Preview Graph
The graph shows how idealized terminal velocity changes as raindrop diameter changes under the current environmental inputs.
Calculator Inputs
Formula Used
This calculator assumes an idealized spherical raindrop and balances weight against drag at terminal velocity.
Frontal area: A = πd² / 4
Volume: V = πd³ / 6
Mass: m = ρwV
Terminal velocity: vt = √((2mg) / (ρaCdA))
Reynolds number: Re = (ρavtd) / μ
Where d is raindrop diameter, ρw is water density, ρa is air density, Cd is drag coefficient, μ is dynamic viscosity, and g is gravitational acceleration.
To estimate travel from a chosen release height, the page also uses the standard quadratic-drag motion model for a drop starting from rest. That gives an estimated fall time and impact speed for the entered height.
How to Use This Calculator
- Enter the raindrop diameter and choose its unit.
- Enter a fall height if you want time and impact estimates.
- Adjust air density, water density, drag coefficient, viscosity, and gravity when needed.
- Click Calculate to show the result box above the form.
- Review the summary table, graph, Reynolds number, and export buttons.
Example Data Table
The sample table below uses the current environmental settings and compares several raindrop diameters.
| Diameter (mm) | Mass (mg) | Terminal velocity (m/s) | Reynolds number | Fall time (s) |
|---|---|---|---|---|
| 0.5 | 0.065253 | 3.364692 | 113.86 | 59.678626 |
| 1 | 0.522028 | 4.758393 | 322.046 | 42.367326 |
| 1.5 | 1.761844 | 5.827817 | 591.636 | 34.730083 |
| 2 | 4.176224 | 6.729384 | 910.883 | 30.196044 |
| 3 | 14.094755 | 8.241779 | 1673.4 | 24.849147 |
| 4 | 33.409791 | 9.516786 | 2576.368 | 21.688157 |
| 5 | 65.253497 | 10.64009 | 3600.583 | 19.548889 |
Frequently Asked Questions
1. What is terminal velocity for a raindrop?
It is the speed reached when drag force balances weight. After that point, the drop stops accelerating and falls at nearly constant speed.
2. Why does diameter change the result so much?
Diameter changes area, volume, and mass together. Larger drops usually gain weight faster than drag area, so their terminal velocity tends to rise.
3. Why can I edit the drag coefficient?
Real droplets do not behave exactly like rigid spheres. The drag coefficient lets you explore different assumptions, experimental fits, or teaching examples.
4. Are real raindrops perfectly spherical?
No. Small ones are nearly spherical, but larger drops flatten and may fragment. That is why large-drop results should be treated as approximations.
5. Why is air density included?
Drag depends on the surrounding fluid. Lower air density generally reduces drag, which can increase the predicted terminal velocity.
6. What does Reynolds number tell me?
It helps describe the flow regime around the drop. Higher values usually indicate inertial effects dominate more strongly than viscous effects.
7. What is the fall-time estimate based on?
It uses a quadratic-drag motion model and assumes the drop starts from rest. The estimate is useful for study, but real cloud processes are more complex.
8. Can I use this for hail or other particles?
You can explore similar physics, but the assumptions fit water drops best. Solid particles may need different shapes, densities, and drag behavior.