12.1 The Causes of Weather MAIN Idea Air masses have different temperatures and amounts of moisture because of the uneven heating of Earth’s surface.. 12.2 Weather Systems MAIN Idea
Trang 1BIG Idea Weather
pat-terns can be observed,
ana-lyzed, and predicted
12.1 The Causes of Weather
MAIN Idea Air masses have
different temperatures and
amounts of moisture because of
the uneven heating of Earth’s
surface.
12.2 Weather Systems
MAIN Idea Weather results
when air masses with different
pressures and temperatures
move, change, and collide.
12.3 Gathering
Weather Data
MAIN Idea Accurate
measure-ments of atmospheric properties
are a critical part of weather
analysis and prediction.
12.4 Weather Analysis
and Prediction
MAIN Idea Several methods
are used to develop short-term
and long-term weather forecasts.
GeoFacts
• The coldest temperature ever
recorded in the United States
was –59.4ºC at McGrath,
Arkansas.
• The sunniest place in the
United States is Yuma, Arizona,
with an average of 4133 hours
of sunshine per year
Meteorology
Strong storm winds Gathering thunderstorm
Fair weather
Trang 2Section 1 • XXXXXXXXXXXXXXXXXX 313
Start-Up Activities
How does a cold air
mass form?
An air mass is a large volume of air that has the
characteristics of the area over which it formed
Procedure
1 Read and complete the lab safety form.
2 Place a full tray of ice cubes on a table
Place a pencil under each end of the tray to raise it off the table.
3 Slide a liquid-crystal temperature strip
under the ice-cube tray.
4 Place two pencils across the top of the tray,
and another temperature strip across them.
5 Record the temperature of each strip at
1-min intervals for about 5 min.
6 Make a graph of the temperature changes
over time for each temperature strip.
Analysis
1 Describe what happened to the
tempera-tures above and below the tray.
2 Explain how this models a mass of cold air.
Types of Fronts Make the
fol-lowing Foldable to help identify the four types of fronts
L
STEP 1 Layer three sheets
of paper so that the top gin or about 3 cm of each sheet can be seen.
STEP 2 Make a 3-cm zontal cut through all three sheets on about the sixth line
hori-of the top sheet.
STEP 3 Make a vertical cut
up from the bottom to meet the horizontal cut.
STEP 4 Place the three sheets on top of a fourth sheet and align the tops and sides of all sheets Label the four tabs
Cold Fronts, Warm Fronts, Stationary Fronts, and Occluded Fronts The Foldable
can be placed in a notebook or stapled along the left edge.
F OLDABLES Use this Foldable with Section 12.2
As you read this section, summarize what you learn about the different fronts Include sketches of air movement and the weather map symbol for each type.
Trang 3■ Figure 12.1 A desert climate is dry
with extreme variations in day and night
temperatures Only organisms adapted to
these conditions, such as this ocotillo, can
survive there.
The Causes of Weather
MAIN Idea Air masses have different temperatures and amounts
of moisture because of the uneven heating of Earth’s surface.
then stepped onto hot pavement on a sunny summer day? Around the world, the Sun heats the different surfaces on Earth to different extents This uneven heating causes weather.
What is meteorology?
What do you enjoy doing on a summer afternoon? Do you like to watch clouds move across the sky, listen to leaves rustling in a breeze, or feel the warmth of sunlight on your skin? Clouds, breezes, and the warmth of sunlight are examples of atmospheric phenomena Meteorology is the study of atmospheric phenomena
The root word of meteorology is the Greek word meteoros, which means high in the air Anything that is high in the sky — raindrops,
rainbows, dust, snowflakes, fog, and lightning — is an example of
a meteor
Atmospheric phenomena are often classified as types of meteors Cloud droplets and precipitation — rain, snow, sleet, and hail — are types of hydrometeors (hi droh MEE tee urz) Smoke, haze, dust, and other particles suspended in the atmosphere are lithometeors (lih thuh MEE tee urz) Examples of electrometeors are thunder and lightning — signs of atmospheric electricity that you can hear or see Meteorologists study these various meteors
Weather versus climate Short-term variations in spheric phenomena that interact and affect the environment and life
atmo-on Earth are called weather These variatiatmo-ons can take place over minutes, hours, days, weeks, months, or years Climate is the long-
term average of variations in weather for a particular area ologists use weather-data averages over 30 years or more to define
Meteor-an area’s climate, such as that of the desert shown in Figure 12.1.
