Weather Factors pg 239
Everybody talks about the weather. It may seem like small
talk, but weather is very important to some people. Pilots,
truck drivers, farmers, and other professionals study the
weather because it can affect their jobs.
What is weather?
You can look out the window and see that it’s raining, or
snowing, or windy. But do you really know what weather is?
Weather is the state of the atmosphere at a specific time
and place. Weather describes conditions such as air pressure,
wind, temperature, and moisture content in the air.
How does the Sun affect weather on Earth?
The Sun provides almost all of Earth’s energy. Energy
from the Sun evaporates water on Earth. Evaporated water
enters the atmosphere and forms clouds. Later, the water
falls back to Earth as rain or snow.
The Sun also heats Earth. Heat from the Sun is absorbed
by Earth’s surface, which then heats the air above it. Because
of differences in Earth’s surface, some places in Earth’s
atmosphere are warmer and other places are cooler. Air
currents and water currents move the heat to different
places around Earth. Weather is the result of heat and
Earth’s air and water.
MARGIN NOTE: the Earth's tilt causes uneven heating of the Earth's surface
What affects temperature? Pg 240
Air is made up of molecules that are always moving
randomly, or without any set pattern, even when there is no
wind. Temperature is a measure of the average amount of
motion of molecules. When the temperature is high, air
molecules move rapidly and it feels warm. When the
temperature is low, air molecules move more slowly and it
What causes wind?
Have you ever flown a kite? What do you need in order to
get the kite off the ground and into the air? Kites fly
because air is moving. Air that moves in one direction is
called wind. The Sun heats Earth unevenly, but wind helps
spread the heat around.
As the Sun warms Earth’s surface, air near the surface is
heated by conduction. The air expands, becomes less dense,
and rises. Warm rising air has low atmospheric pressure.
Cool, dense air sinks, bringing about high atmospheric
pressure. Wind results because air moves from areas of high
pressure to areas of low pressure.
The temperature of air can affect air pressure. When air is
cooler, molecules are closer together, creating high pressure.
When air is heated, it expands and becomes less dense. This
creates lower pressure. Beaches are often windy as a result of
air moving from areas of high pressure to areas of lower
pressure, as shown in the figure below.
What tools are used to measure wind? Pg 241
Some instruments measure wind direction and others
measure wind speed. A wind vane, sometimes seen on
houses or barns, has an arrow that points in the direction
from which the wind is blowing. A wind sock, another tool
that shows wind direction, has an open end to catch the
wind. The wind sock fills and points in the direction toward
which the wind is blowing.
An anemometer (a nuh MAH muh tur) is an instrument
that measures wind speed. Anemometers have four open
cups that catch the wind and cause the anemometer to spin.
The faster the wind blows, the faster the anemometer spins.
What is humidity?
Heat evaporates water into the atmosphere. Where does
the water go? Water vapor molecules fit into spaces among
the molecules that make up air. The amount of water vapor
held in the air is called humidity.
Air does not always hold the same amount of water
vapor. More water vapor can be present when the air is
warm than when it its cool. At warm temperatures, the
molecules of water vapor in the air move quickly. As a
result, the molecules do not come together easily, as shown
on the left in the figure below.
At cooler temperatures, the molecules in air move more
slowly. This slower movement allows the water vapor
molecules to stick together. Droplets of liquid water form,
as shown on the right in the figure above. This process of
liquid water forming from water vapor is called condensation.
If enough water is present in the air for condensation to
take place, the air is saturated.
What is relative humidity? Pg 242
Weather forecasters report the amount of moisture in the
air as relative humidity. Relative humidity is a measure of
the amount of moisture held in the air compared with the
amount of moisture the air can hold at a given temperature.
If the weather forecaster says that the relative humidity is 50
percent, this means that the air contains 50 percent of the
water needed for the air to be saturated at that temperature.
When the temperature drops, less water vapor can be
present in the air. If temperatures are low enough, water
vapor will condense to a liquid or form ice crystals. The
temperature at which the air is saturated and condensation
forms is the dew point. Dew point changes as the amount
of water vapor in the air changes.
