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CK-12 Earth Science Concepts
For Middle School
Dana Desonie, Ph.D.
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AUTHOR
Dana Desonie, Ph.D.
CK-12 Foundation is a non-profit organization with a mission to
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Printed: November 7, 2016
iii
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Forewor
Foreword
Earth science is the study of planet Earth. The CK-12 Earth Science Concept Collection For Middle School presents
Earth science as a set of 14 chapters, each centered around a general area of study, such as plate tectonics or
atmospheric processes. Each chapter is broken down into concepts. Each concept is a mini-lesson that addresses
one specific topic or key objective. The complete CK-12 Earth Science Concept Collection For Middle School is
comprised of 308 concepts.
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Contents
Contents
Forewor
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1
1
2
4
6
8
12
Weather and Climate
1.1
Clouds . . . . .
1.2
Fog . . . . . . .
1.3
Air Masses . . .
1.4
Weather Fronts
1.5
References . . .
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Chapter 1. Weather and Climate
C HAPTER
1
Weather and Climate
Chapter Outline
1.1
C LOUDS
1.2
F OG
1.3
A IR M ASSES
1.4
W EATHER F RONTS
1.5
R EFERENCES
Introduction
What’s the difference between weather and climate?
When you think of Greece, you probably think of sunshine and white buildings. Maybe a beautiful beach. All of
these are typical of Greece, which has a warm Mediterranean climate. In the winter, snow sometimes blankets the
highlands. Rarely, though, it snows in the Greek lowlands. Snow is so uncommon in Athens that when it happens
it brings the city to a standstill. The Acropolis sits on a rocky outcrop above Athens. It’s beautiful, but rare, to see
this ancient ruin covered with snow. The weather in the photo on the right is cold and snowy, but the climate is still
Mediterranean. Climate is the long-term average of weather.
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1.1. Clouds
www.ck12.org
1.1 Clouds
• Explain how clouds form.
• Describe the influence of clouds on weather.
• Describe different types of clouds
Where should you go if you don’t want to see the Sun?
The Pacific Northwest has the most days of heavy cloud cover. 62% of days have more than three-quarters of the
sky covered in clouds in Seattle. Conditions are nearly the same in Portland, Oregon. If you want to see the most
sunshine, the least cloudy cities are in Arizona.
Clouds
Clouds form when air in the atmosphere reaches the dew point. Clouds may form anywhere in the troposphere.
Clouds that form on the ground are called fog.
How Clouds Form
Clouds form when water vapor condenses around particles in the air. The particles are specks of matter, such as dust
or smoke. Billions of these tiny water droplets come together to make up a cloud. If the air is very cold, ice crystals
form instead of liquid water.
Classification of Clouds
Clouds are classified on the basis of where and how they form. Three main types of clouds are cirrus, stratus, and
cumulus; there are other types of clouds, as well (Figure 1.1).
• Cirrus clouds form high in the troposphere. Because it is so cold they are made of ice crystals. They are
thin and wispy. Cirrus clouds don’t usually produce precipitation, but they may be a sign that wet weather is
coming.
• Stratus clouds occur low in the troposphere. They form in layers that spread horizontally and may cover the
entire sky like a thick blanket. Stratus clouds that produce precipitation are called nimbostratus. The prefix
nimbo- means “rain.”
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Chapter 1. Weather and Climate
• Cumulus clouds are white and puffy. Convection currents make them grow upward, and they may grow very
tall. When they produce rain, they are called cumulonimbus.
FIGURE 1.1
Find the cirrus, cirrostratus, and cirrocumulus clouds in the figure. What do they
have in common? They all form high in
the troposphere. Clouds that form in the
mid troposphere have the prefix “alto-”, as
in altocumulus. Where do stratocumulus
clouds form?
Clouds and Temperature
Clouds can affect the temperature on Earth’s surface. During the day, thick clouds block some of the Sun’s rays.
This keeps the surface from heating up as much as it would on a clear day. At night, thick clouds prevent heat from
radiating out into space. This keeps the surface warmer than it would be on a clear night.
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/54320
Summary
• Water vapor condenses on particles in the air to form clouds.
• Clouds block sunlight in the day. Clouds trap heat in the atmosphere at night.
• Cloud types include cirrus, stratus, and cumulus.
Review
1. What happens to turn water vapor into a cloud?
2. What effects to clouds have on temperature?
3. Compare and contrast cirrus, stratus, and cumulus clouds.
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1.2. Fog
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1.2 Fog
• Describe different types of fog.
Where is the most famous fog in the U.S.?
