LESSON 3

Weather and Climate

Weather Patterns
Key Concept
• What causes weather to
What do you think? Read the two statements below and decide change?
whether you agree or disagree with them. Place an A in the Before
column if you agree with the statement or a D if you disagree. After
you’ve read this lesson, reread the statements to see if you have
changed your mind.
Before Statement After
Weather can only be predicted probabilistically.

Interactions of air masses result in changing

weather conditions.

Study Coach

How is weather described? Summarize What You Read 

After you read each
Meteorologists are scientists who study and predict weather.
paragraph, write a sentence
Weather is the atmospheric conditions, along with short-term or two in your own words
changes, of a certain place at a certain time. Meteorologists use describing what you read.
several factors that describe a variety of atmospheric conditions. Use your sentences to
These factors include air temperature, air pressure, wind speed review the lesson.
and direction, humidity, cloud coverage, and precipitation.

Air Temperature Air temperature is a measure of the average Scientific Vocabulary

kinetic energy of molecules in the air. Recall that kinetic energy molecule
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is the energy an object has because it is moving. When the (noun) two or more atoms
temperature is high, molecules have a high kinetic energy. that are held together by
covalent bonds and act
Therefore, molecules in warm air move faster than molecules in
as a unit
cold air. Air temperatures vary with the time of day, season,
location, and altitude. Temperature is measured using a
thermometer.

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 Air Pressure Air pressure is the pressure that a column of air
exerts on the air, or surface, below it. Air pressure decreases as
altitude increases. Therefore, air pressure is greater at low
altitudes than it is at high altitudes. Air pressure is measured
with an instrument called a barometer. Air pressure is measured
in millibars (mb). The term barometric pressure means “air
Academic Vocabulary pressure.” Knowing the barometric pressure of different areas
forecast helps meteorologists forecast the weather.
(verb) to predict or
estimate something in Wind Wind is created as air moves from areas of high pressure
the future to areas of low pressure. Wind direction is given as the direction
from which the wind is blowing. Meteorologists measure wind
speed using an instrument called an anemometer (a nuh MAH
muh tur).
Scientific Vocabulary Humidity Air can only hold a certain amount of gaseous water
vapor vapor (g/m3). The amount of water vapor in the air is called
(noun) tiny drops of a humidity (hyew MIH duh tee). Humidity can be measured in
liquid that float in the air grams per cubic meter of air (g/m3). When the humidity is high,
there is more water vapor in the air. On a day with high
Scientific Vocabulary humidity, your skin might feel sticky because sweat might not
evaporate evaporate quickly from your skin.
(verb) to change from a
Temperature determines how much water vapor air can
liquid to a gas
contain. Warm air can contain more water vapor than cold air
can. Relative humidity is the amount of water vapor present in
Academic Vocabulary the air compared to the maximum amount of water vapor the air
state could contain at that temperature. Relative humidity is
(verb) to express
measured using a psychrometer. It is stated as a percent. A
something in speech or
writing in a definite or relative humidity of 50 percent means that the amount of water
formal way vapor in the air is one-half of the maximum the air can hold at
that temperature.
Precipitation Droplets in clouds form around small solid

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particles in the atmosphere. These particles might be dust, salt,
or smoke. Precipitation occurs when cloud droplets combine
and become large enough to fall to Earth’s surface. Precipitation
is water in liquid or solid form that falls from the atmosphere.

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 Rain, snow, sleet, and hail are all types of precipitation. Rain
is precipitation that reaches Earth’s surface as droplets of water.
Snow is precipitation that reaches Earth’s surface as solid, frozen
crystals of water. Sleet may start out as snow. The snow melts
into rain as it passes through a layer of warm air and refreezes
when it passes through a layer of below-freezing air. Other times,
sleet is just freezing rain. Hail reaches Earth’s surface as large ice
pellets. Hail starts as a small piece of ice that is repeatedly caught
in an updraft within a cloud. A layer of ice is added with each
lifting. When it becomes too heavy, it falls to Earth.

How does Earth’s surface affect the air

above it?
Have you ever noticed that the weather sometimes stays the
same for several days in a row? Air masses are responsible for
this. Air masses are large bodies of air with distinct temperature
and moisture characteristics. An air mass forms when a large, Academic Vocabulary
high-pressure system stays over an area for several days. The air several
circulating in the high-pressure system comes in contact with (adjective) being more
Earth. This air takes on the temperature and moisture than two but not very
characteristics of the surface below it. Air masses can extend for many
a thousand kilometers or more. Sometimes one air mass covers
most of the United States. Air masses affect weather patterns.
Air Mass Classification Air masses are classified by their
temperature and moisture characteristics. Air masses that form
over land are called continental air masses. Air masses that form
over water are called maritime air masses. Air masses that form
near the equator are called tropical air masses. Those air masses Scientific Vocabulary
that form in cold regions are called polar air masses. Air masses region
that form near the poles are called arctic and antarctic air masses. (noun) a broad geographic
area
Arctic Air Masses Arctic air masses form over Siberia and the
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Arctic. These air masses contain bitterly cold, dry air. During the
winter, an arctic air mass can bring temperatures to -40°C.
Continental Polar Air Masses Land cannot transfer as much
moisture to the air as oceans can. Thus, air masses that form over
land are drier than air masses that form over oceans. Continental
polar air masses are fast moving. They bring cold temperatures in
winter and cool temperatures in summer. Polar air masses that
affect North America often form over Alaska and Canada.

