Name
CHAPTER 21
Class
Date
Weather
4 Forecasting the Weather
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• How do weather stations communicate weather data?
• How do meteorologists create weather maps?
• How do computer models help meteorologists
forecast weather?
• What are three types of weather that meteorologists
have tried to control?
When Did Weather Forecasting Begin?
People have tried to predict the weather for thousands
of years. In many early cultures, people believed gods
controlled the weather. Some people tried to use the
positions of the moon and stars to predict weather.
Scientific weather predictions began with the
invention of weather instruments, such as the
thermometer. The invention of the telegraph in 1844
helped meteorologists share weather information. Over
time, national weather services formed.
How Do Meteorologists Collect Weather
Data?
READING TOOLBOX
Make a List As you read
this section, list ways that
meteorologists collect,
display, and analyze weather
data. Discuss your list with a
partner.
READING CHECK
1. Explain When did people
begin to make scientific
weather predictions?
Weather observers are people who watch and record
changes in weather conditions. Weather observers are at
weather stations around the world. They report weather
conditions regularly, often many times per hour. Weather
observers record many things, including
•
•
•
•
•
atmospheric pressure
speed and direction of surface wind
precipitation, temperature, and humidity
type, amount, and height of clouds
visibility and general weather conditions
Automatic observing systems also record data. These
systems, along with weather observers, send data to
a collection center. Weather centers around the world
share the data they collect.
READING CHECK
2. Identify What are two
things that weather observers
record?
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Forecasting the Weather continued
How Do Weather Maps Show Weather Data?
READING CHECK
3. Explain Why do meteorologists use weather maps?
Weather maps display data from weather stations.
Weather maps help meteorologists understand
the current weather and predict future weather.
Meteorologists use symbols and colors to show weather
data on a weather map. Meteorologists around the world
use and understand these symbols.
WEATHER SYMBOLS
Some weather maps have groups of symbols that show
the weather conditions at weather stations. This group
of symbols is called a station model. Weather symbols
describe conditions such as cloud cover and wind speed.
The figure below shows examples of weather symbols
and a station model.
Cloud coverage
(fraction of
sky covered)
LOOKING CLOSER
4. List Identify three pieces
of information you can get
from a station model.
1
Clear
8
Scattered
Calm
1–2
3–7
North
Northeast
3
8
4
5
8
7
Broken
8
Overcast Obscured No data
8
Wind speed
(knots)
8–12
13–17
18–22
23–27
48–52
73–77 103–107
Wind direction
East
Southeast
Weather conditions
South
Southwest
West
Station model
Temperature 27 993
Talk About It
Use Models Draw the
symbols for weather
conditions on index cards.
With a partner, take turns
showing the index cards
and naming the weather
condition. Then discuss
which symbol describes the
current weather conditions in
your area.
Drizzle
Fog
Freezing
rain
Smoke
Hail
Haze
Rain
Shower
Snow Thunderstorm Hurricane Tropical
storm
Weather
conditions
Dew point
26
Northwest
Atmospheric
pressure
Barometric
tendency
Cloud cover
Wind direction
Wind speed
The station model shows the air temperature and the
dew point. The dew point is the temperature at which the
condensation rate equals the evaporation rate. The dew
point shows the level of humidity in the air.
The station model also includes the atmospheric
pressure, a three-digit number in the upper right-hand
section. This number describes pressure in millibars
(mb). The line under the number may be flat or angled.
This line shows whether the pressure is rising, falling, or
steady.
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Forecasting the Weather continued
PLOTTING TEMPERATURE AND PRESSURE
Scientists use lines on weather maps to connect points
of equal measurement. Isotherms are lines that connect
points of equal temperature. Isobars are lines that
connect points of equal atmospheric pressure. Scientists
use the spacing and shape of isobars to understand the
movement of wind, as shown below.
Isobar Spacing and Wind Speed
Spacing
Change in Pressure
Wind Speed
Close together
quick
high
Far apart
gradual
low
LOOKING CLOSER
5. Compare Which would
show stronger winds—widely
spaced isobars or closely
spaced isobars?
Isobars that form circles show areas of high or low air
pressure. High-pressure centers are marked with an H.
Low-pressure centers are marked with an L.
PLOTTING FRONTS AND PRECIPITATION
The table below shows the symbols used to represent
fronts on a weather map.
Type of
Front
Color
Warm
red
Cold
blue
Occluded
purple OR red
and blue
Stationary
red and blue
Example
LOOKING CLOSER
6. Draw Next to each front
symbol, or on a separate
piece of paper, use colored
pencils or markers to draw
the symbols for the four
types of fronts in their correct
colors.
Weather maps can also show the types and amounts
of precipitation in an area. Different weather maps show
precipitation in different ways. Some maps use color to
show the type of precipitation. Others use symbols to
show type of precipitation, and color to show amount of
precipitation.
