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Meteorology
Section 12.1: The Causes of Weather
Section 12.2: Weather Systems
Section 12.3: Gathering Weather Data
Section 12.4: Weather Analysis and Prediction
Section 12.1
The Causes of Weather
Air masses have different temperatures and amounts of
moisture because of the uneven heating of Earth’s
surface.
heat: transfer of thermal energy from a
warmer material to a cooler material
Section 12.1
The Causes of Weather
WHAT IS METEOROLOGY?
Meteorology is the study of atmospheric
phenomena.
The root word of meteorology is the
Greek word meteoros, which means
high in the air.
Section 12.1
The Causes of Weather
WHAT IS METEOROLOGY?
Weather versus climate
Weather is the short-term variations in
atmospheric phenomena that interact and
affect the environment and life on Earth.
Climate is the long-term average (30+
years) of variations in weather for a
particular area.
Section 12.1
The Causes of Weather
HEATING EARTH’S SURFACE
Imbalanced heating
Solar radiation is unequal
partly due to the changing
angle of incidence of the
sunlight. The greater the area
covered by solar radiation,
the smaller the amount of
heat per unit of area.
Section 12.1
The Causes of Weather
AIR MASSES
An air mass is a large volume of air that has
the same characteristics, such as humidity
and temperature, as its source region.
A source region is the area over which an
air mass forms.
Section 12.1
The Causes of Weather
AIR MASSES
Air mass modificationWhen an air mass travels over land or
water that has characteristics different from
those of its source region, the air mass can
acquire some of the characteristics of that
land or water and undergo modification.
Section 12.2
Weather Systems
Weather results when air masses with different pressures
and temperatures move, change, and collide.
convection is the transfer of thermal energy
by the flow of a heated substance
Section 12.2
Weather Systems
GLOBAL WIND SYSTEMS
The directions of Earth’s winds are influenced by Earth’s
rotation.
This Coriolis effect results in fluids and objects moving in an
apparent curved path rather than a straight line.
Section 12.2
Weather Systems
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Section 12.2
Weather Systems
GLOBAL WIND SYSTEMS
The directions of
Earth’s wind systems,
such as the polar
easterlies and the
trade winds, vary with
the latitudes in which
they occur.
Section 12.2
Weather Systems
GLOBAL WIND SYSTEMS
Polar easterlies
The polar easterlies are the wind zones
between 60 N latitude and the north pole,
and 60 S latitude and the south pole.
Section 12.2
Weather Systems
GLOBAL WIND SYSTEMS
Prevailing westerlies
The prevailing westerlies are the wind
systems on Earth located between latitudes
30 N and 60 N, and 30 S and 60 S.
Section 12.2
Weather Systems
GLOBAL WIND SYSTEMS
Trade winds
Between latitudes 30 N and 30 S are
two circulation belts of wind known as
the trade winds.
Section 12.2
Weather Systems
JET STREAMS
A large temperature gradient in upper-level
air combined with the Coriolis effect results
in strong westerly winds called jet streams.
A jet stream is a narrow band of fast, highaltitude, westerly wind.
Section 12.2
Weather Systems
JET STREAMS
Jet streams and weather systems
Storms form along jet streams and can
generate large-scale weather systems.
Jet streams affect the intensity of
weather systems by moving air of
different temperatures from one region
of Earth to another.
Section 12.2
Weather Systems
FRONTS
A collision of two air masses forms a
front—a narrow region between two air
masses of different densities.
Section 12.2
Weather Systems
FRONTS
Cold front
When cold, dense air displaces warm
air, it forces the warm air, which is less
dense, up along a steep slope.
Section 12.2
Weather Systems
FRONTS
Warm front
Advancing warm air displaces cold air
along a warm front. A warm front develops
a gradual boundary slope.
Section 12.2
Weather Systems
FRONTS
Stationary front
When two air masses meet but neither
advances, the boundary between them
stalls. This stationary front frequently
occurs between two modified air masses
that have small temperature and pressure
gradients between them.
Section 12.2
Weather Systems
FRONTS
Occluded front
Sometimes, a cold air mass moves so
rapidly that it overtakes a warm front and
forces the warm air upward. As the warm air
is lifted, the advancing cold air mass collides
with the cold air mass in front of the warm
front.
Section 12.2
Weather Systems
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Section 12.2
Weather Systems
PRESSURE SYSTEMS
Sinking or rising air, combined with the
Coriolis effect, results in the formation of
rotating high- and low-pressure systems
in the atmosphere.
