Lesson 2
What Is Weather?
Fast Fact
Frightening Lightning There is enough energy in a typical lightning bolt to light one million
100-watt bulbs for a week! On average, the world receives 8,640,000 lightning bolts per
day. Lightning usually occurs in connection with thunderstorms. In the Investigate, you
will take a close look at some of the effects of air pressure.
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Air Pressure
Materials
 hot water
 bucket
 0.5-L plastic bottle
 cold water
Procedure
1. CAUTION: Be careful around hot water. Carefully run hot tap water into the bucket
until it is about three-fourths full. Remove the cap from the bottle. Hold the bottle in the hot
water so that the water comes up to its neck. Keep the bottle there for two minutes.
2. Before you remove the bottle from the hot water, screw the cap on tightly.
3. Remove the bottle from the water. Pour the hot water out of the bucket, and replace it with
very cold water. Without removing the cap, hold the bottle in the water. Observe what
happens.
4. Remove the bottle from the water. Hold the neck of the bottle near your ear as you slowly
twist off the cap. What do you hear?
Draw Conclusions
1. When air is warmed, it expands, causing it to put pressure on its container. Was the air
inside the bottle more or less dense than the air outside the bottle when you put on the cap?
2. Did cooling the air in the bottle increase or decrease the pressure inside the bottle?
3. How do you explain what you observed when you cooled the bottle?
4. Inquiry Skill Scientists often make inferences about things they cannot see directly. What
do you infer caused the sound you heard when you opened the bottle?
Investigate Further
Use a weather map to find areas of high and low pressure. Hypothesize what the winds
would be like with each type of pressure. Design and conduct a simple experiment to
test your hypothesis.
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Reading in Science
VOCABULARY
air pressure p. 252
relative humidity p. 253
front p. 254
climate p. 258
SCIENCE CONCEPTS
 how air masses affect weather
 what factors affect climate
READING FOCUS SKILL
CAUSE AND EFFECT Look for causes of weather changes.
Air Masses
Recall that air masses are large bodies of air. Their temperature and humidity are affected by
the areas over which they form. Humidity is the amount of water vapor in the air. For
example, the Gulf of Mexico is a warm sea. An air mass that forms over this area is warm
and wet. Air masses can cover huge areas.
Air masses affect the weather of an area. Weather is the condition of the atmosphere at a
particular time and place. Maritime air masses are humid because they form over the
ocean. Continental air masses are dry because they form over land. Tropical air masses are
warm because they form over the tropics. Polar air masses are cold because they form over
Arctic areas. Air masses are named according to their temperature and humidity. An air
mass that forms over the Gulf of Mexico is a tropical maritime air mass.
Gravity pulls the gases in the atmosphere toward Earth, causing the air to push down on
Earth's surface. Air pressure is the force of the weight of air pressing down on a unit of
area.
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Differences in air pressure at Earth's surface are caused by the unequal heating of Earth. Warm
air masses have fewer air molecules than cold air masses with the same volume. This
means that warm air masses are less dense. They have a lower air pressure than cold air
masses. A change in air pressure is a sign that a different air mass is moving into an area.
The result is a change in weather.
Some air masses bring rain or snow. When air masses hold as much water vapor as possible,
the water vapor condenses to form clouds. The water droplets in clouds grow larger and
heavier until they fall.
Air that is saturated, or is holding as much water vapor as possible, has a relative humidity of
100 percent. Relative humidity is a comparison of the actual amount of water vapor in the
air to the amount of water vapor that would be in the air if it were saturated. Relative
humidity is expressed as a percent.
Relative humidity depends on two factors—the temperature and the amount of water vapor
available. If the amount of water vapor stays the same, reducing the temperature will raise
the relative humidity. Increasing the temperature will lower it.
CAUSE AND EFFECT What causes differences Skin in air pressure at Earth's surface?
Insta-Lab
Air Action
Attach an empty balloon to the end of an empty thread spool. Blow up another balloon, and
twist the neck to keep the air in. Attach the opening to the other end of the spool, and then
untwist the neck. What happens to each balloon? Why?
---see map pg.252
Air masses that form in seven areas affect the weather in the United States. An air mass
has the same temperature, humidity, and air pressure throughout.
