Notes on The Atmosphere: Composition, Structure and Origin

Basic Introductory Concepts

Atmosphere – the envelope of gas that surrounds the planet and is held by the Earth’s gravitational attraction (approximately 10 km)

Meteorology – the study of the atmosphere and atmospheric processes. Meteorologists study the physics of the atmosphere and focus on short-term weather forecasting [Weather – present state of the atmosphere]
Climatology – the study of weather conditions for the longer period. Climatologists focus on average conditions and predict long-range changes [Climate – average of all weather conditions for an area]

Supplies oxygen we breathe and carbon dioxide for plants
Shields Earth from UV rays (ozone layer)
Acts as a blanket to hold in heat (greenhouse gases) and redistributes heat across the globe (wind circulation)
Helps maintain the hydrologic cycle

The Primordial Atmosphere (4.6 billion years ago)
- Hydrogen and Helium
Secondary Primitive Atmosphere (4.5 to 3.8 billion years ago)
- Earth begins to cool and degassing occurs
- Water Vapor (80%), Carbon dioxide (10%), and some Nitrogen
- Heavy Precipitation Period (3.8 billion years ago)
- Water Vapor to Liquid Water
- Carbon dioxide levels dwindle; Nitrogen levels increase
Oxygen Period (3 billion years ago)
- Water Vapor molecules split to Hydrogen and Oxygen
- Oxygen combined with another O molecule – ozone layer
- Photosynthesis occurring in oceans

Atmosphere today is a mixture of solids (dust and ice), liquids (water) and gases
Gases include
- Nitrogen (78%) ⇒ N_2 \approx 0.78
- Oxygen (21%) ⇒ O_2 \approx 0.21
- Argon (0.93%) ⇒ Ar \approx 0.0093
- Carbon dioxide (0.036%) ⇒ ext{CO}_2 \approx 0.00036
- Water vapor (0 – 4%) ⇒ ext{H}_2 ext{O vapor} \in [0, \, 4]\%
- Ozone (variable)
- Trace elements – Neon, Helium, Methane, Krypton, Hydrogen, Nitrous Oxide, Xenon, Hydrogen
- *greenhouse gases

There are both natural and anthropogenic greenhouse gases in our atmosphere (e.g., carbon dioxide, water vapor, methane, etc.)
Greenhouse gases responsible for the actual heating of Earth (without GHGs, the Earth would be below freezing and there would be no liquid water)

Carbon Dioxide Levels
- 1960: 360\ \,\text{ppm}
- 1970: 340\ \,\text{ppm}
- 1980: 320\ \,\text{ppm}
- 1990: 300\ \,\text{ppm}

Troposphere (0-10 km)
- Where most weather takes place
- As elevation increases, temperature and pressure decrease
Stratosphere (10-50 km)
- Temperature inversion – as elevation increases, temperature increases
- Ozone layer shields Earth from UV rays
Mesosphere (50-80 km)
- Temperature decreases with elevation
Thermosphere (80 km and higher)
- Very few molecules – cold; essentially outer space
Ionosphere (60-1000 km)
- Area dominated by ions (particles with electrical charges)
- Important for radio transmission and both northerly and southerly lights

A chart-like layout shows: Troposphere at lowest altitudes with decreasing temperature; Stratosphere above with an inversion; Ozone layer within Stratosphere; Mesosphere and Thermosphere above with respective temperature trends; Ionosphere extending from ~60 km upward and influencing ionization and auroral phenomena

Tropopause – boundary between the troposphere and the stratosphere
Ionosphere – region (60-1000 km) dominated by ions; important for radio transmission; site of auroral activity
Ozone Layer – located within the stratosphere; shields Earth from UV rays

Seeing interaction of solar winds (protons and electrons) interacting with the ions in Earth’s magnetic field
Occurrence correlates with solar flare activity on the sun