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Part 1. Energy and Mass

Chapter 1.

Composition and Structure of the Atmosphere

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IntroductionThe Atmosphere

A mixture of gas molecules, suspended particles, and falling precipitation

The atmosphere strongly affects our day-to-day lives

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MeteorologyThe study of the atmosphere and the processes that cause “weather”

ClimatologyExamines weather elements over long time periods

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Variable Gases

Water VaporMost abundant variable gas

Added/ removed to air through the hydrologic cycle

Concentrations = nearly 0% to nearly 4%

Important to energy balance and many atmospheric processes

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Carbon DioxideA trace gas

• 0.038% of atmosphere’s mass• Important to Earth’s energy balance

Added through biologic respiration, volcanic activity, decay, and natural and human-related combustion

Removed through photosynthesis

Increasing at a rate of 1.8 ppm/year

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Temporal increases due to human activities

Seasonal variations related to biological activity

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Ozone Tri-atomic form of oxygen

Absorbs ultraviolet radiation

Chlorofluorocarbons (CFCs) destroy ozone Destruction peaks over southern hemisphere

Antarctic circumpolar vortex limits latitudinal mixing

• Leads to an O3 “hole”

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Vertical Structure of the Atmosphere

DensityMass (kg) per unit volume (m3)• Sea level average = 1.2 kg/m3

Near surface air is more dense

• Compressibility • Mean free path

– At surface = 0.0001 mm– At 150 km = 10 m

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Compressibility of gases relates to density

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Thermal Layers of the AtmosphereFour distinct atmospheric layers • Troposphere• Stratosphere• Mesosphere• Thermosphere

Each has particular temperature characteristics with height.

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Troposphere Lowest layer

Steady temperature decrease with height • -6.5oC/km (-3.6oF/1000ft)

Virtually all weather processes• Contains 80% of atmospheric mass

Tropopause = top of troposphere

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Violent updrafts maypenetrate cloud tops into the stratosphere. The flattenedtop of this cumulonimbus cloud is in the stratosphere.

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StratosphereLittle actual “weather”

Temperature inversion

• Caused by absorption of UV radiation by O3

Stratopause = top of stratosphere

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Mesosphere and ThermosphereCombined = 0.1% of total mass

The mesosphere• Decreasing temperatures with height • Coldest layer

The thermosphere • Slowly merges into space • Increasing temperatures with height

– “Temperature” = molecular kinetic energy

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Some Weather BasicsWeather information is abundant

Pressure and windWind = horizontal movement of air• Caused by unequal pressures

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Pressure units• Millibar (mb), Kilopascal (kPa)• Isobars

Station Models• Portray weather information

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TemperatureMost obvious weather component• Fronts

HumidityMay be expressed as relative humidity• Dew point temperature

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