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Chapter 10 – Earth’s Weather Patterns
Essential Question: What Makes up the Atmosphere?
The Atmosphere
The atmosphere, the layers that surround Earth are made up of gases. The two
main gases that make up the atmosphere are nitrogen and oxygen. Another important
gas found in the atmosphere is ozone, a form of oxygen that helps protect Earth’s
surface from the dangerous effects of Ultra Violet (UV) radiation.
The atmosphere consists of five layers.
1. troposhere - The layer closest to the Earth’s surface.
o Most of the weather occurs in the troposphere. As the distance from the Earth’s
surface increases the air becomes thinner and colder in the troposphere.
2. stratosphere - As distance increases in the stratosphere the air becomes thinner,
while the temperature increases.
3. mesosphere - coldest layer of the atmosphere.
o Enough friction is caused in the mesosphere for meteorids to burn up. These
chunks of rock move through space at great speeds. On Earth we see them as
meteors, or shooting stars..
4. theromosphere - extremely hot temperatures reaching 2200 F.
5. exosphere - Outermost layer of the atmosphere.
o There is not a true boundary between the exosphere and space, rather the air
continues to become more thin.
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How the Sun Affects the Atmosphere
Since the sun heats the Earth’s atmosphere and surface, the temperature
usually becomes cooler at night.
The sun does not heat Earth’s surface evenly. The area over the equator is
warmest because the sunlight hits the Earth’s surface here almost directly. The area at
the poles is much colder because the indirect sunlight spreads the sun’s engery out
over a larger area.
The uneven heating of the Earth forces the movement of large bodies of air
known as air masses. The movement of these air masses causes changes in
weather.
Essential Question: What is Weather?
Air Masses
Weather is the condition of the atmosphere at a particular time and place. Air
masses effect the weather of an area. An air mass is a large body of air. The
tempearture and humidity, amount of water vapor in the air, is affected by the area
over where an air mass forms.
o Maritime air masses – humid – form over oceans.
o Continental air masses – dry - form over land.
o Tropical air masses – warm – form over the tropics (near the equator)
o Polar air masses – cold – form over Arctic areas (near the poles)
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The air mass types can be combined such as an air mass over the northern parts of
Canada would be a Polar Continental (Arctic and land).
The unequal heating of the Earth causes differences in air pressure. Air
pressure is the weight of air pressing down on a unit of area. A change in air pressure
indicates that a new air mass is moving into the area, causing a change in the weather.
Air that is holding as much water as possible has relative humidity of 100%.
Relative humidity relies on two factors the temperature and the amount of water vapor
available.
Weather Fronts
The boundary between two air masses that collide is called a front. Most often
the collision of moving air masses occurs halfway between one of the poles and the
equator. The front results in a change in weather.
o Warm Front - warm air moves into an area of cooler air. A warm front usually
brings rain.
o Cold Front - cold air moves into an area of warmer air. Cold fronts usually bring
precipitation, but quickly moving cold air masses often cause thunderstorms.
o Stationary Front - boundary between two air masses that are not moving against
one another. A stationary front often brings a long lasting precipitation.
Types of Clouds
A cloud is a mass of tiny water droplets or ice crystals that can be seen in the
atmosphere. Clouds are classified by their appearance and height above sea level.
o Cirrus - high clouds. Mostly ice crystals. Fair weather, may signal a change in
weather. (cirrocumulus, cirrostratus)
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o Altostratus and Altocumulus - mid-level clouds. Made of water droplets white to
gray in color and produce light snow or drizzling rain at times.
o Stratus - low clouds. Made of water droplets. If rain is produced the cloud is
known as nimbostratus.
o Cumulus - first stages of a developing cumulonimbus cloud. These clouds bring
thunderstorms.
Global Wind Patterns
Wind is the movement of air from an area of higher pressure an area of lower
pressure. Global winds are winds that blow across long distance in predictable
patterns.
o Polar Easterlies - move cold air from both poles to the equator
o Prevailing Westerlies - blow towards the poles. Affect most of the weather in the
United States from west to east.
o Jet Stream - a band of very strong wind that blows from east to west high in the
atmosphere. Throughout the winter a jet stream will dip from Canada to the US
bringing cold weather.
o Trade Winds - movement of warm air from the equator over the oceans, cooling,
dropping, and moving back towards the equator.
Factors that Influence Climate
Climate is the average of all weather conditions in an area over a period of time.
Climate is influenced by different factors.
o Winds - affect precipitation by moving air masses. Warm air carries more moisture
than cooler air. Similarly winds that blow over oceans carry more moisture than
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winds that blow across land. The inland areas of continents are often more dry
because they are not affected by winds carrying moisture near the oceans.
o Oceans - influence the temperature of an area. Warm ocean currents warm the
air above them, while cool currents cool the air.
o El Nino - the unusual warming of surface water in the central and eastern Pacific
Ocean. Warm water pushes energy and moisture into the atmosphere, resulting in
a change in global winds and rainfall patterns.
o Humans - have built large cities and polluted air and water resources.
Essential Question: How Can you Track Severe Storms?
Weather Forecasts
A forecast predicts the weather accurately up to five days into the future. The
weather is studied by meterologists who then provide forecasts. Meteorologists use
instruments to measure atmospheric conditions. A thermometer measures
temperature, a barometer measures air pressure, and a anemometer is used to
measure wind speed.
As technological advances increase so does the accuracy of forecasts. Satellite
images are used to show how the weather of an area is changing. A weather map
shows recent weather conditions across a large area.
Thunderstorms and Tornadoes
A thunderstorm is a strong storm with rain, lightning, and thunder. A
thunderstorm forms when warm, humid air rises rapidly. Most thunderstorms last less
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than one hour. Once a thunderstorm is detected meteorologists look for another
severe weather event such as a tornado.
A tornado is a violently rotating column of air that extends downward from
thunderclouds (cumulonimbus) and touches the ground. Every thunderstorm does not
produce a tornado, tornadoes actually only form in less than one percent of
thunderstorms. The wins of a tornado can reach 250 miles per hour. Tornados can last
only a few minutes or a few hours. Tornadoes are more difficult to predict than other
severe storms, however warnings are often given in enough time for people to reach
safety.
Hurricanes
A hurricane is a large, rotating tropical storm system with winds speeds of at
least 74 miles per hour. At first the storm is called a tropical depression, if the winds
reach a consistent speed of 39 miles per hour it is called a tropical storm and given a
name. Nearly half of the tropical storms that form throughout the year develop into
hurricanes.
The calm center of a hurricane is known as the eye. A hurricane in the
Northwest Pacific Ocean is called a typhoon.
Meteorologists are able to track hurricanes; therefore people are given warning if
they need to evacuate their homes.
Blizzards
A winter storm with strong winds and large amounts of heavy, blowing snow is a
blizzard. The heavy snow should accumulate to at least 7 inches per hour and the
winds should blow for at least 3 hours at 35 miles per hour in order to classify the storm
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as a blizzard. During a blizzard the ability to see often becomes less than one quarter
of a mile and is often near zero.
Similar to other storms a blizzard forms because of differences in air pressure.
The intense winds blow falling snow and also pick up snow that has fallen to the ground
and blow that causing whiteout conditions making travel dangerous.
As a result of the extreme cold frostbite, injury to the skin from freezing and
hypothermia, a condition where body temperature falls dangerously low can affect
anyone stranded during a blizzard.
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