🌍 THE ATMOSPHERE
PAGE 1 OF 5, A SHELL OF GAS, HELD BY GRAVITY
AIR = NOT NOTHING
THE ATMOSPHERE IN ONE BREATH
The atmosphere is the envelope of gases bound to Earth by gravity. It is mostly nitrogen (N₂, ~78%) and oxygen (O₂, ~21%) for breathing — plus argon, water vapour, CO₂, and trace gases. It has mass: the weight of the whole column of air presses down on the surface (that is air pressure). Without it, no sound like we know it, no liquid water on the surface, and lethal solar and cosmic radiation. It is our blanket — thin compared to Earth's radius, but everything to life.
🧪 SCALE
If Earth were a basketball, the entire atmosphere would be a thinner skin than a sticker — yet it is roughly 100 km+ to where we say "space" begins for many practical uses.
HOLD!
N₂ / O₂
Mix is almost the same everywhere in the lower layers — water vapour is the variable
1 BAR
Mean sea-level pressure is about 1013 hPa (roughly 1 "atmosphere")