You will read more about Earth’s climates in Chapter 14
Heating Earth’s Surface
As you learned in Chapter 11, sunlight, which is a part of solar radiation, is always heating some portion of Earth’s surface Over the course of a year, the amount of thermal energy that Earth receives is about the same as the amount that Earth radiates back
to space In meteorology, a crucial question is how solar radiation
is distributed around Earth
Objectives
◗ Compare and contrast weather
and climate.
◗ Analyze how imbalances in the
heating of Earth’s surface create
weather.
◗ Describe how air masses form.
◗ Identify five types of air masses.
Review Vocabulary
heat: transfer of thermal energy from
a warmer material to a cooler material
Trang 4■ Figure 12.2 Solar radiation is unequal partly due to the changing angle of incidence of the sunlight In this example it is per- pendicular south of the equator, at the equator it is 60°, and north
Equator
Compare the Angles
of Sunlight to Earth
What is the relationship between the angle of sunlight and amount of heating? The angle at
which sunlight reaches Earth’s surface varies with latitude This results in uneven heating of Earth.
Procedure
1 Read and complete the lab safety form.
2 Turn on a flashlight, and hold it 20 cm above a piece of paper Point the flashlight straight down.
3 Use a pencil to trace the outer edge of the light on the paper This models the angle of sunlight to
Earth at the equator.
4 Keep the flashlight the same distance above the paper, but rotate it about 30°.
5 Trace the outer edge of the light This is similar to the angle of sunlight to Earth at latitudes
nearer the poles.
Analysis
1 Describe how the outline of the light differed between Step 3 and Step 5 Explain why it differed.
2 Compare the amount of energy per unit of area received near the equator to the amount at
lati-tudes nearer the poles.
Imbalanced heating Why are average January
temperatures warmer in Miami, Florida, than in
Detroit, Michigan? Part of the explanation is that
Earth’s axis of rotation is tilted relative to the plane
of Earth’s orbit Therefore, the number of hours of
daylight and amount of solar radiation is greater in
Miami during January than in Detroit
Another factor is that Earth is a sphere and
dif-ferent places on Earth are at difdif-ferent angles to the
Sun, as shown in Figure 12.2. For most of the year,
the amount of solar radiation that reaches a given
area at the equator covers a larger area at latitudes
nearer the poles The greater the area covered, the
smaller amount of heat per unit of area Because
Detroit is farther from the equator than Miami is,
the same amount of solar radiation that heats
Miami will heat Detroit less Investigate this
rela-tionship in the MiniLab on this page
Thermal energy redistribution Thermal
energy areas around Earth maintain about the same
average temperatures over time due to the constant
movement of air and water among Earth’s surfaces,
oceans, and atmosphere The constant movement of
air redistributes thermal energy around the world
Weather — from thunderstorms to large-scale
weather systems — is part of the constant
redistribu-tion of Earth’s thermal energy
Trang 5Careers In Earth Science
Meteorologist A meteorologist
studies air masses and other
atmospheric conditions to prepare
short-term and long-term weather
forecasts An education that includes
physics, Earth science, environmental
science, and mathematics is useful
for a meteorologist To learn more
about Earth science careers, visit
glencoe.com.
Table 12.1 Air Mass Characteristics
Air Mass Type Map Symbol Weather Source Region
Characteristics
Maritime tropical mT Gulf of Mexico, Caribbean Sea, tropical and
subtropical Atlantic Ocean and Pacific Ocean warm, humid hot, humid
Interactive Table To explore more about air masses, visit
glencoe.com.
