Ch+6+lesson+1-+Read+w_+q+%26+A
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Key Concepts
What is weather?
What variables are used
to describe weather?
How is weather related
to the water cycle?
What is weather?
Weather is the atmospheric conditions, along with short-term
changes, of a certain place at a certain time. Have you ever been
caught in a rainstorm on what began as a sunny day? If so, you
know that weather can change quickly. It can also stay the same
for days.
Weather Variables
Variables are things that can change. Temperature and rainfall
are two of the variables used to describe weather. Meteorologists
are scientists who study and predict weather. They use several
variables that describe a variety of atmospheric conditions. These
variables include air temperature, air pressure, wind speed and
direction, humidity, cloud coverage, and precipitation.
Air Temperature
Air temperature is a measure of the average kinetic energy of
molecules in the air. Kinetic energy is the energy an object has
because it is moving. When the temperature is high, molecules
have a high kinetic energy. Therefore, molecules in warm air move
faster than molecules in cold air. Air temperatures vary with the
time of day, season, location, and altitude.
What do you think? Read the two statements below and decide
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
1. Weather is the long-term average of
atmospheric patterns of an area.
2. All clouds are at the same altitude within the
atmosphere.
Summarize What You
Read After you read each
paragraph, write a
sentence or two in your
own words describing what
you read. Use your
sentences to review the
lesson.
Key Concept
Check
1. Define What is
weather?
Describing Weather
Weather LESSON 1
CHAPTER 6
Reading Essentials Weather 121
These variables include air
temperature, air pressure,
wind speed and w-Hill
Education.
direction, humidity, cloud
coverage, and precipitation.
Copyright © McGraw-Hill Education.
Air Pressure
Air pressure is the pressure
that a column of air exerts on the
air, or surface, below it. Look at
the figure. 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
pressure.” Knowing the barometric
pressure of different areas helps meteorologists predict the
weather.
Wind
Wind is created as air moves from areas of high pressure to
areas of low pressure. Wind direction is given as the direction from
which the wind is blowing. For example, winds that blow from
west to east are called westerlies. Meteorologists measure wind
speed using an instrument called an anemometer (a nuh MAH
muh tur).
Humidity
The amount of water vapor in the air is called humidity
(hyew MIH duh tee). Humidity can be measured in 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 humidity, your skin
might feel sticky because sweat might not evaporate quickly from
your skin.
Relative Humidity
A sponge can absorb water. When it becomes full, it cannot
absorb any more water. In the same way, air can hold only a
certain amount of gaseous water vapor. When air is saturated, it
holds as much water vapor as possible. Temperature determines
how much water vapor air can contain. Warm air can contain more
water vapor than cold air can. Relative humidity is the amount of
water vapor present in the air compared to the maximum
amount of water vapor the air could contain at that temperature.
Relative humidity is measured using a psychrometer. It is stated as
a percent. A relative humidity of 50 percent means that the
amount of water vapor in the air is one-half of the maximum the
air can hold at that temperature.
Visual Check
2. Apply What happens
to air pressure as altitude
decreases?
Reading Check
3. Name What instrument
measures air pressure?
Reading Check
4. Compare humidity and
relative humidity.
122 Weather Reading Essentials
A relative humidity of 50 percent
means that the
amount of water vapor in the air is
one-half of the maximum the
air can hold at that temperature.
They get closer together
barometer
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Dew Point
When a sponge becomes saturated with water, the water starts
to drip from the sponge. Likewise, when air becomes saturated
with water vapor, the water vapor condenses and forms water
droplets. When air near the ground is saturated, water vapor
condenses into a liquid. If the temperature is above 0°C, dew
forms. If the temperature is below 0°C, ice crystals, or frost, form.
Higher in the atmosphere, saturated air forms clouds.
When the temperature decreases, air can hold less moisture.
Eventually, the air becomes saturated, condensation occurs, and
dew forms. The dew point is the temperature at which air is
saturated and condensation can occur.
