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Weather
Earth Science Chapter 16
Chapter 16-1
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Objectives:
Explain how solar heating and water vapor
in the atmosphere affect weather.
Discuss how clouds form and how they are
classified.
Describe how rain, hail, sleet and snow
develop.
Meteorology
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Meteorologists are people who study the
weather.
A> Weather is the specific daily condition
of the Earth’s atmosphere.
The atmospheric factors that interact to
cause weather are air pressure, wind,
temperature, and moisture.
Heating the atmosphere
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The Sun’s heat evaporates water and it also
heats the air directly and indirectly.
Temperature is the measure of air molecule
movement.
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a. The Sun energizes molecules to move more
rapidly…temperatures are high and it feels
warm.
When less energy reaches air molecules = cold
Wind = air mass moving in a
specific direction
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The temperature of air affects air
pressure. As the Sun heats air, the air
expands, becomes less dense, rises, and
has low air pressure.
Cooler air is denser and sinks, causing
high atmospheric pressure.
Wind is air that is moving from an area of
high pressure to an area of low pressure.
Wind speed can be measured with an
anemometer.
Humidity
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Water vapor, or moisture, in the air is called
humidity. The amount of moisture varies from
place to place and time to time. Warmer air can
hold more water vapor.
Relative humidity is the percentage of moisture
the air holds relative to the amount it could hold
at a specific temperature.
When the relative humidity is 100 percent, the air
is saturated.
Dew
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When air cools, it can’t hold as much water
vapor, so the vapor condenses into a liquid
or forms ice crystals.
The dew point is the temperature at which
air is saturated and condensation forms.
Clouds
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B> Clouds form as warm air is forced
upward and cools. Then the water vapor
condenses into tiny droplets that stay
suspended in the air.
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The shape and height of clouds vary with
temperature, pressure and water vapor in air.
Three shapes…
Types of Clouds
Stratus
 Cumulus
 Cirrus
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Cloud Types
Cloud height prefixes
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Cirro = high
Alto = middle
Strato = low
Nimbus clouds are very dark and
so full of water that sunlight can’t
penetrate them.
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Cumulonimbus clouds
are thunderstorm clouds.
High winds can flatten
the top of the cloud into
an anvil-like shape.
When we see
cumulonimbus clouds,
we may have heavy rain,
snow, hail, lightning and
even tornadoes.
Stratus Clouds
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Smooth, gray clouds that cover
the whole sky and block out the
sun are called stratus clouds.
They form at an altitude of
about 2.5 kilometers.
Light rain and drizzle are
associated with stratus clouds.
Nimbostratus clouds bring rain
and snow.
When stratus clouds form near
the ground, the result is fog.
Cirrus Clouds
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Feathery or fibrous
clouds are called
cirrus clouds.
Cirrus clouds form
at very high
altitudes.
Cirrus clouds are
usually made of
ice crystals.
Precipitation
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Water vapor that condenses and
forms clouds can fall to the Earth
as rain, sleet, snow or hail, as
determined by air temperature.
Water that falls from the
atmosphere to the Earth is called
precipitation.
Sleet
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When falling snow or rain
passes through an extremely
cold layer of air, they
sometimes freeze into small
ice pellets called sleet. Sleet
reaches the Earth only in
winter.
Snow
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Snow forms when water vapor
changes directly into a solid.
Snowflakes are flat six-sided ice
crystals that have beautiful shapes.
Because they sometimes clump
together, it is hard to see the
separate crystals. No two
snowflakes are exactly alike.
Hail
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Hail is one of the most
damaging forms of
precipitation forms in
cumulonimbus clouds.
Hailstones are small balls of
ice ranging from 5-75 mm in
diameter.
Hailstones are formed when
water droplets hit ice pellets in
a cloud and freeze.
http://www.youtube.com/watch?v=
wZr8jXo1Uso
Chapter 16-2 Weather Patterns
Objectives:
 Describe how weather is associated with
fronts and high and low pressure systems.
 Explain how tornadoes develop from
thunderstorms.
 Discuss the dangers of severe weather.
A. Because air and moisture move in the
atmosphere, weather constantly changes.
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Air masses are large bodies of air that
form over the same area and take on the
properties of that part of Earth’s surface.
Stormy weather is associated with low
pressure.
Fair weather is associated with high
pressure.
Air pressure is measured with a barometer.
B. Fronts
When two air masses that have different
properties meet, they do not mix easily. A
boundary forms between the two air
masses. This boundary is called a front.
 The weather at a front is usually unsettled
and produces clouds, precipitation and
storms.
 There are four types of fronts:
Cold Front
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A cold front forms when
a mass of colder air
meets and pushes under a
mass of warm air. The
cold air mass forces its
way underneath the warm
air mass and pushes it
upward. Violent storms
are associated with a cold
front. Fair, cool weather
usually follows.
Warm Front
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A warm front forms
when a mass of
warmer air overtakes
a colder air mass and
moves over it. Rain
and showers usually
accompany a warm
front. Hot, humid
weather usually
follows.