Air Masses
In Chapter 11, you learned that air over a warm surface can be heated by conduction This heated air rises because it is less dense than the surrounding air On Earth, this process can take place over thousands of square kilometers for days or weeks The result
is the formation of an air mass An air mass is a large volume of
air that has the same characteristics, such as humidity and
temper-ature, as its source region — the area over which the air mass
forms Most air masses form over tropical regions or polar regions
Types of air masses The five types of air masses, listed in
Table 12.1, influence weather in the United States These air masses are common in North America because their source regions are nearby
tropical bodies of water, listed in Table 12.1. In the summer, they bring hot, humid weather to the eastern two-thirds of North America The southwestern United States and Mexico are the source regions of continental tropical air, which is hot and dry, especially in summer
waters of the North Atlantic and North Pacific The one that forms over the North Pacific primarily affects the West Coast of the United States, occasionally bringing heavy rains in winter
Continental polar air masses form over the interior of Canada and Alaska In winter, these air masses can carry frigid air southward
In the summer, however, cool, relatively dry, continental polar air masses bring relief from hot, humid weather
masses.
Trang 6Self-Check Quiz glencoe.com
Surface ( 18 C)
Continental polar air mass
Great Lakes (1 C)
Warming and evaporation
Surface ( 6 C) Snow
■ Figure 12.3 As the cold, nental polar air moves over the warmer Great Lakes, the air gains thermal energy and moisture This modified air cools as it is uplifted because of convec- tion and topographic features, and pro- duces lake-effect snows.
◗ Solar radiation is unequally
distrib-uted between Earth’s equator and its
poles.
◗
◗ An air mass is a large body of air
that takes on the moisture and
tem-perature characteristics of the area
over which it forms.
◗
◗ Each type of air mass is classified by
its source region.
Understand Main Ideas
1 MAIN Idea Summarize how an air mass forms.
2 Explain the process that prevents the poles from steadily cooling off and the
tropics from heating up over time.
3 Distinguish between the causes of weather and climate.
4 Differentiate among the five types of air masses.
Think Critically
5 Predict which type of air mass you would expect to become modified more
quickly: an arctic air mass moving over the Gulf of Mexico in winter or a maritime tropical air mass moving into the southeastern United States in summer.
Earth Science
6 Describe how a maritime polar air mass formed over the North Pacific is modified
as it moves west over North America.
60°N latitude in Siberia and the Arctic Basin are the source regions
of arctic air masses During part of the winter, these areas receive
no solar radiation but continue to radiate thermal energy As a
result, they become extremely cold and can bring the most frigid
temperatures during winter
Air mass modification Air masses do not stay in one place
indefinitely Eventually, they move, transferring thermal energy
from one area to another When 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, as shown in Figure 12.3. When this happens, the
air mass undergoes modification ; it exchanges thermal energy
and/or moisture with the surface over which it travels
Trang 7Cold
Convection current
fl w
Surf
aceow Su
rface
flow
■ Figure 12.4 If Earth did not rotate, two
large convection currents would form as denser
polar air moved toward the equator These
cur-rents would warm and rise as they approached
the equator, and cool as they moved toward
each pole.
Weather Systems
MAIN Idea Weather results when air masses with different
pressures and temperatures move, change, and collide.
However, on a winter day, you might avoid the cold wind Winds are part of a global air circulation system that balances thermal energy around the world.
Global Wind Systems
If Earth did not rotate on its axis, two large air convection currents would cover Earth, as shown in Figure 12.4. The colder and more dense air at the poles would sink to the surface and flow toward the tropics There, the cold air would force warm, equatorial air to rise This air would cool as it gained altitude and flowed back toward the poles However, Earth rotates from west to east, which prevents this situation
The directions of Earth’s winds are influenced by Earth’s
rota-tion This Coriolis effect results in fluids and objects moving in an
apparent curved path rather than a straight line Thus, as trated in Figure 12.5, moving air curves to the right in the north-ern hemisphere and curves to the left in the southern hemisphere
illus-Together, the Coriolis effect and the heat imbalance on Earth create distinct global wind systems They transport colder air to warmer areas near the equator and warmer air to colder areas near the poles Global wind systems help to equalize the thermal energy
on Earth
There are three basic zones, or wind systems, at Earth’s surface
in each hemisphere They are polar easterlies, prevailing westerlies, and trade winds
Objectives
◗ Compare and contrast the three
major wind systems.
