Watershed Management 1 (WSHD1) Practice Test

Fall rate is set by the balance between gravity pulling the drop downward and air resistance slowing it down. The downward pull depends on the drop’s weight, which comes from its mass and thus its volume. For a spherical raindrop, volume scales with the cube of the radius, so weight grows as r^3. The resisting drag depends on how much surface area the air has to push against, which scales with the cross-sectional area ∝ r^2 (and also on air properties and velocity).

Because weight grows faster with size than drag does, larger drops reach a higher terminal velocity and fall faster than smaller ones. The main factors, then, are the drop’s size that determines both weight and drag. Humidity and temperature tweak air density and viscosity (and thus drag) a bit, but they’re not the primary determinants of fall rate. Wind can change the observed speed relative to the ground, but the inherent fall rate comes from the interplay of volume (weight) and cross-sectional area (drag).