Science High School Reviewer | Grade 4 Science | Earth Science | Weather and Climate | Lesson 21: Air and Weather

What is in the air?
When you think of wind, what comes to
mind? Wind is really just air in motion. In fact,
air surrounds Earth like a thin blanket. This
blanket of air is the atmosphere (AT•muhs•feer).
Gases
The atmosphere is a mix of different gases.
You can tell from the pie chart that most of the
atmosphere is made of nitrogen (NYE•truh•juhn)
and oxygen. Without these gases, life could not
exist on Earth!
The atmosphere also has a few other gases,
including carbon dioxide and water vapor. These
gases are important to Earth’s water cycle.

Layers of the Earth’s atmosphere (diagram)

The Troposphere
Earth’s atmosphere is made
up of layers. The layer closest to
Earth’s surface is the troposphere
(TROHP•uh•sfeer). Compared to
the rest of the atmosphere, the
troposphere is very thin. Yet all of
Earth’s life exists here.
The troposphere is also where all
of Earth’s weather takes place. Here
the air is always on the move. Air
that moves from place to place is
called wind. Wind can be as gentle
as a light breeze. It can be as fierce
as a tornado. Any change in the
wind brings a change in the weather.
Other Layers of the Atmosphere
The diagram shows three other
layers of Earth’s atmosphere. The
stratosphere (STRAT•uhs•feer) is the
layer above the troposphere. There,
temperatures get warmer as you go
higher. Temperatures get colder in
the mesosphere (MEZ•uh•sfeer) and
thermosphere (THURM•uh•sfeer).

What are some
properties of weather?
Wind is an important part of
Earth’s weather. Weather is the
condition of the atmosphere at a
given time and place.
Air Temperature
Temperature (TEM•puhr•uh•chuhr)
describes how hot or cold something
is. When the Sun’s energy heats
Earth’s surface, the surface warms
the air above it. The temperature of
the air rises. If the air temperature
changes, the air moves. Winds start
to blow. Temperature affects wind
speed and wind direction.

Humidity
If the air around us feels damp
and sticky, we call the weather
humid (HYEW•mid). Humidity
(hyew•MID•i•tee) is a measure of
how much water vapor is in the
air. Deserts usually have very low
humidity. Rain forests have very
high humidity.
Air always has some amount of
water vapor. Most of it comes from
ocean water that evaporates from a
liquid to a gas. This is why the air
over oceans and lakes is humid.

Air Pressure
We live at the bottom of the
troposphere. Here, the weight of the
entire atmosphere pushes down on
us. The force of air pushing on an
area is called air pressure.
Particles of cool air are closer
together than particles of warm air.
In the same amount of space, cool
air weighs more than warm air. Air
that weighs more has greater air
pressure. Therefore, cool air has
higher air pressure than warm air.
Precipitation
Any form of water that falls
from clouds is precipitation
(pree•sip•uh•TAY•shuhn). The term
includes rain, snow, sleet, and hail.
The amount of precipitation is an
important property of weather.

How can you measure weather?
Weather scientists often collect data from a
place called a weather station. You can set up
your own weather station! All you need are a
few of the tools shown on this page. (diagram SACL Gr4)

A hygrometer
(hye•GROM•i•tuhr)
measures humidity. ▲
◀ A thermometer measures
air temperature in degrees
Celsius (°C) or degrees
Fahrenheit (°F).

A wind vane
points in the
direction from
which the wind
is blowing.

A barometer
measures air
pressure.

An anemometer (an•uh•MOM•i•tuhr)
measures wind speed. The faster
the wind blows, the faster
the cups spin.

A rain gauge (GAYJ) is a
tube that collects water. It
shows the amount of rainfall.