◗ Identify four types of fronts.
◗ Distinguish between
high-and low-pressure systems.
Review Vocabulary
convection: the transfer of thermal
energy by the flow of a heated
Trang 8Section 2 • Weather Systems 319
Figure 12.5 The Coriolis effect results in fluids and objects moving in an apparent curved path rather
than a straight line.
Visualizing the Coriolis Effect
Recall that distance divided by time equals speed The equator has a length of about 40,000 km—Earth’s circumference—and Earth rotates west to east once about every 24 hours This means that things on the equator, including the air above it, move eastward at a speed of about 1670 km/h.
Equator 1670km
/h
However, not every location on Earth moves eastward
at this speed Latitudes north and south of the equator have smaller circumferences than the equator Those objects not on the equator move less distance during the same amount of time Therefore, their eastward speeds are slower than objects on the equator Equator 1670km
approxi-a dapproxi-ay lapproxi-ater, it will be eapproxi-ast of Mapproxi-artinique becapproxi-ause the air was moving to the east faster than the island was moving to the east.
Equator
1670km/h
Martinique
1613 km/h
The result is that air moving toward the poles appears
to curve to the right, or east The opposite is true for air moving from the poles to the equator because the eastward speed of polar air is slower than the east- ward speed of the land over which it is moving.
Equator
To explore more about the Coriolis effect, visit glencoe.com.
Trang 9Polar easterlies
NE trade winds
SE trade winds
in which they occur.
Polar easterlies The wind zones between 60°N latitude and the north pole, and 60°S latitude and the south pole are called the
as dense polar air that sinks As Earth spins, this cold, descending air is deflected in a westerly direction away from each pole In the northern and southern hemispheres, the polar easterlies are typi-cally cold winds Unlike the prevailing westerlies, these polar east-erlies are often weak and sporadic
Between polar easterlies and prevailing westerlies is an area called a polar front Earth has two polar fronts located near lati-tudes 60°N and 60°S Polar fronts are areas of stormy weather
Prevailing westerlies The wind systems on Earth located between latitudes 30°N and 60°N, and 30°S and 60°S are called the
surface winds move in an easterly direction toward each pole, as shown in Figure 12.6. Because these winds originate from the West, they are called westerlies Prevailing westerlies are steady winds that move much of the weather across the United States and Canada
torna-does in the United States.
Trade winds Between latitudes 30°N and 30°S are two
cir-culation belts of wind known as the trade winds, which are shown
in Figure 12.6. Air in these regions sinks, warms, and moves toward the equator in a westerly direction When the air reaches the equator, it rises and moves back toward latitudes 30°N and 30°S, where it sinks and the process repeats
associated with the trade winds creates an area of high pressure
This results in a belt of weak surface winds called the horse tudes Earth’s major deserts, such as the Sahara, are under these high-pressure areas
Common usage: condition of being
passed about and widely known;
distribution
Trang 10■ Figure 12.7 Weather in the dle latitudes is strongly influenced by fast-moving, high-altitude jet streams.
mid-VOCABULARY
ACADEMIC VOCABULARY
Generate (JE nuh rayt)
to bring into existence
Wind is generated as air moves from an area of high pressure to an area of low pressure.