Section 12.2
Weather Systems
PRESSURE SYSTEMS
Low-pressure systems
In surface low-pressure systems, air
rises. When air from outside the
system replaces the rising air, this air
spirals inward toward the center and
then upward.
Section 12.2
Weather Systems
PRESSURE SYSTEMS
In the northern hemisphere, winds move
counterclockwise around a low-pressure
center, and clockwise around a highpressure center.
Low-pressure center
High-pressure center
Section 12.3
Gathering Weather Data
Accurate measurements of atmospheric
properties are a critical part of weather
analysis and prediction.
temperature: the average thermal energy
of the particles that make up a substance
Section 12.3
Gathering Weather Data
DATA FROM EARTH’S SURFACE
Two important factors in weather forecasting
are the accuracy of the data and the amount
of available data.
Section 12.3
Gathering Weather Data
DATA FROM EARTH’S SURFACE
Temperature and air pressure
A thermometer measures temperature using either the
Fahrenheit or Celsius scale.
A barometer measures air pressure.
Section 12.3
Gathering Weather Data
DATA FROM EARTH’S SURFACE
Wind speed and relative humidity
An anemometer measures wind speed.
A hygrometer measures relative humidity.
Section 12.3
Gathering Weather Data
DATA FROM EARTH’S SURFACE
Automated Surface Observing System
The Automated Surface Observing System (ASOS) gathers
data in a consistent manner, 24 hours a day, every day. It
provides essential weather data for aviation, weather
forecasting, and weather-related research.
Section 12.3
Gathering Weather Data
DATA FROM THE UPPER ATMOSPHERE
The instrument used for gathering upper-atmosphere data
is a radiosonde.
A radiosonde’s sensors measure the air’s temperature,
pressure, and humidity.
Section 12.3
Gathering Weather Data
WEATHER OBSERVATION SYSTEMS
Weather radar
A weather radar system detects specific locations of
precipitation.
The Doppler effect is the change in pitch or frequency that
occurs due to the relative motion of a wave, such as sound or
light, as it comes toward or goes away from an observer.
Section 12.3
Gathering Weather Data
WEATHER OBSERVATION SYSTEMS
Weather radar
Analysis of Doppler radar data can be used to
determine the speed at which precipitation moves
toward or away from a radar station.
Section 12.3
Gathering Weather Data
WEATHER OBSERVATION SYSTEMS
Weather satellites
Some weather satellites use infrared imagery to make
observations at night.
Objects radiate thermal energy at slightly different
frequencies. Infrared imagery detects these different
frequencies, which enables meteorologists to map either
cloud cover or surface temperatures.
Section 12.3
Gathering Weather Data
WEATHER OBSERVATION SYSTEMS
Weather satellites
Some satellites use cameras that require visible light to
photograph Earth.
These digital photos are sent back to ground stations, and
their data are plotted on maps. Unlike weather radar,
which tracks precipitation but not clouds, satellites track
clouds but not necessarily precipitation.
Section 12.3
Gathering Weather Data
WEATHER OBSERVATION SYSTEMS
Weather satellites
Another type of satellite imagery that is useful in weather
analysis and forecasting is called water-vapor imagery.
Water-vapor imagery is a valuable tool for weather
analysis and prediction because it shows moisture in the
atmosphere, not just cloud patterns.
Section 12.4
Weather Analysis and Prediction
Several methods are used to develop shortterm and long-term weather forecasts.
Section 12.4
Weather Analysis and Prediction
SURFACE WEATHER ANALYSIS
Station models
A station model is a record of weather
data for a particular site at a particular
time.
Section 12.4
Weather Analysis and Prediction
SURFACE WEATHER ANALYSIS
Station models
Meteorological
symbols are used
to represent
weather data in a
station model.
Section 12.4
Weather Analysis and Prediction
SURFACE WEATHER ANALYSIS
Plotting station model data
To plot data nationwide and globally,
meteorologists use lines that connect
points of equal or constant values.
Section 12.4
Weather Analysis and Prediction
SURFACE WEATHER ANALYSIS
Plotting station model data
Lines of equal pressure are called isobars.
Lines of equal temperature are called
isotherms.
Section 12.4
Weather Analysis and Prediction
TYPES OF FORECASTS
Digital forecasts
A digital forecast is created by applying
physical principles and mathematics to
atmospheric variables and then making a
prediction about how these variables will
change over time.
Section 12.4
Weather Analysis and Prediction
TYPES OF FORECASTS
Analog forecasts
An analog forecast is based on a
comparison of current weather patterns to
similar weather patterns from the past.
