Calculation history
Formula used
Terminal speed occurs when downward effective weight equals drag. Buoyancy reduces the force available for acceleration. The iterative model updates drag until both values agree.
Fd = ½ρfCdAv²
Fg − Fb = (ρd − ρf)Vg
vt = √[4d(ρd − ρf)g ÷ (3ρfCd)]
vStokes = (ρd − ρf)gd² ÷ (18μ)
The empirical option estimates normal terrestrial raindrop speeds. Advanced corrections remain approximations rather than full weather simulations. Verify critical engineering work with specialist experimental data.
How to use this calculator
- Select one drop, a range, or a custom list.
- Enter the drop size and atmospheric conditions.
- Choose automatic modelling for most normal calculations.
- Add wind, height, and optional advanced corrections.
- Submit the form and review convergence notices.
- Copy, print, or download the resulting data.
Start with automatic mode when model selection feels uncertain. Compare several diameters to understand changing aerodynamic behaviour. Treat breakup and evaporation results as scenario estimates only.
Example raindrop data
| Category | Typical diameter | Suggested model | Useful observation |
|---|---|---|---|
| Fog droplet | 0.01 mm | Stokes or automatic | Slip can become measurable. |
| Cloud droplet | 0.02 mm | Automatic | Terminal speed remains very small. |
| Drizzle | 0.2 mm | Automatic | Shape remains nearly spherical. |
| Light rain | 0.8 mm | Automatic | Inertial drag becomes important. |
| Moderate rain | 1.5 mm | Automatic or empirical | Oblate deformation begins. |
| Heavy rain | 3 mm | General or empirical | Shape correction affects speed. |
| Large drop | 5 mm | General or empirical | Breakup risk increases strongly. |
Frequently asked questions
What is terminal velocity?
Terminal velocity is the steady speed reached after drag balances effective weight.
Why do larger drops fall faster?
Their weight grows faster than their frontal area, although deformation eventually limits speed.
Why does the calculator iterate?
Drag coefficient depends on Reynolds number, while Reynolds number depends on velocity.
When should Stokes law be used?
Use it for tiny droplets with very low Reynolds numbers and nearly creeping flow.
Does humidity change terminal velocity?
Humidity changes air density slightly and can also influence real droplet evaporation.
How does altitude affect the result?
Lower air density at altitude usually reduces drag and raises still-air terminal speed.
Can an updraft suspend a raindrop?
Yes. Suspension occurs when upward air speed matches the drop's terminal speed.
Are large raindrops perfectly spherical?
No. Aerodynamic pressure flattens larger drops and can cause oscillation or breakup.
What does the Weber number show?
It compares aerodynamic forces with surface tension and helps flag breakup scenarios.
Is evaporation calculated from full thermodynamics?
No. This page uses a user-selected diameter-loss scenario for transparent comparison.
Can this calculator replace laboratory measurements?
No. It supports estimates, teaching, screening, and comparisons under stated assumptions.