Subtropical jet stream
Polar jet stream 90˚
60˚
30˚
winds from the North and the South meet and join, as shown in
Figure 12.6. The air is forced upward, which creates an area of
low pressure This process, called convergence, can occur on a
small or large scale Near the equator, it occurs over a large area
called the intertropical convergence zone (ITCZ) The ITCZ drifts
south and north of the equator as seasons change In general, it
follows the positions of the Sun from March to September in
rela-tion to the equator Because the ITCZ is a region of rising air, it
has bands of cloudiness and thunderstorms, which deliver
mois-ture to many of the world’s tropical rain forests
Jet Streams
Atmospheric conditions and events that occur at the boundaries
between wind zones strongly influence Earth’s weather On either
side of these boundaries, both surface air and upper-level air differ
greatly in temperature and pressure Recall from Chapter 11 that
warmer air has higher pressure than cooler air, and that the
differ-ence in air pressure causes wind Wind is the movement of air
from an area of high pressure to an area of low pressure
A large temperature gradient in upper-level air combined with
the Coriolis effect results in strong westerly winds called jet
streams A jet stream, shown in Figure 12.7, is a narrow band of
fast, high-altitude, westerly wind Its speed varies with the
temper-ature differences between the air masses at the wind zone
bound-aries A jet stream can have a speed up to 185 km/h at altitudes of
10.7 km to 12.2 km
The position of a jet stream varies with the season It generally
is located in the region of strongest temperature differences on a
line from the equator to a pole The jet stream can move almost
due south or north, instead of following its normal westerly
direc-tion It can also split into branches and re-form later Whatever
form or position it takes, the jet stream represents the strongest
core of westerly winds
Types of jet streams The major jet streams, called the polar
jet streams, separate the polar easterlies from the prevailing
wester-lies in the northern and southern hemispheres The polar jet
streams occur at about latitudes 40°N to 60°N and 40°S to 60°S,
and move west to east The minor jet streams are the subtropical
jet streams They occur where the trade winds meet the prevailing
westerlies, at about latitudes 20°N to 30°N and 20°S to 30°S
Jet streams and weather systems Storms form along jet
streams and generate large-scale weather systems These systems
transport cold surface air toward the tropics and warm surface air
toward the poles Weather systems generally follow the path of jet
streams Jet streams also affect the intensity of weather systems by
moving air of different temperatures from one region of Earth to
another
NASA/CORBIS
Trang 11■ Figure 12.8 The type of front formed depends on the
types of air masses that collide.
Identify the front associated with high cirrus clouds.
Fronts
Air masses with different characteristics can collide and result in dramatic weather changes A collision
of two air masses forms a front — a narrow region
between two air masses of different densities Recall that the density of an air mass results from its tem-perature, pressure, and humidity Fronts can cover thousands of kilometers of Earth’s surface
Cold front When cold, dense air displaces warm air, it forces the warm air, which is less dense, up along a steep slope, as shown in Figure 12.8. This type of collision is called a cold front As the warm air rises, it cools and condenses Intense precipita-tion and sometimes thunderstorms are common with cold fronts A blue line with evenly spaced blue triangles represents a cold front on a weather map
The triangles point in the direction of the front’s movement
Warm front Advancing warm air displaces cold air along a warm front A warm front develops a grad-ual boundary slope, as illustrated in Figure 12.8. A warm front can cause widespread light precipitation
On a weather map, a red line with evenly spaced, red semicircles pointing in the direction of the front’s movement indicates a warm front
Stationary front When two air masses meet but neither advances, the boundary between them stalls This front — a stationary front, as shown in
Figure 12.8 — frequently occurs between two fied air masses that have small temperature and pressure gradients between them The air masses continue moving parallel to the front Stationary fronts sometimes have light winds and precipitation
modi-A line of evenly spaced, alternating cold- and front symbols pointing in opposite directions, repre-sents a stationary front on a weather map
warm-Occluded front Sometimes, a cold air mass moves so rapidly that it overtakes a warm front and forces the warm air upward, as shown in Figure 12.8.
As the warm air is lifted, the advancing cold air mass collides with the cold air mass in front of the warm front This is called an occluded front Strong winds and heavy precipitation are common along an occluded front An occluded front is shown on a weather map as a line of evenly spaced, alternating purple triangles and semicircles pointing in the direction of the occluded front’s movement
Cold front
Cold air Warm air
Trang 12Self-Check Quiz glencoe.com
L
Surface
Rising air
■ Figure 12.9 In the northern sphere, winds move counterclockwise around a low-pressure center, and clockwise around a high-pressure center.
hemi-H
Surface
Subsiding air
◗◗ The three major wind systems are
the polar easterlies, the prevailing
westerlies, and the trade winds.
◗
◗ Fast-moving, high-altitude jet
streams greatly influence weather in
the middle latitudes.
◗
◗ The four types of fronts are cold
fronts, warm fronts, occluded fronts,
and stationary fronts.
◗
◗ Air moves in a generally circular
motion around either a high- or
low-pressure center.
