THE AIR AROUND US, AND THE PRESSURE IT CREATES

Weather is the condition of Earth’s atmosphere at a particular time andplace. Earth’s atmosphere is the envelope of gases that surrounds the planet.

- The Air Around You

Composition of the AtmosphereEarth’s atmosphere is made up of nitrogen, oxygen, carbon dioxide, water vapor, and many other gases, as well as particles of liquids and solids.

•Nitrogen makes up about three fourths of theair.•Oxygen is the second most abundant gas in air.• Plants and animals take oxygen directly fromair and use it to release energy from food.Oxygen also is needed for fire to burn.•Most oxygen molecules have two oxygenatoms.•A form of oxygen called ozone, which containsthree oxygen atoms in each molecule instead ofthe usual two, is sometimes found in air. It formswhen lightning interacts with oxygen in the air.

•Carbon dioxide is very important because plants need it to survive.•Plants and animals give off carbon dioxide as a waste product when they break down food to produce energy. •The burning of fuels such as gasoline and coal also produces carbon dioxide.•Water vapor is invisible—it is water in the form of a gas. •The amount of water vapor in the air may vary greatly depending on atmospheric conditions. •Water vapor is important in weather. It produces clouds and precipitation.

Pure air contains only gases. In the real world, air also contains tiny solid and liquid particles of dust, smoke, salt, and other chemicals.

oEarth’s atmosphere makes conditions on Earth suitable for living things. oThe atmosphere contains oxygen and other gases that living things need. oThe atmosphere also traps energy from the sun, which keeps Earth’s surface warm and Earth’s water in liquid form, another requirement of living things. oIn addition, the atmosphere protects Earth from dangerous radiation from the sun and from meteoroids, which are chunks of rock from outer space.

- Air Pressure

Air Pressure

There is a column of air above you all the time.

The weight of the air in the atmosphere causes air pressure.

- Air Pressure

Measuring Air PressureAir pressure pushes down on the surface of the mercury in the dish, causing the mercury in the tube to rise. The air pressure is greater on the barometer on the right, so the mercury is higher in the tube.

- Air Pressure

Measuring Air PressureThis diagram shows an aneroid barometer. Changes in air pressure cause the walls of the airtight metal chamber to flex in and out. The needle on the dial indicates the air pressure.

- Air Pressure

Air Pressure and Altitude

Air pressure decreases as altitude increases.

- Air Pressure

Altitude and DensityThe density of air decreases as altitude increases. Air at sea level has more gas molecules in each cubic meter than air at the top of a mountain.

Identifying Main Ideas

Because air has mass, it also has density and pressure.

Density is the amount of mass of a

substance in a given volume.

Pressure is the force pushing on an area or

surface.

- Air Pressure

Detail Detail

Main Idea