---see pictures pg.253
Fog is actually a stratus cloud that rests on or near the ground.
An instrument called a hygrometer measures the amount of water vapor in the air.
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Weather Fronts
As air masses move across the surface of the Earth, they may collide. These collisions usually
happen halfway between one of the poles and the equator. Most of the United States lies in
one of those regions. The two air masses do not mix much when they collide. Each keeps
its own temperature, humidity, and air pressure. As a result, a boundary forms. The
boundary between two air masses that collide is called a front. The air masses are usually
moving at different speeds. The faster air mass pushes against the slower one. No matter
which air mass pushes, the warmer, less-dense air will always be pushed up over the
cooler, denser air. The result is that the weather changes.
Scientists classify fronts by the characteristics and movements of the two air masses. When a
warm air mass moves into an area of cooler air, the boundary between the air masses is
called a warm front. The warmer, less-dense air slides up over the cooler, denser air in a
wide, gentle slope. As the warm air cools, its relative humidity increases, and the water
vapor condenses into clouds. A warm front usually brings a steady rain.
When a cold air mass moves into an area of warmer air, the boundary between the air masses is
called a cold front. The colder, denser air slides under the warmer, less-dense air. As a
result, the warmer air is pushed up. As it cools, the water vapor condenses and forms
clouds that bring precipitation.
Cold air masses can move rapidly. When they do, they push warm air up so quickly that heavy
clouds form. Water vapor condenses in the clouds, and thunderstorms often result.
The boundary between two air masses that are not moving against each other is called a
stationary front. Some of the warmer air mixes with the cooler air, causing clouds to form.
As the clouds are pushed upward, they cool and produce light rain or snow. Because the
front is not moving, the precipitation can last for a long time.
CAUSE AND EFFECT What causes thunderstorms?
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---see pictures pgs. 254 & 255
Cumulonimbus clouds are towering clouds with flat tops. They often produce
thunderstorms.
At a warm front, a warm air mass moves up over a cold air mass.
At a cold front, a cold air mass pushes a warm air mass upward.
The lines with triangles indicate the location of a cold front. The lines with half circles
indicate warm fronts. What is the symbol for a stationary front?
Warm fronts often bring nimbostratus clouds, which form in layers and cover large areas
of the sky.
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Types of Clouds
A cloud is a mass of tiny water droplets or ice crystals visible in the atmosphere. Clouds
form when water vapor condenses on the surfaces of tiny particles in the air. Clouds
are classified by their appearance and their height above sea level.
---see pictures
Cirrus
Cirrocumulus
Cirrostratus
High clouds are made mostly of ice crystals. Cirrus clouds are usually seen in fair
weather, but they may signal a change in the weather.
Altostratus
Altocumulus
Midlevel clouds are made up of water droplets and are white to gray. They may produce
light snow or drizzling rain.
Stratus
Low clouds are made up of water droplets. Stratus clouds produce gray days. If there is
rain, the cloud is a nimbostratus.
Cumulus
Cumulus with Development
Cumulonimbus
As cumulus clouds develop upward, they form cumulonimbus
(kyoo•myoo•loh•NIM•buhs) clouds. These towering clouds bring thunderstorms.
They may also produce hail.
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Global Wind Patterns
Wind is the movement of air from an area of higher pressure to an area of lower pressure.
Recall that these differences in air pressure are caused by the uneven heating of the Earth.
As air cools, air pressure increases. As air warms, air pressure decreases.
Global winds are winds that blow across long distances in predictable patterns. These winds
carry air masses from one region to another. Each of these winds always blows in the same
direction. Global winds do not follow a straight path as they blow from the
poles toward the equator or from the equator toward the poles. They curve clockwise in the
Northern Hemisphere and counterclockwise in the Southern Hemisphere because of Earth's
rotation on its axis. This curving of the winds is called the Coriolis effect.
A jet stream is a band of very strong wind that blows from east to west high in the atmosphere,
usually above 6000 m (20,000 ft). Jet streams form along the upper boundaries of large air
masses when warm air from the tropics meets cold air from the poles. The sudden
temperature change causes a huge difference in air pressure. This results in wind speeds of
up to 498 km/hr (310 mi/hr).
