Vary in size and shape. The size-distribution functions are concentration as a function of particle radius.

Height-distribution functions are concentration as a function of altitude

There are two classes of particles:

• -Aerosol (radius < 1 m): -Suspended in the atmosphere (most near surface of earth) -Increases scattering over molecular scattering
• -Hydrometers (radius > 1 m): -Water-dominated particles -Shorter lifespan