APES Video Notes 4.4 - Atmosphere

Earth’s Atmosphere

The atmosphere’s layers are defined by how temperature changes with altitude. Gas becomes less dense farther from Earth.

Atmospheric Composition

• Nitrogen (N₂): about 78% Most atmospheric nitrogen is not directly usable by plants. It must be fixed by soil bacteria or through human processes, such as producing synthetic fertilizer.

• Oxygen: about 21% Animals use oxygen for respiration, which breaks down glucose to produce ATP, the body’s main energy source. The instructor recommends using “respiration” rather than simply saying animals breathe oxygen in an AP Environmental Science response.

• Argon: about 0.93% An inert, non-reactive noble gas.

• Water vapor: roughly 0–4% Its concentration varies by region and conditions. Water vapor is a greenhouse gas, but it moves through the atmosphere quickly and is part of the water cycle.

• Carbon dioxide (CO₂): about 0.04% Despite its small share of the atmosphere, carbon dioxide traps heat and can affect temperatures on land and in the oceans.

Layers of the Atmosphere

1. Troposphere

   • The layer closest to Earth, extending to about 16 km.
   • The densest layer and the location of weather.
   • Contains most atmospheric water vapor.
   • Temperature decreases with altitude because Earth’s surface is warmed by the Sun, while air farther from the surface is colder.
   • Ground-level ozone is an air pollutant. It contributes to smog and can harm human respiratory systems, lungs, and plant stomata.

2. Stratosphere

   • The second atmospheric layer.
   • Contains the dense ozone layer, which absorbs much of the Sun’s harmful UV radiation and helps protect life.
   • Temperature increases with altitude because UV radiation warms the upper stratosphere.

3. Mesosphere

   • The middle layer.
   • Temperature decreases with altitude because its sparse gas molecules do not absorb solar energy effectively.
   • The mesopause is described as one of the coldest places in the atmosphere, at about −150°F.

4. Thermosphere

   • Absorbs high-energy UV rays and X-rays, causing temperatures to rise dramatically—up to about 3,100°F.
   • Charged gas molecules in this layer can produce the Aurora Borealis, or northern lights, when affected by high-energy solar radiation.

5. Exosphere

   • The outermost layer, where the atmosphere merges with outer space.

Temperature Pattern to Remember

As altitude increases, the temperature trend alternates across the layers:

• Troposphere: decreases
• Stratosphere: increases
• Mesosphere: decreases
• Thermosphere: increases

As you climb upward from Earth’s surface, it first gets colder. The temperature trend then reverses in the next layer.

Science Practice

Use a diagram to identify an atmospheric layer where temperature and altitude have an inverse relationship, then explain why that relationship occurs.

Speaker and Source

The instructor and presenter is Mr. Smees. The video is from Jordan Dischinger-Smedes’s AP Environmental Science video series.