Understand Main Ideas
1 MAIN Idea Summarize information about the four types of fronts Explain how
they form and lead to changes in weather
2 Distinguish among the three main wind systems.
3 Describe the Coriolis effect.
4 Explain why most tropical rain forests are located near the equator.
5 Describe how a jet stream affects the movement of air masses.
6 Compare and contrast high-pressure and low-pressure systems.
In Chapter 11, you learned that at Earth’s surface, sinking air is
associated with high pressure and rising air is associated with
low pressure Air always flows from an area of high pressure to
an area of low pressure Sinking or rising air, combined with
the Coriolis effect, results in the formation of rotating high- and
low-pressure systems in the atmosphere Air in these systems
moves in a circular motion around either a high- or
low-pressure center
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
Air in a low-pressure system in the northern hemisphere moves
in a counterclockwise direction, as shown in Figure 12.9. The
opposite occurs in the southern hemisphere for a low-pressure
system As air rises, it cools and often condenses into clouds
and precipitation Therefore, a low-pressure system, whether in
the northern or southern hemisphere, is often associated with
cloudy weather and precipitation
High-pressure systems In a surface high-pressure system,
sinking air moves away from the system’s center when it reaches
Earth’s surface The Coriolis effect causes the sinking air to move
to the right, making the air circulate in a clockwise direction in
the northern hemisphere and in a counter clockwise direction in
the southern hemisphere High-pressure systems are usually
asso-ciated with fair weather They dominate most of Earth’s
subtropi-cal oceans and provide generally pleasant weather
Trang 13◗ State the importance of accurate
weather data.
◗ Summarize the instruments used
to collect weather data from Earth’s
surface.
◗ Analyze the strengths and
weak-nesses of weather radar and weather
satellites.
Review Vocabulary
temperature: the measurement of
how rapidly or slowly particles move
Gathering Weather Data
MAIN Idea Accurate measurements of atmospheric properties are
a critical part of weather analysis and prediction.
must accurately assess the patient’s state of health This usually includes suring body temperature and blood pressure Similarly, in order to forecast the weather, meteorologists must have accurate measurements of the atmosphere.
mea-Data from Earth’s Surface
Meteorologists measure atmospheric conditions, such as ture, air pressure, wind speed, and relative humidity The quality
tempera-of the data is critical for complete weather analysis and precise predictions Two important factors in weather forecasting are the accuracy of the data and the amount of available data
Temperature and air pressure A thermometer, shown in
Figure 12.10, measures temperature using either the Fahr en heit
or Celsius scale Thermometers in most homes are in-glass or bimetallic-strip thermometers Liquid-in-glass ther-mom eters contain a column of either mercury or alcohol sealed in
liquid-a glliquid-ass tube The liquid expliquid-ands when heliquid-ated, cliquid-ausing the column
to rise, and contracts when it cools, causing the column to fall A bimetallic-strip thermometer has a dial with a pointer It contains a strip of metal made from two different metals that expand at dif-ferent rates when heated The strip is long and coiled into a spiral, making it more sensitive to temperature changes
A barometer measures air pressure Some barometers have a
column of mercury in a glass tube One end of the tube is merged in an open container of mercury Changes in air pressure change the height of the column Another type of barometer is an aneroid barometer, shown in Figure 12.10. It has a sealed, metal
sub-chamber with flexible sides Most of the air is removed, so the chamber contracts or expands with changes in air pressure A sys-tem of levers connects the chamber to a pointer on a dial
Bimetallic-strip thermometer
Liquid-in-glass thermometer Aneroid barometer
■ Figure 12.10 Thermometers and
barometers are common weather
instruments.
Trang 14Section 3 • Gathering Weather Data 325
Wind speed and relative humidity An
Figure 12.11, measures wind speed The simplest
type of anemometer has three or four cupped arms,
positioned at equal angles from each other, that
rotate as the wind blows The wind’s speed can be
calculated using the number of revolutions of the
cups over a given time Some anemometers also
have a wind vane that shows the direction of the
wind
A hygrometer (hi GRAH muh tur), such as the
one in Figure 12.11, measures relative humidity
This type of hygrometer has wet-bulb and dry-bulb
thermometers and requires a conversion table to
determine relative humidity When water
evapo-rates from the wet bulb, the bulb cools The
tem-peratures of the two thermometers are read at the
same time, and the difference between them is
cal-culated The relative humidity table lists the
spe-cific relative humidity for the difference between
the thermometers
the amount of moisture in air and the temperature of
the wet bulb in a hygrometer.
