
      ::天气天气

    Introduction
    ::导言

    If someone in a distant place were to ask what your weather is like today, what would you say? How would you describe the weather right now where you are? Is it warm or cold? Sunny or cloudy? Calm or windy? Clear or rainy? What features of weather are important to mention?
    ::如果远方的人问你今天的天气如何,你会怎么说?你现在会怎样描述天气?你现在在哪里?是温暖还是寒冷?晴天还是阴暗?平静还是风?清风还是下雨?天气的特征很重要吗?

    What Is Weather?
    ::气象是什么?

    What do temperature, clouds, winds, and rain have in common? They are all part of weather.  Weather  refers to the conditions of the atmosphere at a given time and place.
    ::温度、 云、 风和雨有什么共同之处? 它们都是天气的一部分。 天气是指特定时间和地点的大气状况 。

    All  weather  takes place in the atmosphere. Nearly all of it takes place in the lower atmosphere.  Weather  refers to the conditions of the atmosphere at a given time and place.  Climate  is the average of weather over a long time.
    ::所有天气都发生在大气中,几乎所有天气都发生在低层大气中。天气是指特定时间和地点的大气状况。气候是长期平均天气。

    Imagine your grandmother who lives in a distant place calls you up. She asks what your weather is like today. What would you say? Is it warm or cold? Is it sunny or cloudy? Is it calm or windy? Is it clear or rainy? What features of weather are important to mention?
    ::你想象一下,住在远处的祖母,把你叫来,她问你今天的天气是怎样的。你说:是温暖的还是寒冷的呢?是晴天的呢?还是阴凉的呢?是宁静的呢?还是暴风的呢?是清澈的呢?还是下雨的呢?天气的特征很重要呢?

    What Causes Weather?
    ::什么原因导致天气?

    Weather occurs because of unequal heating of the atmosphere. The source of heat is the sun. The general principles behind weather can be stated simply:
    ::天气发生的原因是大气的热量不平等,热源是太阳。 天气的一般原则可以简单地说:

    • The Sun heats Earth’s surface more in some places than others.
      ::太阳使地球表面在有些地方比在另一些地方更热。
    • Where it is warm, heat from the sun warms the air close to the surface. If there is water at the surface, it may cause some of the water to evaporate.
      ::在温暖的地方,太阳的热能使靠近表面的空气温暖,如果表面有水,可能会使一些水蒸发。
    • Warm air is less dense, so it rises. When this happens, more dense air flows in to take its place. The flowing surface air is wind.
      ::温暖的空气较不稠密,因此它会上升。一旦发生这种情况,更稠密的空气流将取代它。流出的地表空气是风。
    • The rising air cools as it goes higher in the atmosphere. If it is moist, the water vapor may condense. Clouds may form, and precipitation may fall.
      ::不断上升的空气随着大气的升高而冷却。 如果潮湿,水蒸气可能会凝结。 云会形成,降水也会下降。

    Weather and the Water Cycle
    ::天气和水循环

    The water cycle plays an important role in weather. When liquid water evaporates, it causes humidity. When water vapor condenses, it forms clouds and precipitation. Humidity, clouds, and precipitation are all important weather factors.
    ::水循环在天气中起着重要作用,当液态水蒸发时,会造成湿度;当水蒸发时,会形成云和降水;湿度、云和降水都是重要的天气因素。

    Humidity and Heat 
    ::湿度和热度

    Humidity  is the amount of water vapor in the air. High humidity increases the chances of clouds and precipitation.
    ::湿度是空气中的水蒸发量,高湿度增加了云层和降水的可能性。

    People often say, “It’s not the heat but the humidity.” Humidity can make a hot day feel even hotter. When sweat evaporates, it cools your body. But sweat can’t evaporate when the air is saturated with water vapor. It is like trying to dry yourself off with a soaking wet towel. It doesn't work very well.
    ::人们常说 : “ 不是热,而是湿湿。 ”潮湿会让热天更热。 当汗水蒸发时,会冷却你的身体。 但空气充斥着水蒸气时,汗水不会蒸发。 这就像用湿湿的毛巾擦干自己一样。 汗水效果不好。

    Dew Point 
    ::下点

    You’ve probably noticed dew on the grass on a summer morning. Why does dew form? Remember that the land heats up and cools down fairly readily. So when night comes, the land cools. Air that was warm and humid in the daytime also cools overnight. As the air cools, it can hold less water vapor. Some of the water vapor condenses on the cool surfaces, such as blades of grass. The temperature at which water vapor condenses is called the  dew point . If this temperature is below freezing, ice crystals of frost form instead of dew. The dew point occurs at 100 percent relative humidity. Can you explain why?
    ::你可能注意到夏天早上在草地上露水。为什么露水会形成?记住陆地热和冷却相当容易。所以当夜晚到来时,土地会冷却。白天温暖和潮湿的空气也会在夜间冷却。随着空气冷却,水蒸气会吸收较少的水蒸气。有些水蒸气在冷却的表面凝结,例如草叶。水蒸气凝结的温度被称为露水点。如果温度低于冰冻,冰冻的冰晶而不是露水。露水点在100%相对湿度上。你能解释为什么吗?

