- 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
- Ballard, Robert Duane (1942- )
- Banded Iron Formations
- Barrier Islands
- Barringer Meteor Crater
- Basalt
- Basin and Range Topography
- Batholith
- Bathymetric Mapping
- Beach and Shoreline Dynamics
- Beaufort Wind Scale
- Beaufort, Sir Francis (1774-1857)
