Part 1. Energy and Mass

Chapter 1.

Composition and Structure of the Atmosphere

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

ClimatologyExamines weather elements over long time periods

IntroductionThe Atmosphere

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

The atmosphere strongly affects our day-to-day lives

The Thickness of the AtmosphereDensity decreases rapidly with height • The top of the atmosphere is undefined• Majority of mass is compressed near the

surface

Gases and particles are exchanged between the surface and atmosphere

A gas molecule that is input slowly into the atmosphere has a long average residence time.

A gas molecule that is input rapidly into the atmosphere has a short average residence time. Average residence time of CO2 is about 100

years.

Evolution of the AtmosphereEarly atmosphere = hydrogen (H2) and helium (He) -- 4 billion years ago

Secondary atmosphere formed from volcanic outgassing (produced CO2 rich atmosphere) -- 4 to 3 billion years ago

CO2 replaced by O2 through photosynthesis (by life!) and dissolution in water -- by 3.4 billion years ago; significant free O2 by 2.5 billion years ago

N2 (inert) slowly grew to present day levels

Composition of the Atmosphere

Variable Gases

Water VaporMost abundant variable gas

Added/ removed to air through the hydrologic cycle

Concentrations = nearly 0% to nearly 4% (by volume)

Important to energy balance and many atmospheric processes (it is a greenhouse gas)

Water vapor image showingbroader distribution of moisture than the image of actual clouds (below)

Water vapor image

Cloud image

Dry air

Moist air

Carbon DioxideA trace gas

• 0.038% of atmosphere’s mass• Important to Earth’s energy balance (it is a

greenhouse gas)

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

Temporal increases due to human activities

Seasonal variations related to biological activity

2006 value: about 380 ppm

MethaneA variable gas in small but recently increasing concentrations

Increases from burning of fossil fuels, livestock digestion, and agriculture cultivation (esp. rice)

Effective absorber of terrestrial radiation (it is a greenhouse gas)• Plays a role in near-surface warming

The annual increases in atmospheric methane

Aerosols (particulates)Any solid and/or liquid particle, other than water

Both natural (sea spray, dust, combustion) and human (combustion) sources

Long residence times for some types

Acts as condensation nuclei

Will cool atmosphere if emitted in large amounts (such as volcanic eruptions)

Vertical Structure of the AtmosphereDensity

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

kg/m3

Near surface air is more dense

• Compressibility of air • Mean free path (of an air

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

Elevationin the atmosphere

Higher density

Lower density

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

Each has particular temperature characteristics with height.

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

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

StratosphereLittle actual “weather”

Temperature inversion

• Caused by absorption of UV radiation by O3

(ozone) in the ozone layer

Stratopause = top of stratosphere

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”

The reduction of ozone over Antarcticaover time. Area in redindicates the “ozone hole.”

MesosphereDecreasing temperatures with height

Coldest layer

ThermosphereSlowly merges into space

Increasing temperatures with height• “Temperature” = molecular kinetic energy

Combined Mesosphere and Thermosphere = 0.1% of total mass of atmosphere

IonosphereA layer of electrically charged particles (ions)• In the meso- and thermosphere• D-, E-, and F-layers with increasing height

Interactions with subatomic solar particles cause • The aurora borealis (northern lights) • The aurora australis (southern lights)

The ionosphere reflects radio waves, allowing radio transmissions over long distances around the Earth.

TemperatureEnergy of the atmosphere, measured in degrees (Fahrenheit, Celsius or Kelvin)

HumidityWater content of the atmosphere, often expressed as relative humidity

PressureForce of the atmosphere, measured in millibars (mb) or kilopascals (kPa)

Weather Measurements and Units

A Brief History of Meteorology

GalileoPrototype thermometer in 1593• Fahrenheit (1714) and Celsius (1736)

temperature scales

Torricelli Barometer (1643)

Instruments to measure water vapor introduced in late 1700s