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SCIENCE ~ CHAPTER 8
WEATHER
Miss Nelson
SECTION 1
Water in the Atmosphere
ANTICIPATORY SET
What happens to the mirror in your
bathroom when you take a hot shower?
ANTICIPATORY SET
What happens to the mirror in your
bathroom when you take a hot shower?
Warm, moist air from the shower comes in
contact with cool surface of the mirror.
The air cools and can hold less water vapor.
As a result, water vapor condenses on the
mirror.
Clouds form in the same way!
STANDARDS
S 6.4.e –
Students know differences in pressure, heat, air
movement, and humidity result in change in
weather
THE BIG IDEA
What is humidity and how is
it measured?
How do clouds form?
What are the three main
types of clouds?
KEY TERMS
 Water cycle – the continual movement of water
among Ear th’s atmosphere, oceans, and land
through evaporation, condensation, and
precipitation
 Evaporation – the process by which water molecules
in liquid water escape into the air as water vapor
 Humidity – the amount of water vapor in a given
volume of air
 Relative humidity - the percentage of water vapor in
the air compressed to the maximum amount of
water vapor that air can contain at a given
temperature
 Psychrometer – an instrument used to measure
relative humidity, consisting of a wet -bulb
thermometer and a dr y -bulb thermometer
KEY TERMS
 Condensation – the process by which molecules
of water vapor in the air become liquid water
 Dew point – the temperature at which
condensation begins
 Cirrus – wispy, feathery clouds made mostly of
ice crystals that form at high levels
 Cumulus – fluf fy, white clouds, usually with flat
bottoms that look like rounded piles of cotton
 Stratus – clouds that form in flat layers and
often cover much of the sky
THE WATER CYCLE
View the diagram on page 301 of
your textbook
Read Water in the Atmosphere on
page 300 of your textbook
HUMIDIT Y
Humidity is a measure of the
amount of water vapor in the air
Air’s ability to hold water vapor
depends on its temperature
Warm air = more water vapor
than cool air
RELATIVE HUMIDIT Y
Weather reports usually refer to the water
vapor in the air as relative humidity
Relative humidity is the percentage of water
vapor that is actually in the air compared to
the maximum amount of water vapor the air
can hold at a particular temperature
Air with a relative humidity of 100 percent is
said to be saturated
MEASURING RELATIVE HUMIDIT Y
Relative humidity can be
measured with an instrument
called a psychrometer
HUMIDIT Y
Read Humidity on pages 301-302 of your
textbook
HOW CLOUDS FORM
Clouds form when water vapor in
the air condenses to form liquid
water or ice crystals
Process is called condensation
HOW CLOUDS FORM
Two conditions are required for
condensation:
Cooling of the air
Presence of particles in the air
THE ROLE OF COOLING
As air cools, the amount of water vapor it can
hold decreases
The water vapor condenses into tiny droplets
of water or ice crystals
The temperature at which condensation
begins is called the dew point
 Dew point above freezing = water droplets
 Dew point below freezing = ice crystals
THE ROLE OF PARTICLES
For water vapor to condense, tiny particles
must be present
The water vapor needs a surface on which to
condense
In cloud formation, most of these particles
are:
 Salt crystals
 Dust from soil
 Smoke
T YPES OF CLOUDS
Scientists classify clouds into
three main types based on their
shape:
Cirrus
Cumulus
Status
Clouds are further classified by
their altitude
CIRRUS CLOUDS
 Wispy, feathery clouds
 Comes from the word meaning
“a curl of hair”
 Form only at high levels where temperatures are very low
 As a result, are made of ice crystals
 Have feathery “hooked” ends
 Look like the scales of a fish
CUMULUS CLOUDS
 Look like fluffy, rounded piles of
cotton
 Word means “heap” or “mass” in
Latin
 Usually indicate fair weather
 When you add the suffix –nimbus,
which means “rain,” you get clouds
that produce thunderstorms
STRATUS CLOUDS
 Form in flat layers
 Strato means “spread out”
 Usually cover most of the sky
 Are a uniform, dull, gray color
 As they thicken they produce
rain or snow
 Called nimbostratus clouds
ALTOCUMULUS AND ALTOSTRATUS
 Part of a cloud’s name may be based on its height
 Clouds that form between 2-6 kilometers above the
surface have the prefix alto- (means high)
 Altocumulus and altostratus are the two main types
of these clouds
FOG
Clouds that form near the ground are
called fog
Often forms when the ground cools at
night after a warm, humid day
More common near bodies of water or
low-lying marshy areas
T YPES OF CLOUDS
Read Types of Clouds on pages 304 – 306
of your textbook
CHECK FOR UNDERSTANDING
What is humidity?
How is it measured?
CHECK FOR UNDERSTANDING
What is humidity?
Humidity is a measure of the amount of water
vapor in the air.
How is it measured?
Humidity is measured using “relative humidity,”
which is the percentage of water vapor that is
actually in the air compared to the amount of
water vapor the air can hold at a particular
temperature. Relative humidity can be measured
using a psychrometer.
GUIDED PRACTICE
How do clouds form?
What are the three main types of
clouds?
GUIDED PRACTICE
How do clouds form?
Clouds form when water vapor in the air
condenses to form liquid water or ice
crystals.
What are the three main types of clouds?
The three main types of clouds are
cirrus, cumulus, and stratus.
INDEPENDENT PRACTICE
Complete Weather 8-1 Independent
Practice