Clouds and Fog
Think about what happens when you exhale warm air on a cold
day. The warm air you exhale cools. If it reaches its dew point, you
can see the water vapor condense into a foggy cloud in front of
your face. This also happens when warm air containing water
vapor cools as it rises in the atmosphere. When the cooling air
reaches its dew point, water vapor condenses on small particles in
the air and forms droplets. The droplets block and reflect light.
This makes them visible as clouds.
Clouds are water droplets or ice crystals suspended in the
atmosphere. Clouds can have different shapes. Clouds can form at
different altitudes within the atmosphere. Read the table below
that describes different types of clouds. As clouds move, water and
thermal energy are transported from one location to another.
Recall that clouds reflect some of the Sun’s incoming radiation.
A cloud that forms near Earth’s surface is called fog. Fog is a
suspension of water droplets or ice crystals close to or at Earth’s
surface. Fog reduces visibility. Visibility is the distance a person can
see into the atmosphere.
Types of Clouds
Stratus Clouds
flat, white, and
layered
altitude up to
2000 m
Cumulus Clouds
fluffy, heaped, or
piled up
2,000 to 6,000 m
altitude
Cirrus Clouds
• wispy
above 6,000 m
Visual Check
5. Illustrate Look out
the window and find a
cloud. Sketch the shape of
the cloud in the space
above. Then highlight in the
table the type of cloud you
drew.
Reading Check
6. Describe What is fog?
Make a two-tab book and
use it to collect information
about the similarities and
differences between clouds
and fog.
Reading Essentials Weather 123
O
Fog is a
suspension of water droplets or
ice crystals close to or at Earth’s
surface. Fog reduces visibility.
Copyright © McGraw-Hill Education.
Visual Check
7. Compare What is the
difference between snow
and sleet?
Key Concept
Check
8. Identify What
variables are used to
describe weather?
Precipitation
Droplets in clouds form around small solid 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. Types of
precipitation—rain, snow, sleet, and hail—are shown in the figure
above. 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.
The Water Cycle
Precipitation is an important process in the water cycle, shown
at the top of next page. Evaporation and condensation are also
important processes in the water cycle. The water cycle is the
series of natural processes in which water continually moves
among oceans, land, and the atmosphere.
Types of Precipitation
Rain
Sleet
Snow
Hail
124 Weather Reading Essentials
Types of
precipitation—rain, snow, sleet,
and hail—are shown in the gure
above. 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.
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.
Copyright © McGraw-Hill Education.
Most water vapor enters the atmosphere through evaporation.
Water vapor forms as water is heated at the ocean’s surface. Water
vapor cools as it rises in the atmosphere. The cooled water vapor
condenses back into liquid. Clouds form from droplets of liquid
and solid water. Clouds produce precipitation. The precipitation
falls to Earth’s surface and later evaporates, continuing the cycle.
V isual Check
9. Identify Circle the
name of the process in
which liquid water changes
into water vapor.
Key Concept
Check
10. Describe How is
weather related to the
water cycle?
Reading Essentials Weather 125
o
Clouds produce
precipitation. The
precipitation falls to Earth’s
surface and later
evaporates, continuing the
cycle.
Copyright © McGraw-Hill Education.
Mini Glossary
1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains how
relative humidity and dew point are related.
2. Complete the graphic of the water cycle by writing the terms below in the correct order. You may
begin with any process.
air pressure: the pressure that a column of air
exerts on the air, or surface, below it
dew point: the temperature at which air is fully
saturated because of decreasing temperatures
while holding the amount of moisture constant
humidity (hyew MIH duh tee): the amount of
water vapor in the air
precipitation: water, in liquid or solid form, that falls
from the atmosphere
relative humidity: the amount of water vapor
present in the air compared to the maximum
amount of water vapor the air could contain at
that temperature
water cycle: the series of natural processes in
which water continually moves among oceans,
land, and the atmosphere
weather: the atmospheric conditions, along with
short-term changes, of a certain place at a certain
time
cloud formation
condensation
evaporation
precipitation
The Water Cycle c.
b.d.
a.
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 dis-
agree. Did you change your mind?
What do you think
END OF
LESSON
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126 Weather Reading Essentials
All of it has to do with the weather