Characteristics of the Atmosphere

The Composition of the Atmosphere

• The atmosphere is made of 78% Nitrogen gas and about 21% oxygen.

• The atmosphere also contains small particles such as dust, volcanic ash, sea salt, dirt, smoke, liquid and solid water

Atmosphere Facts

• The atmosphere is held around the Earth by gravity.

• Air pressure is caused by gravity pulling the gas molecules in the atmosphere toward the Earth’s surface.

Structure of the AtmosphereStructure of the Atmosphere

• The atmosphere is divided into four layers– Troposphere

• The lowest layer is where most of the weather occurs

– StratosphereStratosphere– MesosphereMesosphere– thermospherethermosphere

The Troposphere: The Layer in Which We Live

• Layer we live in• Tropo means “turning” or “change”• Called this because differences in air

pressure and temperature cause gases in the troposphere to move continuously

• Lowest layer• Most dense layer (90% of the atmosphere’s

mass)• All weather occurs in this layer

The Stratosphere: Home of the Ozone Layer

• The atmospheric layer above the troposphere is called the stratosphere.– Strato means “layered”– Very thin with little moisture– Lowest layer is extremely cold (-60° C)– Temperatures rise with altitude because the

ozone layer absorbs UV from the sun– Pilots fly planes here to avoid weather

disturbances and/or on very long flights.

The Mesosphere: The Middle Layer

• The mesosphere is the middle layer of the atmosphere. It is also the coldest layer.– Meso means “middle”– Temperature decreases as altitude increases

(-93°C at the top)

The Thermosphere: The Edge of the Atmosphere

• The uppermost atmospheric layer is called the thermosphere.– Thermo means heat; high temps (1,000° C or

higher) because Nitrogen and Oxygen atoms absorb solar radiation and release thermal energy

– Temperature increases with altitude– Particles move fast but it does not feel hot!– Density so low particles do not collide to

transfer heat

The Ionosphere: Home of the Auroras

• In the upper thermosphere and the lower ionosphere, nitrogen and oxygen atoms absorb harmful solar energy. – Gas particles become electrically charged

(ions) – Can radiate energy as shimmering lights

called auroras

Atmospheric Pressure

• Air Pressure is the measure of the force with which air molecules push on a surface.

• Air pressure decreases as altitude (height) increases

Air PressureAir Pressure

• Is caused by the weight of the atmosphere

• Above each square inch of Earth’s surface is a column of air that weighs 14.7 pounds

Air Pressure FactorsAir Pressure Factors• If air is cooled, it contracts and becomes

denser– This causes pressure to rise

• If air is heated, it expands and becomes less dense– This causes pressure to fall

• Humid air is lighter than dry air– This is because water

molecules are lighter than the gasses they displace (replace) in the air

Measuring Air PressureMeasuring Air Pressure

• A barometer is an instrument used to measure air pressure.

• Meteorologists measure air pressure in millibars

• On a weather map, On a weather map, isobarsisobars connect places that have the connect places that have the same air pressuresame air pressure