Atmospheric Science: Evolution, Composition, and Properties

Atmospheric Evolution during Earth History

• Primordial Atmosphere (First Atmosphere):

  • Occurred between $4.56$ and $4.54\,Ga$ (billion years ago).

  • The atmospheric mix was similar to the solar nebulae.

  • Composition included hydrogen ($H_2$), helium ($He$), ammonia ($NH_3$), and methane ($CH_4$).

  • Drivers of Change: This composition was altered by solar wind, the lunar-forming collision, volcanic activity, and comet impacts.

• The Second Atmosphere:

  • Occurred between $4.0$ and $3.9\,Ga$.

  • This period followed the end of the Late Heavy Bombardment.

  • The planet's surface cooled, allowing liquid water to accumulate.

  • Atmospheric Composition: Primarily nitrogen ($N_2$), with approximately $20\%$ carbon dioxide ($CO_2$).

• The Third Atmosphere:

  • The first organisms appeared approximately $3.85\,Ga$.

  • Photosynthetic cyanobacteria began extracting $CO_2$ and releasing oxygen ($O_2$).

  • Great Oxygenation Event: Occurred between $2.4$ and $1.8\,Ga$.

  • Resulting Composition: The atmosphere became predominantly nitrogen ($N_2$) and oxygen ($O_2$), characterizing the atmosphere as it exists today.

Atmospheric Composition and Components

• Gaseous Composition of Today's Atmosphere:

  • Nitrogen ($N_2$): $78\%$

  • Oxygen ($O_2$): $21\%$

  • Argon ($Ar$): $0.9\%$

  • Carbon Dioxide ($CO_2$): $0.04\%$

  • Trace amounts of various other gases.

  • Note: Water is excluded from these percentage totals because its concentration varies locally.

• Water in the Atmosphere:

  • Water exists in three states: water vapor (gas), water droplets in clouds (liquid), and ice crystals in clouds (solid).

• Atmospheric Aerosols:

  • Definition: Tiny solid or liquid particles that remain suspended in the air.

  • Inorganic Aerosols: Mineral dust, salts, sulfates, volcanic fine ash, and soot.

    • Dust Examples: Sahara dust blowing over the Atlantic Ocean.

    • Smoke and Soot Examples: Rising from forest fires or industrial emissions.

  • Organic Aerosols: Pollen, bacteria, molds, and viruses.

  • Photochemical Smog: Specifically found over urban areas like Los Angeles, containing particles from power plants, factories, and vehicle exhausts.

Describing and Measuring Atmospheric Properties

• Key Properties:

  • Air temperature.

  • Atmospheric pressure.

  • Relative humidity ($RH$).

  • Wind speed and direction.

  • Visibility.

  • Cloud cover.

  • Precipitation.

• Air Temperature:

  • Represents the average speed of molecules in the air.

  • Faster molecule movement results in higher temperatures.

  • Measured using a thermometer.

• Atmospheric Pressure:

  • Definition: The force applied by the overlying air on a specific surface area (e.g., a square meter).

  • Measurement: Meteorologists use barometers (e.g., the aneroid barometer).

  • Relationship with Elevation: Pressure and air density decrease as elevation increases. Approximately $50\%$ of the air mass is located below an elevation of $5.6\,km$ ($3.5\,miles$).

• Relative Humidity ($RH$):

  • Definition: The amount of water vapor in the air divided by the air’s total capacity for holding water vapor.

  • Capacity is temperature-dependent; warmer air can hold more moisture.

• Dewpoint Temperature:

  • The lowest temperature to which air can be cooled before reaching saturation ($100\%$ relative humidity).

  • Below this temperature, excess water forms droplets (liquid) or frost (solid).

• Wind Speed and Direction:

  • Wind is the horizontal movement of air.

  • Wind Direction: The direction from which the wind originates.

  • Anemometer: Instrument used to measure wind speed.

  • Wind Vane: Instrument used to indicate wind direction.

• Visibility and Cloud Cover:

  • Visibility: The distance at which a person with normal vision can identify objects through the air.

  • Cloud Cover: An estimate of the portion of the sky obscured by clouds at a given time.

• Precipitation:

  • Definition: Water that condenses to a liquid or solid state and falls from the sky.

  • Precipitation Rate: Measure of the speed of falling rain or snow.

  • Total Precipitation: The cumulative amount (measured in liquid form) over a specific time period.

• Weather Stations:

  • Automated sites monitoring temperature, dew point, wind speed/direction, visibility, cloud type/coverage, and precipitation type/intensity.

Vertical Structure of the Atmosphere

• Thermal Layers (Bottom to Top):

  • Troposphere: Temperature decreases with increasing elevation.

  • Stratosphere: Temperature increases with increasing elevation. This layer contains the highest concentration of ozone ($O_3$).

  • Mesosphere: Temperature decreases with increasing elevation.

  • Thermosphere: Temperature increases with increasing elevation.

• The Ozone Layer ($O_3$):

  • Located in the stratosphere.

  • Protects life by absorbing most ultraviolet ($UV$) radiation from the Sun.

  • The "ozone hole" refers to a decrease in ozone concentration caused by human activities in recent decades.

• Layers Defined by Composition and Charge:

  • Homosphere: The well-mixed layers of gases found in the troposphere, stratosphere, and mesosphere.

  • Heterosphere: Layers in the thermosphere where gases are sorted by molecular weight.