San Francisco, the city by the bay, is known for its fog. There’s an old joke: "I spent the coldest winter of my life
in San Francisco one summer." That’s because of the fog! There is a big difference in temperature between the two
sides of the city. On the Pacific side, where there is fog, temperatures are low. On the bay side, where there isn’t fog,
temperatures are much higher.
Fog
Fog (Figure 1.2) is a cloud on the ground. Fog forms when humid air near the ground cools below its dew point.
Each type of fog forms in a different way.
• Radiation fog forms at night. Skies are clear, and the relative humidity is high. The ground cools as
temperature goes down. Air near the ground cools below its dew point. Tule fog is an extreme form of
radiation fog. This type of fog strikes the Central Valley of California in winter.
• Advection fog is famous in San Francisco, California. Warm, moist air comes in from the Pacific Ocean. The
air blows over the cold California current. When the air cools below its dew point, fog forms. Sea breezes
bring the fog onshore.
• Steam fog appears in autumn when cool air moves over a warm lake. Water evaporates from the lake surface.
As the water vapor cools, it condenses. The fog appears like steam.
• Warm humid air travels up a hillside and cools below its dew point to create upslope fog.
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Chapter 1. Weather and Climate
FIGURE 1.2
(a) Tule fog in the Central Valley of California. (b) Advection fog in San Francisco.
(c) Steam fog over a lake in Algonquin
Park, Canada. (d) Upslope fog around the
peak of Sanqing Mountains in China.
Fog levels are declining along the California coast as climate warms. The change in fog may have big ecological
changes for the state.
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/116506
Summary
• Fog forms when there is a difference in temperature between the land and the air.
Review
1. Why does fog form?
2. What makes tule fog distinctive?
3. Compare and contrast the different types of fog.
5
1.3. Air Masses
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1.3 Air Masses
• Explain how air masses form, move, and influence weather.
Pick an air mass!
A cold dry air mass forms over the interior of Alaska. The mountain is Mt. McKinley in Denali National Park. A
warm wet air mass forms over the ocean. It sneaks onto the coastal area. Which region would you like to visit?
Air Masses
An air mass is a large body of air that has about the same conditions throughout. For example, an air mass might
have cold dry air. Another air mass might have warm moist air. The characteristics of an air mass depend on where
the air mass formed. The air must stay over that location long enough to pick up those characteristics.
Formation of Air Masses
Most air masses form over polar or tropical regions. They may form over continents or oceans. Air masses are moist
if they form over oceans. They are dry if they form over continents. Air masses that form over oceans are called
maritime air masses. Those that form over continents are called continental air masses. The image below shows air
masses that form over or near North America (Figure 1.3).
An air mass takes on the conditions of the area where it forms. For example, a continental polar air mass has cold
dry air. A maritime polar air mass has cold moist air. Which air masses have warm moist air? Where do they form?
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Chapter 1. Weather and Climate
FIGURE 1.3
North American air masses.
Movement of Air Masses
When a new air mass moves over a region it brings its characteristics to the region. This may change the area’s
temperature and humidity. Moving air masses cause the weather to change when they contact different conditions.
For example, a warm air mass moving over cold ground may cause an inversion.
Why do air masses move? Winds and jet streams push them along. Cold air masses tend to move toward the Equator.
Warm air masses tend to move toward the poles. The Coriolis effect causes them to move on a diagonal. Many air
masses move toward the northeast over the U.S. This is the same direction that global winds blow.
Summary
• An air mass has roughly the same temperature and humidity.
• Air masses form over regions where the air is stable for a long enough time. The air takes on the characteristics
of the region.
• Air masses move when they are pushed by high level winds.
Review
1. What is an air mass?
2. Why do air masses form where the air stays in one place for a while?
3. What happens when an air mass moves over a new region?
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1.4. Weather Fronts
www.ck12.org
1.4 Weather Fronts
• Define different types of fronts.
• Explain how fronts create changes in weather.
What happens when one air mass meets another?
This sight is common when one air mass meets another. You can almost see one air mass being pushed up over the
other. When two air masses meet, the result is often a storm. Have you been to the Midwestern United States in the
spring? This is a common sight.
Fronts
When cold air masses move south from the poles, they run into warm air masses moving north from the tropics.
The boundary between two air masses is called a front. Air masses usually don’t mix at a front. The differences
in temperature and pressure cause clouds and precipitation. Types of fronts include cold, warm, occluded, and
stationary fronts.
Cold Fronts
A cold front forms when a cold air mass runs into a warm air mass (Figure 1.4). The cold air mass moves faster
than the warm air mass. So the cold air mass lifts the warm air mass out of its way. As the warm air rises, its water
vapor condenses. Clouds form, and precipitation falls. If the warm air is very humid, precipitation can be heavy.