You’ve probably seen water droplets form on the outside
of a can of cold soda. The cold can cooled the air around it
to its dew point. The water vapor in the air condensed,
forming water droplets on the soda can. Something similar
occurs when you see dew. Air near the ground cools to its
dew point, and then water vapor condenses and forms dew. If
temperatures are near 0° C, frost may form.
Clouds form as warm air is forced upward, expands, and
then cools, as shown in the figure below. When the air
cools, the water vapor molecules in the air come together
around particles of dust or salt in the air. These tiny water
droplets are not heavy enough to fall to Earth. So, they stay
suspended in the air. Billions of these droplets form a cloud.
Classifying Clouds pg 243
Clouds are grouped, or classified, by shape and height.
Some clouds are tall and rise high into the sky. Some clouds
are low and flat. Dense clouds can bring snow or rain. Thin
clouds usually appear on sunny days. Three main factors
determine the shape and height of clouds—temperature,
pressure, and the amount of water vapor in the air.
What are the different types of clouds?
Stratus clouds are layered in smooth, even sheets across the
sky and may be seen on fair, rainy, or snowy days. Usually
stratus clouds form low in the sky. Fog is a stratus cloud that
forms when air is cooled to its dew point near the ground.
Cumulus (KYEW myuh lus) clouds are large, white, puffy
clouds that are often flat on the bottom and sometimes
tower high into the sky. Cumulus clouds can be seen either
in fair weather or in thunderstorms.
Cirrus (SIHR us) clouds are thin, white, feathery clouds.
They form high in the atmosphere and are made of ice
crystals. Although cirrus clouds are linked with fair weather,
they sometimes appear before a storm.
How is height used to name clouds?
Cloud names are sometimes given prefixes to describe the
height of the cloud base. Three common cloud prefixes are
cirro-, alto- and strato-. Cirro- describes high clouds. Alto- is
used for clouds that form at middle levels. Strato- is used
for clouds that form closer to the ground.
Cirrostratus clouds are made of ice crystals and form high
in the air. Usually cirrostratus clouds are a sign of fair
weather. Sometimes they signal a storm is on the way.
Altostratus clouds form at middle levels. If these clouds are
not too thick, sunlight can filter through them.
What types of clouds produce rain and snow?
Dark clouds that contain rain or snow are called nimbus
clouds. Nimbus is a Latin word meaning “dark rain cloud.”
The water content of nimbus clouds is so high that only a
little sunlight can pass through them.
When a cumulus cloud grows into a thunderstorm, it is
called a cumulonimbus (kyew myuh loh NIHM bus) cloud.
These high clouds can tower almost 18 km. Nimbostratus
clouds are layered clouds that usually bring long, steady rain
Precipitation pg 244
Precipitation is water falling from clouds. Precipitation
occurs when cloud droplets combine and grow large enough
to fall to Earth. The cloud droplets form around tiny
particles like salt and dust in the air.
Why are some raindrops bigger than others?
You have probably noticed that some raindrops are bigger
than others. One reason for this size difference is the
strength of updrafts in a cloud. If strong updrafts of wind
keep drops in the air longer, they can combine with other
drops. As a result, they grow larger.
Another factor which affects raindrop size is the rate of
evaporation as the drop falls to Earth. If the air is dry, the
raindrop will get smaller as it falls. Sometimes the raindrop
will evaporate completely before it even hits the ground.
How does temperature affect precipitation?
Air temperature determines what kind of precipitation
will fall—rain, snow, sleet, or hail. How air temperature
affects precipitation is shown in the figures below. When the
air temperature is above freezing, water falls as rain. If the
air temperature is so cold that water vapor changes to a
solid, it snows. Sleet forms if raindrops fall through a layer
of freezing air near Earth’s surface, forming ice pellets.
During thunderstorms, hail forms in cumulonimbus
clouds. Hailstones form when water freezes around tiny centers
of ice. Hailstones get larger as they’re tossed up and
down by rising and falling air. Most hailstones are small, but
sometimes they can get larger than softballs. Of all forms
of precipitation, hail causes the most damage.