1: Atmosphere composition and layers

Atmosphere: Earth's Protective Blanket

Imagine Earth wrapped in a thin, invisible shield – that's our atmosphere. It's a layer of gases held by gravity, vital for life as we know it. Understanding its makeup and structure helps us grasp weather, climate, and how life survives.

What's It Made Of?
The air we breathe isn't just one gas; it's a mixture. The two main players are Nitrogen ($N_2$) – making up about 78% – and Oxygen ($O_2$) – providing roughly 21%. Nitrogen is relatively inert and acts as a buffer, while oxygen fuels life and fire. The remaining 1% is crucial and includes:

• Argon (Ar): An inert gas, about 0.9%.
• Carbon Dioxide ($CO_2$): A tiny fraction (around 0.04%), vital for plant life but significantly impacts climate.
• Trace Gases: Neon, helium, methane, ozone, and water vapor. Water vapor levels fluctuate greatly and drive weather patterns.

Layers Stacked Like an Onion
The atmosphere isn't uniform; it's divided into distinct layers based on temperature changes:

1. Troposphere: This is our home layer, extending roughly 8-15 km high (thinner at the poles, thicker at the equator). Almost all weather happens here. Air gets steadily colder as you go higher. Planes fly in the upper troposphere.
2. Stratosphere: Sitting above the troposphere, reaching up to about 50 km. Here, temperature increases with height due to the Ozone Layer ($O_3$). This layer absorbs harmful ultraviolet (UV) radiation from the sun, acting as Earth's sunscreen. Jet streams and weather balloons operate here.
3. Mesosphere: The middle layer, stretching up to about 85 km. Temperature decreases again, making it the coldest part of the atmosphere (down to $-90^\circ$C). This is where most meteors burn up as "shooting stars" due to friction.
4. Thermosphere: Extending from about 85 km up to 600 km or more. Temperatures soar dramatically (over $1000^\circ$C) because solar radiation hits sparse gas molecules. However, it wouldn't feel hot because molecules are so far apart. This is where the auroras (northern and southern lights) dance and where the International Space Station orbits. Radio waves bounce off parts of this layer.
5. Exosphere: The outermost layer, gradually fading into the vacuum of space, starting around 600 km. It's extremely thin, composed mostly of hydrogen and helium atoms. This is where satellites in high orbits reside.