- Asteroids
- Asthenosphere
- Astrolabe
- Astronomy
- Atmospheric Chemistry
- Atmospheric Circulation
- Atmospheric Composition and Structure
- Atmospheric Inversion Layers
- Atmospheric Lapse Rate
- Atmospheric Pollution
- Atmospheric Pressure
- Atom
- Atomic Mass and Weight
- Atomic Number
- Atomic Theory
- Aurora Borealis and Aurora Australialis
- Australia
- Avalanche
- Aviation Physiology