Temperature and pressure differences between the two air masses cause winds. Winds may be very strong along a
cold front.
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Chapter 1. Weather and Climate
FIGURE 1.4
Cold fronts often bring stormy weather.
As the fast-moving cold air mass keeps advancing, so does the cold front. Cold fronts often bring sudden changes in
the weather. There may be a thin line of storms right at the front that moves as it moves. In the spring and summer,
the storms may be thunderstorms and tornadoes. In the late fall and winter, the storms may bring snow. After a cold
front passes, the cold air mass behind it brings cooler temperatures. The air is likely to be less humid as well. Can
you explain why?
Warm Fronts
When a warm air mass runs into a cold air mass, it creates a warm front (Figure 1.5). The warm air mass is moving
faster than the cold air mass. The warm air mass then flows up over the cold air mass. As the warm air rises, it cools.
This brings about clouds and sometimes light precipitation. Warm fronts move slowly and cover a wide area. After
a warm front passes, the warm air mass behind it brings warmer temperatures. The warm air is also likely to be more
humid.
FIGURE 1.5
Warm fronts generally bring cloudy
weather.
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1.4. Weather Fronts
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Occluded Fronts
With an occluded front, a warm air mass becomes trapped between two cold air masses. The warm air is lifted up
above the cold air (Figure 1.6). Cloudy weather and precipitation along the front are typical.
FIGURE 1.6
How does an occluded front differ from a
warm or cold front?
Stationary Fronts
Sometimes two air masses stop moving when they meet. These stalled air masses create a stationary front. Such a
front may bring clouds and precipitation to the same area for many days.
Summary
• Much of the weather occurs at fronts, where air masses meet.
• In a warm front, a warm air mass slides over a cold air mass. In a cold front, a cold air mass slides under a
warm air mass.
• An occluded front has three air masses: cold, warm, and cold.
Review
1.
2.
3.
4.
What characteristics give warm fronts and cold fronts their names?
Describe a warm front. What weather is found with a warm front?
Describe a cold front. What weather is found with a cold front?
How does an occluded front form?
Explore More
Use the resource below to answer the questions that follow.
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Chapter 1. Weather and Climate
MEDIA
Click image to the left or use the URL below.
URL: https://www.ck12.org/flx/render/embeddedobject/1601
1.
2.
3.
4.
5.
6.
7.
8.
What is a front?
How does a cold front form?
What forms along a cold front?
How does a warm front form?
What type of clouds form at warm fronts?
What type of precipitation is produced from a warm front?
What is a stationary front?
What type of weather can occur at an occluded front?
Summary
Weather conditions include air temperature, humidity, wind speed and direction, and precipitation. Weather is
temporary. People in many states have the joke, "if you don’t like the weather, wait five minutes and it will change."
One way to change the weather is to have a different air mass move over the region. Interactions between air masses
bring about a lot of weather. For example, thunderstorms and tornadoes form along a front. A front is where
air masses meet. Fronts can bring about blizzards, thunderstorms, tornadoes and other types of weather. Weather
prediction is much better than it was in past years. This is due in part to information from satellites. Climate is the
long-term average of weather. The climate of a location depends on its latitude, position relative to the atmospheric
circulation cells, position on a continent, altitude, and position relative to mountains. Areas with roughly the same
climate make up a climate zone. The organisms that live within a climate zone create a unique biome. Earth’s
climate has changed throughout the planet’s history. Earth was frigid at the end of the Pleistocene ice age, but has
been warming since then. Since the Industrial Revolution the pace of warming has increased. In the past few decades
the rate has really intensified. This is related to greenhouse gas emissions, primary from fossil fuel burning. The
destruction of forests also increasing warming. The effects of global warming are already being seen. Treaties and
voluntary actions are needed to reduce emissions to lessen the amount of global warming that will take place in the
future.
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1.5. References
www.ck12.org
1.5 References
1. Courtesy of Christopher M. Klaus, US Department of Energy. Picture showing classification of clouds .
Public Domain
2. (a) marya (Flickr:emdot); (b) Flickr:different2une; (c) Flickr:3rdparty!; (d) Jordan (Flickr:jshansen). Pictures
of tule, advection, steam, and upslope fog . CC BY 2.0
3. Courtesy of National Oceanic and Atmospheric Administration; labels added by CK-12 Foundation. Map of
North American air masses . Public Domain
4. Christopher Auyeung. Diagram of a cold front . CC BY-NC 3.0
5. Christopher Auyeung. Diagram of a warm front . CC BY-NC 3.0
6. Christopher Auyeung. Diagram of an occluded front . CC BY-NC 3.0
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