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 Maritime Polar Air Masses Air masses that form over the
northern Atlantic and Pacific Oceans are maritime polar air
masses. These air masses are cold and humid. Maritime polar air
masses often bring cloudy, rainy weather.
Continental Tropical Air Masses Air masses forming in the
tropics over dry, desert land are continental tropical air masses.
These hot and dry air masses bring clear skies and high
temperatures. Continental tropical air masses usually form only
during summer.
Maritime Tropical Air Masses These air masses form over the
Gulf of Mexico, the Caribbean Sea, and the eastern Pacific
Ocean. Maritime tropical air masses are moist air masses. They
bring hot, humid air to the southeastern United States in
summer. During the winter, they can bring heavy snowfall.
Air masses can change as they move over the land and
ocean. Warm, moist air can lose its moisture and become cool.
Cold, dry air can move over water and become moist and warm.

How do differences in pressure affect

weather?
Recall that air pressure is the weight of the molecules in a
large mass of air. Cool air molecules are closer together than
warm air molecules. Cool air masses have high pressure, or
more weight, than warm air masses do. Warm air masses have
low pressure, or less weight.
Pressure Systems A high-pressure system is a large body of
Scientific Vocabulary
circulating air with high pressure at its center and lower
dense pressure outside of the system. Heavy, high-pressure air inside
(adjective) having a high
the system moves away from the center. A low-pressure system
mass per unit of volume
is a large body of circulating air with low pressure at its center

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and higher pressure outside of the system. Air on the outside of
Scientific Vocabulary
the system will spiral in toward the center.
sink
(adjective) to move down Weather Conditions and Pressure Systems The dense air
inside a high pressure system sinks and brings clear skies and fair
Scientific Vocabulary weather. Air inside a low-pressure system rises. The rising air cools
condense and the water vapor condenses. Clouds form, and sometimes
(verb) to make or become precipitation, such as rain or snow, also forms.
more close, compact,
concise, or dense

14  Weather and Climate Reading Essentials

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 What happens when air masses meet?
In 1918, Norwegian Jacob Bjerknes (BYURK nuhs) and his
coworkers developed a new method for forecasting the weather.
Bjerknes noticed that specific types of weather occur at the
boundaries between different air masses. He used the word Science Use v. Common Use
front, to describe this boundary. A weather front is the boundary front
between two air masses. As wind carries an air mass away from Science Use a boundary
the area where it formed, the air mass will eventually bump into between two air masses
another air mass. Common Use a boundary
between opposing armies
Changes in Weather at Frontal Boundaries How the
weather changes depends on the type of front. Changes that
occur at frontal boundaries include the following:
• Sharp
 temperature changes over a relatively short distance
• Change
 in moisture content (humidity)
• Rapid
 shifts in wind speed and direction
• Pressure
 changes
• Clouds
 and precipitation patterns
There are four types of weather fronts: cold fronts, warm
fronts, stationary fronts, and occluded fronts. Refer to the
figures as you read about these types of fronts.
Cold Front A cold front forms when a colder air mass moves
toward a warmer air mass. Cold air is denser than warm air. As
a result, the cold air pushes underneath the warm air mass. The
warm air rises and begins to cool. Water vapor in the air
condenses, and clouds form. Rain showers and thunderstorms
often form along cold fronts. It is common for temperatures to
decrease. The wind becomes gusty and changes direction. In
many cases, cold fronts give rise to severe storms.
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Warm air
Cold air

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 Warm Front The figure below shows a warm front. A warm
front forms when less dense, warmer air moves toward colder,
denser air. The warm air rises above the cold air mass. When the
water vapor in the warm air condenses, a wide blanket of clouds
forms. These clouds often bring steady rain or snow for several
Scientific Vocabulary hours or days. A warm front brings warmer temperatures and
shift causes the wind to shift directions.
(verb) to move to a
different position

Cold air

Warm air
Scientific Vocabulary
stall
(verb) to stop moving or
developing Stationary Front Sometimes a front stalls, or stops, for several
days. Warm air is located on one side of the front and cold air
Scientific Vocabulary on the other side. When the boundary between two air masses
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stationary stalls, the front is called a stationary front. Cloudy skies and
(adjective) a fixed position light rain are common along stationary fronts.
or course

Warm air

Cold air

Occluded Front Cold fronts move faster than warm fronts.

When a cold air mass moves so rapidly that it overtakes a warm

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front, it forces the warm air upward. As the warm air is lifted, the
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Scientific Vocabulary advancing cold air mass collides with another cold air mass that
occlude was in front of the warm air. This is called an occluded front. As
(verb) to close up or shown below, occluded fronts usually bring precipitation.
block off

Warm air

Cold air

Cold air

16  Weather and Climate Reading Essentials

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 How do meteorologists predict the
weather?
The first step in making a weather forecast is to measure the
conditions of the atmosphere. A variety of instruments at many
different locations are used to measure weather variables. Recall
that a thermometer measures temperature. A barometer
measures air pressure. A psychrometer measures relative
humidity, and an anemometer measures wind speed.
Meteorologists use the data from these instruments to make
weather forecasts.
Forecasting Modern weather forecasts are made with the help
of computer models. Computer models are detailed computer
programs that use weather data to solve a set of complex
mathematical formulas to predict the weather. The quality of the Academic Vocabulary
data is critical for complete weather analysis and precise precise
predictions. The formulas created from computer models predict (adjective) very accurate
what temperatures and winds might occur, when and where it and exact
will rain or snow, and what types of clouds will form. The most
detailed forecasts are short term forecasts. However, even short
term forecasts are still only able to be predicted probabilistically.
Long term forecasts are more difficult to make because weather
conditions are constantly changing.

Reread the statements at the beginning of the lesson. Fill in

the After column with an A if you agree with the statement or a
D if you disagree. Did you change your mind?
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Reading Essentials Weather and Climate  17

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