How Do Meteorologists Forecast Weather?
Meteorologists use computers to mark weather
systems on maps. The meteorologists compare the most
recent weather map with older maps. This helps them
track large weather systems and forecast the weather.
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Forecasting the Weather continued
WEATHER DATA
READING CHECK
7. Identify Name two
weather instruments that
provide data for computer
models.
Doppler radar and satellite images give data about
weather patterns. Meteorologists put these data into
computers and create weather models. These models
can show the possible weather conditions for many days.
However, the models are based on general patterns.
Meteorologists must interpret the models to make
accurate forecasts.
Meteorologists use many sets of data to make a
forecast. Some computer models may be better at
predicting certain weather conditions. Other computer
models may be better at predicting weather in one area.
Meteorologists must compare models to make an accurate
forecast. If two or more models give the same information,
meteorologists are more certain about the forecast.
TYPES OF FORECASTS
Meteorologists make different types of forecasts:
READING CHECK
8. Identify Which type of
forecast predicts tomorrow’s
weather?
•
•
•
•
•
Nowcasts use radar to track current weather conditions.
Daily forecasts predict weather for a 48-hour period.
Extended forecasts look ahead 3 to 5 days.
Medium-range forecasts look ahead 3 to 7 days.
Long-range forecasts look ahead weeks or months.
Meteorologists can make accurate weather forecasts
for 0 to 5 days. However, accuracy decreases with each
day. Computers help meteorologists make extended
forecasts. These computers analyze changes in air
movements to predict future weather conditions.
SEVERE WEATHER WATCHES AND WARNINGS
Critical Thinking
9. Infer Is severe weather
more likely during a severe
weather watch or during a
severe weather warning?
Severe weather can destroy buildings and hurt people
if they are not ready. For this reason, meteorologists try
to forecast severe weather early. Meteorologists may
issue, or give, warnings and watches when they forecast
severe weather. Meteorologists issue a watch when
weather conditions make severe weather possible. They
give a warning when they expect severe weather within
24 hours.
Meteorologists use these alerts to tell people how to
be safe during severe weather. The table on the next page
shows some ways to be safe during severe weather.
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Forecasting the Weather continued
Severe Weather Safety Tips
Type of Weather
How to Stay Safe
Thunderstorm
Listen to weather updates. Stay or go inside. Avoid
electrical objects, running water, metal pipes, and phone
lines. If you are outside, avoid tall objects and bodies of
water. Get into a car, if possible.
Tornado
Listen to weather updates. Stay or go inside. Go to
a basement or small, inner room or closet with no
windows. Avoid areas with flying debris. If you are
outside, lie in a low area. Protect your head and neck.
Hurricane
Listen to weather updates. Be ready to follow an
evacuation route. Stay inside. Avoid areas with flying
debris.
Blizzard
Listen to weather updates. Stay or go inside. Dress
warmly. Avoid walking or driving in icy conditions.
LOOKING CLOSER
10. Summarize Name two
things you should do to stay
safe during a tornado.
PREPARING FOR SEVERE WEATHER
People can prepare for severe weather by having a
storm preparedness kit. This kit should include a radio,
batteries, flashlights, rain gear, blankets, water, canned
food, and medicine. People can also practice safety routes
so that they know where to go during severe weather.
Can People Control the Weather?
Some meteorologists are studying ways to control
rain, hail, and lightning. One method for producing rain
is cloud seeding. In this process, scientists add certain
particles to clouds. These particles cause precipitation
to fall from the clouds. Cloud seeding can also prevent
severe precipitation. Some scientists have used cloud
seeding on possible hail clouds. This caused rain to fall
instead of hail.
READING CHECK
11. Define What is cloud
seeding?
HURRICANE CONTROL
Scientists have also seeded hurricanes. The scientists
added frozen nuclei to hurricanes to reduce their
intensity. This process does not always work. Most
scientists have stopped trying to control hurricanes
because the technology is limited.
LIGHTNING CONTROL
Some scientists have tried to control lightning. They
added silver-iodide nuclei to possible lightning storms.
However, scientists are not sure if this process really
affects lightning.
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Section 4 Review
SECTION VOCABULARY
station model a pattern of meteorological
symbols that represents the weather at
a particular observing station and that is
recorded on a weather map
1. Explain How do weather stations communicate weather data? Use the term
station model in your answer.
2. Organize Complete the concept map below with the terms warning, isotherm,
station model, isobar, watch, and weather map.
weather forecasts
display data on
predict storms with
severe weather
forecasts
mark weather
conditions with
issue
3. Describe How do meteorologists use computer models to forecast weather?
4. List Name three types of weather that meteorologists have tried to control.
5. Make Inferences Why might people want to use cloud seeding to produce rain?
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Holt McDougal Earth Science
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Weather