Section 12.4
Weather Analysis and Prediction
SHORT-TERM FORECASTS
The most accurate and detailed forecasts are
short term because weather systems change
directions, speeds, and intensities over time.
Section 12.4
Weather Analysis and Prediction
LONG-TERM FORECASTS
Because it is impossible for computers to
model every variable that affects the
weather at a given time and place, all
long-term forecasts are less reliable than
short-term forecasts.
CHAPTER
13
Table Of Contents
Section 13.1 Thunderstorms
Section 13.2 Severe Weather
Section 13.3 Tropical Storms
Section 13.4 Recurrent Weather
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SECTION
13.1
Thunderstorms
How thunderstorms form
• For a thunderstorm to form, three conditions
must exist: a source of moisture, lifting of the
air mass, and an unstable atmosphere.
SECTION
13.1
Thunderstorms
TYPES OF THUNDERSTORMS
• Thunderstorms are often classified according
to the mechanism that causes the air mass
that formed them to rise.
• There are two main types of thunderstorms:
air-mass and frontal.
SECTION
13.1
Thunderstorms
TYPES OF THUNDERSTORMS
Air-mass thunderstorms
• When air rises because of unequal heating of
Earth’s surface beneath one air mass, the
thunderstorm is called an air-mass
thunderstorm.
• There are two kinds of air-mass
thunderstorms.
SECTION
13.1
Thunderstorms
TYPES OF THUNDERSTORMS
Air-mass thunderstorms
• Mountain thunderstorms occur when an air
mass rises by orographic lifting, which
involves air moving up the side of a
mountain.
SECTION
13.1
Thunderstorms
TYPES OF THUNDERSTORMS
Air-mass thunderstorms
• Sea-breeze thunderstorms are local air-mass
thunderstorms that occur because land and
water store and release thermal energy
differently.
• During the day, the temperature of land
increases faster than the temperature of
water. At night, conditions are reversed.
SECTION
13.1
Thunderstorms
TYPES OF THUNDERSTORMS
Frontal thunderstorms
• Frontal thunderstorms are produced by
advancing cold fronts and, more rarely, warm
fronts.
SECTION
13.1
Thunderstorms
THUNDERSTORM DEVELOPMENT
• A thunderstorm usually has three stages: the
cumulus stage, the mature stage, and the
dissipation stage. The stages are classified
according to the direction the air is moving.
Cumulus stage
• In the cumulus stage of a thunderstorm, air
starts to rise vertically. This creates updrafts.
SECTION
13.1
Thunderstorms
• Lightning is the transfer of electrical charge
caused by the rapid rushes of air in a
cumulonimbus cloud.
SECTION
13.1
Thunderstorms
LIGHTNING FACTS
• Lightning kills on average 60 people a year.
• The thunder you hear is the sound produced
as this superheated air rapidly expands and
contracts.
SECTION
13.2
Severe Weather
SEVERE THUNDERSTORMS
Supercells
• Severe thunderstorms can develop into selfsustaining, extremely powerful storms called
supercells.
• These furious storms can last for several
hours and can have updrafts as strong as
240 km/h.
SECTION
13.2
Severe Weather
STRONG WINDS
• Violent downdrafts that are concentrated in a
local area are called downbursts.
• Based on the size of the area they affect,
downbursts are classified as either
macrobursts or microbursts.
SECTION
13.2
Severe Weather
HAIL
• For hail to form, water droplets rise to the
heights of a cumulonimbus cloud where the
temperature is below freezing, encounter ice
pellets, and freeze on contact with the pellets,
which causes the ice pellets to grow larger.
SECTION
13.2
Severe Weather
HAIL
• The second characteristic that allows hail to
form is an abundance of strong updrafts and
downdrafts moving side by side within a
cloud.
SECTION
13.2
Severe Weather
TORNADOES
• A tornado is a violent, whirling column of air in
contact with the ground.
• When a tornado does not reach the ground, it
is called a funnel cloud.
SECTION
13.2
Severe Weather
Tornadoes
Development of tornadoes
• A tornado forms when wind speed and
direction change suddenly with height, a
phenomenon called wind shear.
• Although tornadoes rarely exceed 200 m in
diameter and usually last only a few minutes,
they can be extremely destructive.
SECTION
13.2
Severe Weather
TORNADOES
Tornado classification
• The Enhanced Fujita Tornado Damage scale,
which ranks tornadoes according to their
destruction and estimated wind speed, is
used to classify tornadoes.