    Clouds 
    ::云云

    Clouds form when air in the atmosphere reaches the dew point. Clouds may form anywhere in the troposphere. Clouds that form on the ground are called  fog .
    ::当大气中的空气到达露点时,云会形成云。云会在对流层的任何地方形成。地面上的云会被称为雾。

    How Clouds Form
    ::云云如何形成

    Clouds form when water vapor condenses around particles in the air. The particles are specks of matter, such as dust or smoke. Billions of these tiny water droplets come together to make up a cloud. If the air is very cold, ice crystals form instead of liquid water.
    ::当水蒸气在空气中凝聚颗粒时,云会形成。这些粒子是物质的斑点,例如尘埃或烟雾。这些微小的水滴中有数十亿聚集在一起组成云层。如果空气非常寒冷,冰晶会形成,而不是液态水。

    Classification of Clouds (Advanced Topic)
    ::云的分类(高级专题)

    Clouds are classified on the basis of where and how they form. Three main types of clouds are cirrus, stratus, and cumulus.  Figure    shows these and other types of clouds.
    ::云是根据其形成地点和方式加以分类的,主要有三种云是环状云、矩形云和积聚云。图显示了这些云和其他类型的云。

    • Cirrus clouds  form high in the troposphere. Because it is so cold they are made of ice crystals. They are thin and wispy. Cirrus clouds don’t usually produce precipitation, but they may be a sign that wet weather is coming.
      ::云层云层在对流层中形成高度。 云层冷得要命,因此由冰晶组成。 云层是薄的,微弱的。 云层云层通常不会降水,但可能是潮湿天气即将来临的迹象。
    • Stratus clouds  occur low in the troposphere. They form in layers that spread horizontally and may cover the entire sky like a thick blanket. Stratus clouds that produce precipitation are called nimbostratus. The prefix  nimbo-  means “rain.”
      ::斯特拉图斯云层在对流层低端出现。 云层由水平分布的层组成, 并可能像厚厚的毯子一样覆盖整个天空。 产生降水的斯特拉图斯云被称为 nimbostratus 。 前缀 nimbo 是指“ 雨 ” 。
    • Cumulus clouds  are white and puffy. Convection currents make them grow upward and they may grow very tall. When they produce rain, they are called cumulonimbus.
      ::云是白的,是浮肿的,对流的流水使云向上增长,而且可能很高。当雨水降下时,它们被称为云。
    lesson content
     
    Find the cirrus, cirrostratus, and cirrocumulus clouds in the figure. What do they have in common? They all form high in the troposphere. Clouds that form in the mid troposphere have the prefix “alto-”, as in altocumulus. Where do stratocumulus clouds form?

    Clouds and Temperature
    ::云和温度

    Clouds can affect the temperature on Earth’s surface. During the day, thick clouds block some of the Sun’s rays. This keeps the surface from heating up as much as it would on a clear day. At night, thick clouds prevent heat from radiating out into space. This keeps the surface warmer than it would be on a clear night.
    ::云层可以影响地球表面的温度。 在白天,厚云堵住了太阳的一些射线。 这使得表层的热量保持在晴朗的白天。 在晚上,厚云能防止热向太空散热。 这使得表层的温度比晴朗的夜晚更暖和。

    Precipitation
    ::降降

    Clouds are needed for precipitation. This may fall as liquid water, or it may fall as frozen water, such as snow.
    ::降水需要云。 降水可能像液体水一样落下,也可能像雪等冰冻水一样落下。

    Why Precipitation Falls
    ::为何降水瀑布

    Millions of water molecules in a cloud must condense to make a single raindrop or snowflake. The drop or flake falls when it becomes too heavy for updrafts to keep it aloft. As a drop or flake falls, it may collect more water and get larger.
    ::云层中的数百万水分子必须凝结, 才能产生一滴雨或雪花。 当云层变大时, 云层中的水分子会掉落, 其体积会变得太重, 无法保持升降。 当云层落下时, 它会收集更多的水, 并变大 。

    Types of Precipitation
    ::降水类型

    Why does it snow instead of rain? Air temperature determines which type of precipitation falls. Rain falls if the air temperature is above freezing (0° C or 32° F). Frozen precipitation falls if the air or ground is below freezing. Frozen precipitation may fall as snow, sleet, or freezing rain. You can see how the different types form in  Figure   .
    ::为何降雪而不是降雨?空气温度决定哪一种降水。如果空气温度高于冰冻度(摄氏度或32°F),雨就会降下。如果空气或地面低于冰冻度,冰冻降水就会降下。冷冻降水会像雪、水里或冻雨一样降下。您可以看到图表中不同种类的降水情况。

    lesson content
     
    Frozen precipitation may fall as snow, sleet, or freezing rain.

