Lesson 4
How Can Weather Be Predicted?
Fast Fact
A Winter Wonderland Ice storms deposit massive amounts of ice over everything. In
fact, during a severe ice storm about 45,000 kilograms (99,000 lb) of ice can pile
up on a I5-meter (50-ft) pine tree! In the Investigate, you will make and use a
weather instrument used to help predict weather.
Making a Barometer
Materials
 plastic jar
 scissors
 large round balloon
 safety goggles
 large rubber band
 tape
 wooden craft stick
 large index card
 ruler
Procedure
1. CAUTION: Wear safety goggles. Be careful when using scissors. Use the scissors
to cut the neck off the balloon.
2. Have your partner hold the jar while you stretch the balloon over the open end.
Secure the balloon with the rubber band.
3. Tape the craft stick to the top of the balloon. More than half of the craft stick should
extend beyond the jar's edge.
4. On the blank side of an index card, draw a line and label it Day 1. Tape the card to a
wall. The line should be at the same height as the stick on your barometer. Next to
it, record the current weather.
5. Air pressure is the force of air pressing down on Earth. Measure air pressure by
marking the position of the wooden stick on the index card for the next four days.
Label the marks Days 2-5. Record the pressure and weather each day.
Draw Conclusions
1. How did the air pressure change? What might cause changes in air pressure?
2. Inquiry Skill Scientists use instruments to measure weather data. Infer how a
barometer works.
Investigate Further
Track changes in air pressure and weather for five more days. What can you infer
is the relationship between air pressure and type of weather?
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Reading in Science
VOCABULARY
air mass p. 254
cold front p. 256
warm front p. 256
barometer p. 260
anemometer p. 260
SCIENCE CONCEPTS
 what makes an air mass
 how to read a weather map
READING FOCUS SKILL CAUSE AND EFFECT Look for the causes of changes
in weather.
Air Masses
Have you ever wondered why the weather can be sunny one day and rainy the next?
Movements of air masses cause weather changes. An air mass is a large body of
air. All of the air in an air mass has a similar temperature and moisture level.
Moisture level means the amount of water that is in air.
The temperature and moisture level of an air mass depend on where the air mass
formed. Air masses that form over land are dry. Air masses that form over water
have a lot of moisture in them. In the United States, cold air masses come from the
north. Warm air masses come from the south.
The temperature and moisture level of an air mass affect the kind of weather the air
mass brings. Cold, wet air masses can bring snow to an area. But cold, dry air
masses can bring cool weather
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with little or no precipitation. Warm air masses with a lot of moisture usually bring
precipitation. But warm, dry masses can bring warm weather with little or no
precipitation.
As air masses move, they tend to stay separate from each other. That's because warm air
is lighter than cold air. When they come in contact with each other, warm air
masses are pushed upward and cold air masses sink.
CAUSE AND EFFECT What causes the weather to change?
---see picture pg 254
The map shows where the air masses that affect North America form. Cool air
masses are in blue. Warm air masses are in red.
---see picture pg 255
Air masses do not mix very much with each other. Instead, they stay separate as
they move.
Insta-Lab
Making an Air Mass
Fill a cup halfway with ice cubes. Wait five minutes. With one hand, pour chilled water
into the cup. Hold the other hand over the cup as you pour the water. What do you
feel? If the air you felt were an air mass, how would you describe it? In a cold
front, the air is colder behind the front than ahead of it.
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Fronts
When air masses move, they come into contact with other air masses. The border
between one air mass and another is called a front. Most storms happen at fronts.
There are two main types of fronts: cold fronts and warm fronts. A cold front forms
where a cold air mass moves under a warm air mass. This causes the warm air mass
to move upward. As the warm air mass moves up, it begins to cool. Remember that
water vapor condenses when it cools. The condensing water vapor in the upwardmoving air mass forms clouds. It might begin to rain along the front.
Thunderstorms will often develop. Also, the air temperature will become cooler as
the cold air mass moves forward.
A warm front forms where warm air moves over cold air. The warm air slides up over
the cold air as it moves forward. Warm fronts generally move slowly. Because of
this, warm fronts bring steady rain instead of thunderstorms. Warm fronts are then
followed by clear, warm weather as the warm air mass moves over the area.
Fronts do not always move. A front that stays in one place for many days is called a
stationary front. Stationary fronts happen when the two air masses along a front do
not have enough energy to move. The weather along a stationary front is often
cloudy and wet. This kind of front can leave many inches of snow or cause
flooding rains.