SECTION
13.2
Severe Weather
TORNADOES
Tornado distribution
• Most tornadoes—especially violent
ones—form in the spring during the late
afternoon and evening, when the
temperature contrasts between polar air
and tropical air are the greatest. This type
of large temperature contrast occurs most
frequently in the central United States.
SECTION
13.2
Severe Weather
TORNADOES
Tornado distribution
• More than 1000 tornadoes that touch down in
the United States each year occur in a region
called “Tornado Alley,” which extends from
northern Texas through Oklahoma, Kansas,
and Missouri.
SECTION
13.3
Tropical Storms
OVERVIEW OF TROPICAL CYCLONES
• During summer and fall, the tropics experience
conditions ideal for the formation of large,
rotating, low-pressure tropical storms called
tropical cyclones.
SECTION
13.3
Tropical Storms
OVERVIEW OF TROPICAL CYCLONES
Cyclone formation
• Tropical cyclones require two basic
conditions to form: an abundant supply of
warm ocean water and some sort of
mechanism to lift warm air and keep it rising.
SECTION
13.3
Tropical Storms
OVERVIEW OF TROPICAL CYCLONES
Cyclone formation
• When wind speeds around the low-pressure
center of a tropical depression exceed 62
km/h, the system is called a tropical storm.
• If air pressure continues to fall and winds
around the center reach at least 119 km/h, the
storm is officially classified as a cyclone.
SECTION
13.3
Tropical Storms
OVERVIEW OF TROPICAL CYCLONES
Cyclone formation
• Once winds reach at least 119 km/h, another
phenomenon occurs—the development of a
calm center of the storm called the eye.
• The eye of the cyclone is often 30 to 60 km of
calm weather and blue sky.
SECTION
13.3
Tropical Storms
OVERVIEW OF TROPICAL CYCLONES
Cyclone formation
• The strongest winds in a hurricane are
usually concentrated in the eyewall—a
tall band of strong winds and dense
clouds that surrounds the eye.
SECTION
13.3
Tropical Storms
OVERVIEW OF TROPICAL CYCLONES
Cyclone formation
• A hurricane will last until it can no longer
produce enough energy to sustain itself. This
usually happens when the storm has moved
either over land or over colder water.
SECTION
13.3
Tropical Storms
HURRICANE HAZARDS
• The Saffir-Simpson
Hurricane Wind
scale classifies
hurricanes
according to wind
speed, which
implies potential for
flooding and
potential for
property damage.
SECTION
13.4
Recurrent Weather
FLOODS
• An individual thunderstorm can unleash
enough rain to produce floods, and hurricanes
also cause torrential downpours, which result
in extensive flooding.
• Floods can also occur when weather patterns
cause even mild storms to persist over the
same area.
SECTION
Recurrent Weather
13.4
Floods

Low-lying areas are most susceptible to
flooding, making coastlines particularly
vulnerable to storm surges during hurricanes.

Rivers in narrow-walled valleys can rise
rapidly, creating high-powered and
destructive walls of water.
SECTION
13.4
Recurrent Weather
DROUGHTS
• Droughts are extended periods of well-belowaverage rainfall.
• Droughts are usually the result of shifts in
global wind patterns that allow large, highpressure systems to persist for weeks or
months over continental areas.
SECTION
13.4
Recurrent Weather
DROUGHTS
• Because the sinking air prevents humid air
from rising, condensation cannot occur, and
drought sets in until global patterns shift
enough to move the high-pressure system.
SECTION
13.4
Recurrent Weather
DROUGHTS
Heat waves
• An unpleasant side effect of droughts often
comes in the form of heat waves, which are
extended periods of above-average
temperatures.
• Heat waves can be formed by the same highpressure systems that cause droughts.
SECTION
13.4
Recurrent Weather
COLD WAVES
• The opposite of a heat wave is a cold wave,
which is an extended period of below-average
temperatures.
• Cold waves are also brought on by large, highpressure systems. However, cold waves are
caused by systems of continental polar or
arctic origin.
SECTION
13.4
Recurrent Weather
Cold Waves
• Because of the location and the time of year in
which they occur, winter high-pressure
systems are much more influenced by the jet
stream than are summer high-pressure
systems.
SECTION
13.4
Recurrent Weather
Cold Waves
• The winter location of the jet stream can
remain essentially unchanged for days or even
weeks. This means that several polar highpressure systems can follow the same path
and subject the same areas to continuous
numbing cold.
SECTION
13.4
Recurrent Weather
Cold Waves
Windchill index
• The effects of cold air on the human body are
magnified by wind. Known as the windchill
factor, this phenomenon is measured by the
windchill index.