    Snow falls when water vapor condenses as ice crystals. The air temperature is below freezing all the way to the ground, so the ice crystals remain frozen. They fall as flakes.  Sleet  forms when snow melts as it falls through a layer of warm air and then refreezes. It turns into small, clear ice pellets as it passes through a cold layer near the ground.
    ::当水蒸气像冰晶一样凝结时,雪就落下。空气温度一直到地面都冻在下面,所以冰晶仍然冻结。它们像片片一样落下。当雪从温暖的空气层滑落,然后再冷却时,它们会流出水来。当它穿过地面的冰层时,它会变成小的、清晰的冰块。

    Freezing rain  falls as liquid water. It freezes on contact with cold surfaces near the ground. It may cover everything with a glaze of ice. If the ice is thick, its weight may break tree branches and pull down power lines.
    ::冻雨像液体水一样落下,它与地面附近的冷面接触时冻住,它可能用冰块覆盖一切。如果冰厚,它的重量可能断裂树枝并拉下电线。

    Hail  is another type of frozen precipitation. Hail forms in thunderstorms when strong updrafts carry rain high into the troposphere. The rain freezes into balls of ice called hailstones. This may happen over and over again until the hailstones are as big as baseballs. Hail forms only in cumulonimbus clouds.
    ::冰雹是另一种冷冻的降水。 当强力上升的雨将高雨带入对流层时, 冰雹会形成海风。 雨会冻结在冰块的球中, 称为冰雹。 这可能一次又一次地发生, 直到冰雹和棒球一样大。 冰雹只会在黑云中形成。

    Lesson Summary
    ::经验教训摘要

    • Weather refers to conditions of the atmosphere at a given time and place. It occurs because of unequal heating of the atmosphere. Humidity, clouds, and precipitation are important weather factors.
      ::天气是指特定时间和地点的大气状况,由于大气供暖不均而发生。湿度、云和降水是重要的天气因素。
    • Humidity is the amount of water vapor in the air. Relative humidity is the percent of water vapor in the air relative to the total amount the air can hold. The total amount depends on temperature.
      ::湿度是指空气中的水蒸发量,相对湿度是指空气中水蒸发量相对于空气中总量的百分比。总量取决于温度。
    • Clouds form when water vapor condenses in the air around specs of matter. Clouds are classified on the basis of where and how they form. Types of clouds include cirrus, stratus, and cumulus clouds.
      ::当水蒸气在空气中围绕物质形状凝结时,云会形成云。云会根据云的形成地点和方式进行分类。云的类型包括螺旋、stratus和积聚云。
    • Precipitation is water that falls from clouds. It may fall as liquid or frozen water. Types of frozen precipitation include snow, sleet, freezing rain, and hail.
      ::降水来自云层,可能像液体或冷冻水一样落下,冷冻降水类型包括雪、流水、冷雨和冰雹。

    Lesson Review Questions
    ::经验回顾问题

    Recall
    ::回顾

    1. What is weather? 
      ::什么是天气?
    1. List three weather factors.
      ::列出三个天气因素。
    1. What is humidity?
      ::什么是湿度?
    1. How do clouds form?
      ::云是怎样形成的?
    1. Identify sleet, freezing rain, and hail.
      ::冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹 冰雹

    Apply Concepts
    ::应用概念

    1. Classify the clouds pictured in  Figure   . (Advanced Topic)
      ::将图.(高级专题)所描绘的云层分类。

    lesson content

     

    Think Critically
    ::仔细仔细思考

    1. Explain how dew point is related to air temperature and humidity.
      ::解释露水点与空气温度和湿度的关系。
    1. You are lying in your sleeping bag on a cold morning. Your sleeping bag is wet with water. You know it didn't rain last night. What happened?
      ::你知道昨晚没下雨,怎么了?
    1. Infer why hail forms only in cumulonimbus clouds.
      ::推论为什么冰雹只出现在 积聚的云层中

    Points to Consider
    ::需要考虑的要点

    A clear sky can quickly become covered with clouds. The clouds may bring a change in the weather.
    ::晴朗的天空会很快被云笼罩,云会改变天气。

    • Why does a clear day turn cloudy?
      ::为什么一个晴朗的一天会变成阴云?
    • What causes weather to change?
      ::是什么原因导致天气发生变化?