Measuring and reporting hail
What counts as severe, what to measure, and which reference objects the NWS asks you not to use.
The two thresholds
The chart, verbatim
Reproduced exactly from the NWS New Orleans and Baton Rouge office, which publishes the fullest version and is the nearest office to this platform that does.
Verbatim, including the entry for marble. NWS guidance elsewhere tells spotters not to use objects whose size varies, and names marbles among them, so the agency contradicts itself here. Use the coins. The contradiction is flagged for the reviewer rather than silently corrected, because this is a verbatim reproduction.
How to measure it
- Wait until it is safe. Not while it is still falling, and not until the thunder has stopped. Lesson 7.
- Find the largest stone, not a typical one. The size question means the maximum.
- Measure the longest axis with a ruler or calipers. Stones are not spheres and the longest dimension is what is asked for.
- Photograph it beside something of known size, a coin or the ruler itself. This is the single thing that turns your estimate into somebody else's measurement.
- Handle it as little as possible, and get it out of the sun. It is melting while you look at it.
Then report it with the four things every report needs: that you are a trained spotter, where it happened, the time it happened rather than the time you are calling, and whether the size was measured or estimated. Module 1 lesson 9 covered why the time clause decides whether your report verifies anything.
Hailpads, and what a careful observer records
The Community Collaborative Rain, Hail and Snow NetworkA volunteer precipitation observing network. Its hail protocol is the fullest widely used standard for recording what fell, beyond just the largest stone. A foil-wrapped square left outdoors to be dented by hail, so the stones can be sized afterwards from the dents. The CoCoRaHS standard is 12 inches square. is a 12-inch square of foil-wrapped surface left out to be dented. Afterwards the dents give the largest, the average or most common, and the smallest stone size, plus the total number and the duration, and the observer notes whether rain fell with it.
You are unlikely to own one. It is here because it shows what a complete hail observation looks like, and because almost every field of it is something you could add to a report from memory: how long it lasted, roughly how much fell, whether it came with rain, and how deep it lay. Size is the headline and it is not the whole observation.
You measure the largest stone at exactly one inch across its longest axis. Severe?
The chart marks one entry as the criterion itself.One inch is the criterion, not the value below it, and quarter size is the reference object. The three-quarter-inch penny threshold in the third option was replaced on 5 January 2010 and is the most common outdated figure in spotting.