Global winds and jet streams can affect local weather. For example, the prevailing westerlies
move from the Pacific Ocean to the Pacific Coast of the United States. They bring moist,
mild weather. The westerlies bring most of the weather changes across the country from
west to east. During the winter, a jet stream often dips south into the United States,
bringing cold weather from Canada.
CAUSE AND EFFECT What causes wind?
---see diagram
Global Winds
Polar Easterlies
These winds move cold air from both poles toward the equator.
Prevailing Westerlies
These winds blow toward the poles, in a direction opposite to that of the trade winds.
They affect most of the weather in the United States.
Trade Winds
Warm air from the equator moves at high altitudes over the Atlantic Ocean and other
oceans to about the 30° north and south latitudes. There it cools, descends, and begins
to flow back toward the equator. These air movements are known as the trade winds.
When the winds meet near the equator, they form a relatively calm area over the
ocean called the doldrums, which are caused by the rising movement of air rather
than by the horizontal movement.
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Factors That Influence Climate
Climate is the average of all weather conditions in an area over a period of time. Precipitation,
temperature, and ocean currents are three factors that influence climate.
Winds affect precipitation by moving air masses. Warm air carries more moisture than cooler
air. Winds that blow across oceans tend to pick up and hold more moisture than winds that
blow across land. Areas near oceans tend to be cooler and wetter than inland areas. There
may be more cloud formations as cool air masses and warm air masses meet. The interiors
of continents tend to be drier because they are not affected by winds carrying moisture
from the oceans.
Oceans also influence the temperature of an area. Warm ocean currents heat the air above them.
Cool currents cool the air. Winds blow the warmed and cooled air onshore, affecting the
climate along coastal areas. The warm Gulf Stream begins in the warm waters south of
Florida. It carries warm air masses near the United Kingdom and northern Europe.
Therefore, the winter temperatures onshore are somewhat mild. They are able to cultivate
plants that require mild temperatures such as azaleas.
A unique weather event takes place in the central and eastern Pacific Ocean every three to
seven years. This event, known as El Nino, results from unusual warming of surface water
in that area. The warmer water pushes energy and moisture into the atmosphere, changing
global wind and rainfall patterns. In the past, El Nino years have brought tornadoes to
Florida, mudslides to California, smog to Indonesia, and forest fires to Brazil.
Other events—some as sudden as volcanic eruptions and asteroid impacts and others as gradual
as glacier formation—also influence climate. Even humans have caused climate changes
by building large cities and by polluting the air and water.
CAUSE AND EFFECT What effects have El Nino events had on South America?
---see pictures
A thermograph continuously records temperatures. It can help scientists predict and
track weather disturbances such as an El Nino.
The changes in weather patterns that an El Nino brings can spell disaster for some parts
of the world. While South America suffered flooding in 1982-1983, Australia and
Indonesia experienced severe drought.
1. CAUSE AND EFFECT Copy and complete this graphic organizer.
2. SUMMARIZE Summarize this lesson in a short paragraph. Begin the first sentence with
Weather is caused by...
3. DRAW CONCLUSIONS Why should a farmer in New Zealand be concerned that an El
Nino event is predicted in Peru?
4. VOCABULARY Write a sentence that explains what a front is.
Test Prep
5. Critical Thinking You see cumulonimbus clouds forming. What can you conclude has
caused them?
6. Which term describes an air mass that forms over land?
A. continental
B. maritime
C. polar
D. tropical
Writing
Expressive Writing
A haiku is a poem with three lines. The first and third lines each have five syllables, and the
second has seven syllables. Write a haiku about the weather. It should have a seasonal
word as part of the poem.
Math
Solve a Problem
Warm air cools 15°C (27°F) for every 305 m (1000 feet) it is pushed up over a cold air mass. If
air with a temperature of 27°C (80°F) is forced up 1525 m (5000 feet), what is the new
temperature? Show the steps you used to find the solution.
Art
Cloudy Impressions
Use cloth, cotton, and other materials to make a display showing the different types of clouds.
Use library resources to find the names of other clouds not in your text. To go with the
display, write a report in which you identify the weather the clouds would bring.
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