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  • 湿度

    Lesson
    ::经验教训

     

    Side-by-side view of Phoenix, Arizona mountains and Houston, Texas skyline with greenery.
     

    Would you rather spend a summer day in Phoenix or in Houston?
    ::你想在凤凰城或休斯顿过暑假吗?

    In Phoenix, Arizona in July, the average high   is 106°F (41°C). That's hot!  But people who live in Phoenix are told that summer isn't so bad because "it's a dry  heat ." What does that mean? Imagine that one  day , both Phoenix, Arizona and Houston, Texans have a temperature of 90°F (32°C). In Phoenix, the  relative humidity  is 20%. In Houston, the relative humidity is 90%. So in Phoenix it feels like it's 90°, but in Houston it feels like it's 122 (50°C)!  Humidity can make a big difference!
    ::7月在亚利桑那州凤凰城,平均气温为106摄氏度(41摄氏度),这是很热的。但生活在凤凰城的人被告知夏天并不坏,因为“这是干热。”这意味着什么?想象有一天,菲尼克斯、亚利桑那和休斯顿,德克萨斯的气温为90摄氏度(32摄氏度)。在凤凰城,相对湿度为20%。在休斯敦,相对湿度为90%。在凤凰城,这感觉好像是90摄氏度,但在休斯敦,它感觉是122摄氏度(50摄氏度)!潮湿度可以大不一样!

    Humidity
    ::湿度

    Humidity  is the amount of  water vapor  in the air. High humidity increases the chances of   and  .
    ::湿度是空气中的水蒸发量,高湿度增加机会和机会。

    Relative Humidity
    ::相对湿度

    Humidity usually refers to  relative humidity . This is the percent of water vapor in the air relative to the total amount the air can hold. How much water vapor can the air hold? That depends on temperature. Warm air can hold more water vapor than cool air ( Figure ).
    ::湿度通常指相对湿度。 这是空气中水蒸发量相对于空气能保持总量的百分比。 空气中有多少水蒸发量? 这取决于温度。 温暖的空气能容纳更多的水蒸发量, 而不是冷空气( 图表 ) 。

    Graph showing maximum water vapor capacity in g/m³ at temperatures up to 40°C.
     
    How much water vapor can the air hold when its temperature is 40° C?

    Humidity and Heat
    ::湿度和热度

    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 when the air contains as much water vapor as it can hold,  water  condenses on your skin just as fast as it evaporates, providing no cooling benefit. The  heat index  ( Figure ) is a measure of what the temperature feels like because of the humidity.
    ::人们常说 , “ 不是热,而是湿度。 ”湿度会使炎热的一天更热。 当汗水蒸发时,它会冷却你的身体。 但当空气中蓄水量越多,水就会像蒸发一样快速地凝结在你的皮肤上,不会带来冷却的好处。 热指数(Figure)是测量湿度导致的温度的尺度。

    Heat index chart showing apparent temperature based on air temperature and relative humidity.
     
    How hot does it feel when the air temperature is 90°F? It depends on the humidity.

    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 over night. 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 ( Figure ). 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。空气冷却时,水蒸气会保持较少的水蒸气。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。如果温度低于冰冷,冰冻的冰晶形式而不是露水(figure ) 。露水点在百分之百的相对湿度上。你能解释为什么吗?

    Grass on the left covered with dew; grass on the right covered with frost.
     
    The grass on the left is covered with dew. The grass on the right is covered with frost. The difference is the temperature of the grass.

    Science Friday: Snowflake Safari
    ::科学星期五:雪花Safari

    Next snowstorm, grab a magnifying glass and look carefully at snowflakes. Bullet rosettes, stellar  plates  and capped  columns  are just a few of the many varieties of  snow  crystals. In this video by  Science  Friday, physicist Kenneth Libbrecht shares secrets about snowflakes.
    ::下一场雪暴,拿起放大镜,仔细看雪花。 子弹玫瑰、恒星板和圆柱只是许多雪晶种类中的少数。 在科学周五的这段影片中,物理学家肯尼斯·利布雷赫特分享了雪花的秘密。

    Summary
    ::摘要

    • Air reaches its dew point when humidity increases or temperature decreases.
      ::当湿度升高或温度下降时,空气达到其露水点。
    • Water droplets form when the air reaches 100% humidity. If the temperature is cold enough, frost will form.
      ::当空气达到100%湿度时,水滴会形成。如果温度足够寒冷,结霜就会形成。
    • Relative humidity is how the air feels at its temperature and humidity.
      ::相对湿度是空气在温度和湿度时的感受。

    Review
     ::回顾

    1. What is humidity? What is relative humidity?
      ::什么是湿度 相对湿度是什么
    1. Explain what heat index is.
      ::解释一下热指数是什么。
    1. Why does water come out of the air at its dew point?
      ::为什么水在露水点从空气中流出?

    Explore More
    ::探索更多

    Use the resource below to answer the questions that follow.
    ::利用以下资源回答以下问题。

    1. What is humidity?
      ::什么是湿度?
    1. What is water vapor?
      ::什么是水蒸气?
    1. What is absolute humidity?
      ::什么是绝对湿度?
    1. What two factors does relative humidity consider?
      ::相对湿度考虑的两个因素是什么?
    1. What are you likely to see when the humidity is 100%?
      ::当湿度达到100%时,你会看到什么?