Occluded Front
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A cold front travels faster than
a warm front. When a cold
front overtakes a warm front,
an occluded front forms. As
the warm air is pushed
upward, the cold air meets
cool air. An occluded front
may also occur when cool air
overtakes a cold front and
warm air is pushed upward.
An occluded front produces
less extreme weather than a
cold or warm front.
Stationary Front
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When a warm air
mass meets a cold air
mass and no
movement occurs, a
stationary front
forms. Rain may fall
in an area for many
days when a
stationary front is in
place.
C. Severe Weather
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A storm is a violent disturbance in the
atmosphere. It is marked by sudden
changes in air pressure and rapid air
movements. Some storms may cover a
huge area, whereas others cover only a
small area.
1. Thunderstorms
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Occur inside warm, moist air masses and at
fronts.
 Warm, moist air is forced rapidly upwards
where it cools and condenses.
 Strong updrafts of warm air and sinking,
rain-cooled air cause strong winds.
2. Lightning
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Movement of air inside storm
clouds can cause parts of cloud to
have be oppositely charged.
Current flows between the
oppositely charged regions,
forming a lightning bolt.
Lightning, an electrical discharge,
striking the ground is the leading
cause of forest fires.
Lightning
3. Thunder
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When lightning superheats the air,
it expands rapidly and then
contracts. This sudden expansion
and contraction of the air results in
sound waves, or thunder.
4. Tornado
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A tornado is whirling, funnel-shaped
cloud. Tornadoes often form from a
type of cumulonimbus cloud called a
wall cloud. They are usually formed
from severe thunderstorms.
The air pressure at the bottom of the
funnel of swirling air is extremely
low. When this low-pressure area
touches the ground, it acts like a giant
vacuum cleaner.
A tornado over water is called a water
spout. They can travel 95 km per
hour.
Tornadoes 101
Fujita Scale
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The Fujita scale ranks
tornadoes according
to how much damage
they cause.
Only 1% is classified
as an F5.
Cyclones and Anticyclones
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Air pressure has a great effect on the
weather. An area of low pressure that
contains rising warm air is called a
cyclone. In a cyclone, cooler air
moves in to take the place of the
warm air and spiral around and into
the center of the cyclone.
As the winds blow into a low pressure
area, they swirl counterclockwise
direction in the Northern Hemisphere.
Cyclones usually cause rainy, stormy
weather.
The weather caused by high pressure
systems, called anticyclones, is
usually clear, dry and fair.
5. Hurricane
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A hurricane is a powerful cyclone
that forms over tropical oceans.
Hurricanes that form over the
western Pacific Ocean are called
typhoons. In the Indian Ocean they
are called cyclones.
Hurricanes that affect the United
States move from the Eastern
Atlantic near Africa to the north west
striking the East Coast, usually
between Florida and North Carolina.
Hurricanes 101
Formation of Hurricanes
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Warm, moist air begins
to rise rapidly. Cooler
air moves in and the air
begins to spin.
As the air pressure in
the center drops more
air is drawn into the
spinning system. The
system begins to spin
faster.
Damage of a Hurricane
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The high waves and strong winds
of a hurricane often cause great
damage, especially in coastal
areas. Heavy rain may also cause
serious flooding.
Meteorologists can track the path
of a hurricane and issue watches
or warnings. A typical hurricane
lasts for about 9 days. Some may
last 3 or 4 weeks.
6. Blizzard
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A blizzard is a winter storm with strong
winds, cold temps, and low visibility that
lasts more than three hours.
Severe Weather Safety
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When severe weather threatens, the
National Weather Service will issue a
watch or a warning.
During a watch conditions are favorable for
severe weather to develop. Stay tuned to a
radio or television for updates/instructions.
During a warning, severe conditions
already exist; you should immediate action.
Chapter 16-3
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Objectives:
Explain how data are collected for weather
maps and forecasts.
Identify the symbols used in a weather
map.
Weather Forecasts
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Meteorologists study and predict weather.
Weather data from 300 local stations are
used by the National Weather Service to
prepare daily weather maps. The
information is recorded in the form of
numbers and symbols.
Symbols are used to show wind speed and
direction, cloud cover, precipitation,
position and direction of fronts and areas
of high and low pressure.
How to Read a Weather Map
1. Station Models show weather
conditions at a specific location.
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Meteorologists interpret
weather information from
local weather observers,
balloons, satellites, and
weather stations around the
world.
When meteorologists
gather data it is recorded
on a map using a
combination of symbols
forming a station model.
Isotherms
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Isotherms are lines
that connect locations
with the same
temperature. The
number on the end of
an isotherm indicates
the temperature at all
point along the
isotherm.
Isobars
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Isobars are lines that join places
on a weather map that have the
same air pressure.
The number at the end of an
isobar represents the air pressure
recorded at each observation.
The air pressure can be given in
millibars, inches of mercury or
both.
Frontal Movement
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In North America, weather fronts usually
moved from west to east. Why?