Automated surface observing system
Meteorologists need a true “snapshot” of the
atmo-sphere at one particular moment to develop an
accurate forecast To obtain this, meteorologists
analyze and interpret data gathered at the same
time from weather instruments at many different
locations Coordinating the collection of this data
was a complicated process until late in the
twenti-eth century With the development of reliable
auto-mated sensors and computer technology,
instantaneously collecting and broadcasting
accu-rate weather-related data became possible
In the United States, the National Weather
Ser-vice (NWS), the Federal Aviation Administration,
and the Department of Defense jointly established
a surface-weather observation network known as
the Automated Surface Observing System (ASOS)
It gathers data in a consistent manner, 24 hours a
day, every day It began operating in the 1990s
and more than doubled the number of full-time
observation sites, such as the one shown in
Figure 12.12.ASOS provides essential weather
data for aviation, weather forecasting, and
weather-related research
■ Figure 12.12 This ASOS station in the United Kingdom consists of several instruments that measure atmospheric conditions.
Anemometer
■ Figure 12.11 Anemometers are used to measure wind speed based on the rotation of the cups as the wind blows Hygrometers measure relative humidity based on temperature difference between the wet bulb and the dry bulb
Hygrometer
(tcr)Aaron Haupt, (tr)Casella CEL Ltd, (br)Martin Bond/Photo Researchers, Inc.
Trang 15Data from the Upper Atmosphere
While surface-weather data are important, the weather is largely the result of changes that take place high in the troposphere To make accurate forecasts, meteorologists must gather data at high altitudes, up to 30,000 m This task is more difficult than gathering surface data, and it requires sophisticated technology
The instrument used for gathering upper-atmospheric data is a
con-sists of a package of sensors and a battery-powered radio ter These are suspended from a balloon that is about 2 m
transmit-in diameter and filled with helium or hydrogen A radiosonde’s sensors measure the air’s temperature, pressure, and humidity
Radio signals constantly transmit these data to a ground station that tracks the radiosonde’s movement If a radiosonde also mea-sures wind direction and speed, it is called a rawinsonde
(RAY wuhn sahnd), radar + wind + radiosonde.
Tracking is a crucial component of upper-level observations
The system used since the 1980s has been replaced with one that uses Global Positioning System (GPS) and the latest computer tech-nology Meteorologists can determine wind speed and direction by tracking how fast and in what direction a rawinsonde moves The various data are plotted on a chart that gives meteorologists a pro-file of the temperature, pressure, humidity, wind speed, and wind direction of a particular part of the troposphere Such charts are used to forecast atmospheric changes that affect surface weather
Weather Observation Systems
There are many surface and upper-level observation sites across the United States However, data from these sites cannot be used to locate exactly where precipitation falls without the additional help
of data from weather radars and weather satellites
Weather radar A weather radar system detects specific
loca-tions of precipitation The term radar stands for radio detecting
and ranging How does radar work? A radar system generates
radio waves and transmits them through an antenna at the speed
of light Recall that radio waves are electromagnetic waves with wavelengths greater than 10‒3 m The transmitter is programmed
to generate waves that only reflect from particles larger than a cific size For example, when the radio waves encounter raindrops, some of the waves scatter Another antenna receives these scattered waves or echoes because an antenna cannot send and receive sig-nals at the same time An amplifier increases the received wave sig-nals, and then a computer processes and displays them on a monitor From these data, meteorologists can compute the distance
spe-to precipitation and its location relative spe-to the receiving antenna
■ Figure 12.13 Radiosondes gather
upper-level weather data such as air
temperature, pressure, and humidity.
VOCABULARY
ACADEMIC VOCABULARY
Compute (kuhm PYEWT)
to perform mathematical operations
Jane used a calculator to compute the
answers for her math homework.