For this reason, stationary fronts can be dangerous.
Different kinds of fronts move differently. Because of this, they cause different kinds of
clouds to form. The types of clouds in an area can help you predict the weather.
CAUSE AND EFFECT What are the effects of a cold front?
---see pictures
In a warm front, the air is warmer behind the front than ahead of it.
In a cold front, the air is colder behind the front than ahead of it.
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---see pictures
STRATUS CLOUDS
Stratus clouds often occur along warm fronts.
Stratus clouds can develop into nimbostratus clouds. Nimbostratus clouds bring
light rain or snow showers.
CUMULUS CLOUDS
Cumulus (KYOO•myuh•luhs) clouds are common on clear, warm days.
Cumulus clouds can develop into cumulonimbus, or thunderstorm, clouds.
CIRRUS CLOUDS
Cirrus (SIR•uhs) clouds usually indicate cooler, fair weather.
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Weather Maps
Have you ever used a street map to find a friend's house? Have you ever used a trail
map while hiking? Another kind of map you can use is a weather map. A weather
map helps you know what the weather is like in an area.
Weather maps use symbols to show the weather. A sun symbol means it is sunny in the
area. A symbol of a cloud with rain means it is raining in the area.
Fronts are also shown on weather maps. The symbol for a warm front is a red line with
half circles along it. A blue line with triangles shows a cold front.
Many weather maps show temperature. Sometimes the temperature is written on the
map. In the United States, the temperature is given in degrees Fahrenheit. Almost
all other countries give the temperature in degrees Celsius. When the temperature is
not written on the weather map, it may be shown using colors. When an area is
warm, it will be colored red (very hot), orange (warm), or yellow (mild).
---see maps
A line with triangles is the symbol for a cold front. The triangles point in the
direction of movement.
A line with half circles is the symbol for a warm front. The half circles point in the
direction the front is moving.
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When an area is cold, it will be colored green (cool) or blue (very cold).
Other information you may see on a weather map includes wind speed and direction, air
pressure, and the highest and lowest temperature in an area for that day.
Where does all the information on a weather map come from? Weather information is
collected at thousands of weather stations across the country.
A weather station is a place that has many different instruments that measure
weather. The information from the weather stations is reported to the National
Weather Service (NWS). The NWS then studies the weather data from all the
weather stations. Each day, the NWS makes weather maps based on the
information collected at all the weather stations.
CAUSE AND EFFECT How would the weather map above look if a warm front
were moving through Arizona?
Math in Science
Interpret Data
---see map
Use the key to read the weather map. What is the weather near Washington, D.C.?
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Measuring Weather
When you say that it is hot or cold outside, you are describing one part of weather—the
temperature. The most accurate way to describe weather is to use data from weather
instruments. In the Investigate, you built one kind of weather instrument—a
barometer. Barometers measure air pressure. Another weather instrument is an
anemometer. Anemometers (an•uh.MAHM.uht•erz) measure wind speed. Other
common weather instruments are wind vanes and rain gauges.
CAUSE AND EFFECT While reading a thermometer, you notice that the
temperature has fallen throughout the day. What might be causing this?
---see picture
This school weather station collects data for students. The data is shared with
other schools.
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Reading Review
1. CAUSE AND EFFECT Copy and complete the following graphic organizer.
---see chart
2. SUMMARIZE Write a paragraph explaining how air masses, fronts, and the water
cycle are related.
3. DRAW CONCLUSIONS Why might it have been more difficult to predict weather
years ago?
4. VOCABULARY Use each vocabulary term in the lesson in a sentence.
Test Prep
5. Critical Thinking You hear on the radio that a cold front is headed toward your
town. What type of weather can you expect?
6. Which of the following instruments measures wind speed?
A. anemometer
B. barometer
C. rain gauge
D. thermometer
Writing
Narrative Writing
Use what you have learned in this chapter to write a short poem about weather and the
water cycle. Use these words in your poem: air mass, front, rain, clouds.
Math
Subtract Decimals
Suppose you record a rainfall of 0.3 cm in the gauge in the morning. You don't empty
the gauge. In the afternoon, the gauge reads 1.5 cm. How much new rain fell?
Language Arts
Be a Weather Forecaster
Make up a weather map of your state. Present your forecast to the class. Be sure to use
the correct vocabulary for the weather you are describing.
For more links and activities, go to www.hspscience.com
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