  • Ionosphere: A region (within the thermosphere) containing a high concentration of ions (charged subatomic particles). It reflects certain radio waves back to Earth.

• Auroras:

  • High-energy particles from the Sun channeled by magnetic field lines to the poles.

  • They interact with molecules to emit light.

  • Aurora Borealis: Northern hemisphere.

  • Aurora Australis: Southern hemisphere.

Clouds and Precipitation

• Cloud Characteristics:

  • Composed of water droplets and/or ice crystals.

  • Optics: They reflect, scatter, and absorb light. Thin clouds appear white; thick clouds appear gray or dark.

  • Formation: Air containing water vapor rises, expands, and cools. If it cools below the dew point, water condenses/freezes on aerosols.

• Lifting Mechanisms for Cooling:

  • At a front (frontal lifting).

  • At a mountain range (orographic lifting).

  • From buoyancy due to local surface warming.

• Classification of Clouds:

  • High-Altitude Clouds (above $6\,km$ or $20,000\,feet$): Known as Cirrus clouds. Composed entirely of ice crystals, often wisp-like. They do not produce precipitation.

  • Mid-Altitude Clouds ($3$ to $6\,km$ or $10,000$ to $20,000\,feet$): Known as Alto clouds (e.g., altostratus, altocumulus). Usually too thin for precipitation.

  • Low-Altitude Clouds (below $3\,km$ or $10,000\,feet$): Widespread, layered clouds like Stratus or Stratocumulus. These can produce precipitation.

  • Towering Clouds: Not defined by a single altitude range; they span multiple levels. Example: Cumulonimbus (puffy raincloud).

  • Unusual Clouds: Includes lenticular, wall, shelf, mammatus, and roll clouds.

• Fog:

  • Occurs when the cloud base is at the Earth's surface.

  • Radiation Fog: Cooled ground absorbs heat from the air above, cooling the air to saturation.

  • Advection Fog: Warm, humid air flows over a cooler surface (e.g., over a snow field or cold ocean water).

  • Evaporation Fog: Surface water evaporates into the air until it becomes saturated (e.g., over a warm lake).

• Precipitation Processes:

  • Warm-Cloud Precipitation: Temperatures stay above freezing. Droplets collide and merge (collision-coalescence) until heavy enough to fall.

  • Cold-Cloud Precipitation: Ice particles combine to form snowflakes or graupel (soft ice spheres). Depending on air temperature during descent, they fall as snow, hail, or rain.

• Latent Heat:

  • Energy absorbed or released during a phase change (e.g., vapor to liquid).

  • The temperature of the water remains constant during the phase change itself.

Radiation and Optical Effects

• Electromagnetic Radiation:

  • Energy traveling through space in electric and magnetic fields (light). All objects emit some radiation.

  • Blackbody Radiation: Concept where all radiation absorbed must be re-radiated to maintain thermal equilibrium. Warmer objects emit more energy and have shorter peak wavelengths.

• Greenhouse Effect:

  • Visible light passes through the atmosphere and is absorbed by the surface.

  • The surface re-radiates energy at longer wavelengths (infrared).

  • The atmosphere absorbs these longer wavelengths, acting as insulation to trap heat and allow liquid water to exist.

• Optical Phenomena:

  • Blue Sky: Nitrogen/air molecules cause light scattering. Short wavelengths (blue) scatter more effectively than long ones (red).

  • Mirages: Images formed when light bends through the atmosphere.

    • Inferior Mirage: Appears below the actual path as light bends upward.

    • Superior Mirage: Appears above the actual path as light bends downward; often appears upside down.

  • Rainbows: Produced by sunlight being refracted and reflected through water droplets or ice crystals.

Questions & Discussion

• Class Question 1: Which of the following gases is the most abundant in the Earth’s atmosphere?

  • Answer: Nitrogen.

• Class Question 2: Which of the following are tiny, organic, or inorganic particles small enough to stay suspended in air?

  • Answer: Aerosols.

• Class Question 3: When did oxygen begin to accumulate at more than trace quantities in the Earth’s atmosphere?

  • Answer: Great Oxygenation Event ($2.4-1.8\,Ga$).

• Class Question 4: The amount of water vapor in the air divided by the air’s capacity for holding water vapor is called the…

  • Answer: Relative humidity.

• Class Question 5: Wind speed and direction properties are measured with a(n)…

  • Answer: Anemometer and wind vane.

• Class Question 6: Which layer of the atmosphere is characterized by temperatures that decrease as elevation increases?

  • Answer: Troposphere.

• Class Question 7: A cumulonimbus cloud is a…

  • Answer: Towering, puffy raincloud.

• Class Question 8: Which type of fog forms as warm air flows over a cold snow field?

  • Answer: Advection fog.

• Class Question 9: The greenhouse effect is important because it…

  • Answer: All of the above (allows liquid water, keeps surface from radiative equilibrium, allows surface to be warmer than it would be otherwise).

• Class Question 10: The sky is blue because…

  • Answer: Short-wavelength light scatters more effectively, and blue wavelengths arrive in our eyes from somewhere in the sky.

• Class Question 11: When saturated air is cooled, clouds form because…

  • Answer: Water molecules accumulate on aerosols to form liquid drops or ice particles.

• Think-Pair-Share: Why is the sky blue? Why are some sunsets more colorful? Where geographically would colorful sunsets occur and why? (Note: This refers to scattering effects and aerosol concentrations.)