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LESSON 3
PREDICTING WEATHER
Chapter 7,
Weather and Climate
OBJECTIVES
• Describe high- and low- pressure systems
and the weather associated with each.
• Explain how technology is used to study
weather.
MAIN IDEA
• To predict weather, scientists study air’s
properties and movement.
VOCABULARY
• isobar - lines that connect places with equal
air pressure
• air mass - a large region of the atmosphere in
which the air has similar properties throughout
• front - the boundary between two air masses
• cold front - cold air moves in under a warm
air mass
• warm front - warm air moves in over a cold
air mass
• occluded front - a weather front where a
cold front catches up with a warm front and
then moves underneath the warm front
producing a wedge of warm air between two
masses of cold air
WHAT ARE HIGHS AND LOWS?
• Scientists predict weather by studying how
wind moves, from areas of high pressure to
areas of low pressure.
• A region’s air pressure is shown on weather
maps which include isobars, measured in
millibars, to show places with equal air
pressure.
• A low pressure system is illustrated by an (L)
and has isobar readings that decrease
towards the center.
• A high pressure system (H) has, at its center,
higher air pressure than its surroundings.
• Wind speed is fastest where air pressure
differences are greatest.
• Closely spaced isobars show a large change
over a small area, which indicates high wind
speeds.
• Gentle winds are shown by widely spaced
isobars.
AIR MOVEMENT AROUND
HIGHS AND LOWS
• Air flows outward from the center of a high
pressure system.
• The air leaving rotates clockwise in the Northern
Hemisphere because of the Coriolis effect.
• In a low pressure system, the opposite occurs; air
flows in and rotates counterclockwise.
•
In the Southern Hemisphere the opposite
occurs; the Coriolis effect bends moving air to
the left.
AIR PRESSURE AND WEATHER
• Fair weather is usually found in areas of high
pressure.
• Clouds and precipitation are found in lowpressure areas.
• A drop in air pressure is usually followed by
storms and rain; this can be predicted by a
drop in barometer readings.
QUICK CHECK
Cause and Effect
What kind of weather would be caused
by a drop in air pressure?
The types of weather caused by a drop in air
pressure includes: clouds, rainfall
(precipitation), and storms.
Critical Thinking
In what direction would air move
around a low in the Southern
Hemisphere? Explain.
In the Southern Hemisphere air moves
clockwise around a low pressure system. Air
would move in toward the low and curve to the
right. This is the opposite of the type of motion
in the Northern Hemisphere.
WHAT ARE WEATHER FRONTS?
• Weather conditions vary due to different air
masses.
• Each air mass has its own characteristics.
• Air masses that form over
• land tend to be dry,
• water – humid,
• tropics – hot,
• poles – cold.
• Global winds such as westerlies, west to east,
move air masses.
• Air masses meet at a front.
• One of the three basic types of moving fronts
is a cold front. Cold fronts usually have quick,
heavy storms, followed by clear, cool, and dry
weather.
• Warm fronts usually have light, steady
precipitation, followed by warm and humid
weather.
• In an occluded front the storm is usually brief.
INTERPRETING WEATHER MAPS
• Factors that influence weather are air
pressure, humidity, and temperature.
• Scientists compile this and other information
and use computers to analyze the information
which is then summarized on weather maps.
• On a weather map look at the high and low air
pressure systems.
• High means fair weather and lows mean precipitation.
• Then look at the fronts.
• Fronts always come out of lows.
• In the Northern Hemisphere, fronts rotate
counterclockwise around a low.
• If you are northeast of an approaching front, the
winds will blow from the southeast.
QUICK CHECK
Cause and Effect
Suppose the temperature dropped and
a storm brought heavy rains.
Which type of front might have caused
this?
Cold fronts bring heavy storms.
Critical Thinking
Why does weather change along a
front?
Weather changes along a front because one
type of air mass is replaced by another air
mass.
HOW IS TECHNOLOGY USED TO
STUDY WEATHER?
• Weather stations record temperature, wind
direction, wind speed, and humidity.
• Weather balloons rise up 22 miles (35 km)
into the stratosphere collecting data.
• Replacing balloons is expensive, so satellites
are used.
• Satellites take pictures of earth and weather
conditions.
• Radar, using radio signals, pulses of energy,
detects precipitation.
• Doppler radar measures wind speed.
• Using all these instruments and data,
scientists form detailed pictures of weather
conditions.
QUICK CHECK
Cause and Effect
What effect has technology had on
weather prediction?
Technology allows us to be more accurate in
predicting the weather.
Satellites and radar provide data for large areas.
Computers can process it, so scientists can
predict weather more quickly and accurately.
Critical Thinking
Why is it better to obtain weather
information from more than one
source?
Additional sources provide a “backup” to ensure
accuracy.