Raven spreads a printed table across the lab table. "This is from the Water Science School," she says. "Drop sizes for every kind of rain."
Rocket runs his finger down the numbers. "Drizzle drops are about one millimeter. Cloudburst drops are almost three."
"What do you notice about the speed column?" Raven asks.
"Bigger drops fall faster," Rocket says. "A lot faster."
Nova hovers over the table and highlights one row in green. "Now compare the rain that fell on Station Rooftop," she says. "Which row did it match?"
Rocket checks the log and grins. "Moderate rain. The jar agrees with the sky."
The United States Geological Survey Water Science School publishes a table of typical raindrop sizes and speeds.
The numbers below are copied from that table. Diameter is the width of a typical drop in millimeters.
Intensity is how deep the rain would pile up in one hour if none drained away.
| Kind of rain | Typical drop width (mm) | Intensity (inches per hour) | Fall speed (feet per second) |
|---|---|---|---|
| Drizzle | 0.96 | 0.01 | 13.5 |
| Light rain | 1.24 | 0.04 | 15.7 |
| Moderate rain | 1.60 | 0.15 | 18.7 |
| Heavy rain | 2.05 | 0.60 | 22.0 |
| Excessive rain | 2.40 | 1.60 | 24.0 |
| Cloudburst | 2.85 | 4.00 | 25.9 |
Small raindrops, less than about one millimeter in radius, are round like tiny spheres.
At about two to three millimeters across, the bottom of a falling drop flattens and dents inward.
At about four to five millimeters, a drop breaks apart into smaller drops.
So the biggest drops in the table are near the limit of what a raindrop can be.
The National Weather Service counts rain as measurable when it reaches 0.01 inch.
One inch of rain on a single acre of land is 27,154 gallons of water.
With a grown-up, find the rainfall your nearest Weather Service office reported for your last rain day.
Your jar may read a little higher or lower. Rain is patchy, and a jar is rougher than a measuring tube.
Excellent data reading. Tomorrow you connect precipitation to everyday life and look ahead to air masses.