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

feels cold.


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.


Dew Point

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.


Forming Clouds

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

or snowfall.


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.