EARTH SCIENCE
Weather and Climate
2.4 Water in the
Atmosphere
March 16th, 2020
Objectives:
Students will be able to:
 Describe humidity and how it is measured.
 Explain how clouds form.
 Name the three main types of clouds.
Key Terms &
Vocabularies
• water cycle
水循环
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evaporation
humidity
relative humidity
psychrometer
condensation
dew point
cirrus
cumulus
stratus
蒸发
湿度
相对湿度
干湿表
冷凝
露点
卷云
积云
层云
Water in the
Atmosphere
Water cycle 水循环-
movement of water between the
atmosphere and earth’s surface.
Water in the
Atmosphere
Evaporation蒸发 –
the process by which water molecules in
liquid water escape into the air as water
vapor.
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Humidity湿度
Humidity湿度 – a measure of the amount of
water vapor in the air.
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Relative Humidity相对湿度 – the
percentage of water vapor in the air
compared to the maximum amount the air
could hold.
Depends on temperature!
Measuring Relative Humidity
• Measured with
a
psychrometer
干湿表
How does it work?
–The dry bulb tells the temperature
–The wet bulb cools depending on the
humidity in the air
•If it is very wet, the water will not
evaporate much…the temperature of the
wet bulb is close to the dry
•If it is very dry, the water will evaporate a
lot…the temperature of the wet bulb will
drop lower than the dry
How Clouds Form
• Clouds form when water vapor in the air
condenses冷凝 to form liquid water or ice
crystals冰晶.
• Condensation冷凝- the process of
molecules of water vapor in the air
become liquid water
• Two conditions required by
condensation:
 Cooling of the air
Dew Point
 The presence of particles in the air
The Role of Cooling
• Dew Point露点-The temperature at
which condensation begins.
Dew point >
o
0
Dew point < 0o
water droplets
ice crystals
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The Role of Particles
For water to condense, tiny particles must
be present so the water has a surface on
which to condense
Particles are usually salt crystals, dust from
soil, and smoke.
Dew露水 is water that condenses above the
freezing point冰点 and land on a solid
surface such as a blade of grass.
Frost霜 is ice that has been deposited
directly onto a surface that is below the
freezing point冰点 .
How Clouds Form
Types of Clouds
• Scientists classify clouds into three
main types based on their shape:
 Cirrus 卷云
 Cumulus 积云
 Stratus 层云
 Clouds are further classified by their
altitude海拔.
Cirrus Clouds卷云
• Wispy, feathery
clouds
• High altitudes
• Mostly made of ice
crystals
• No rain or snow
 Cirrocumulus卷积云-rows of cotton balls
indicating a storm暴风雨 is on the way.
Cumulus Clouds积云
• Means heap or mass
• Look like fluffy, cotton
piles
• Indicate nice weather
when not tall
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Cumulonimbus clouds积雨云
(nimbus means dark) produce
thunderstorms雷阵雨.
Stratus Clouds 层云
• Flat layers
• Often cover the sky,
dull gray color
 As they thicken, they may produce
drizzle毛毛雨, rain or snow, then called
nimbostratus clouds雨层云.
Altocumulus and
Altostratus
高积云和高层云
• Alto means high
• Middle–level
• Higher than regular cumulus积云 and stratus clouds
层云
• Lower than cirrus卷云 and other “high” clouds
Fog 雾
Form at or near the ground
Form when the ground cools at
night after a warm humid day.
More common in areas near
bodies of water or low lying
marshy湿地的 areas.
Homework 作业
1.Please upload your notes
which should be in Chinese
and English.
上传笔记（中英文）；
2.Finish the worksheet.
完成作业单。