Measuring and reporting hail
- View
What counts as severe, what to measure, and which reference objects the NWS asks you not to use.
In this lesson you will
- State that spotters report all hail sizes, and that severe is 1.00 inch or larger.
- Measure the largest stone's longest dimension and photograph it against a reference object of known size.
- Produce a report carrying spotter status, location, time of occurrence and whether the size was measured or estimated.
- Describe hailpad methodology and the quantities it records.
The two thresholds
Report all hail sizes, whatever they are. Severe is 1.00 inch, a quarter, or larger. The one-inch criterion took effect on 5 January 2010 and replaced an older 0.75 inch penny threshold that is still in circulation. If you learned the penny, relearn the quarter.
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.
| Hail diameter size | Description |
|---|---|
| 1/4" | Pea Size |
| 1/2" | Marble Size |
| 3/4" | Penny Size |
| 7/8" | Nickel Size |
| 1" (Severe Criteria) | Quarter Size |
| 1 1/4" | Half Dollar Size |
| 1 1/2" | Walnut/Ping Pong Ball Size |
| 1 3/4" | Golf Ball Size |
| 2" | Hen Egg Size |
| 2 1/2" | Tennis Ball Size |
| 2 3/4" | Baseball Size |
| 3" | Teacup Size |
| 4" | Grapefruit Size |
| 4 1/2" | Softball Size |
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.
NWS: "Objects that can vary in size such as balloons, pebbles or marbles do not make good comparisons." Coins are fixed sizes and everybody has one. Pea, penny, nickel, quarter, half dollar, golf ball, hen egg, tennis ball, baseball, softball.
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 CoCoRaHS hailpad 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.
Check yourself
You measure the largest stone at exactly one inch across its longest axis. Severe?
- Yes. One inch, quarter size, is the severe criterion
- No. Severe begins above one inch, so this is just under
- No. The threshold is three quarters of an inch and this exceeds it, so it is significant severe
- Only if more than one stone reached that size
Hint
The chart marks one entry as the criterion itself.
Answer
Yes. One inch, quarter size, is the severe criterion
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.
Sources
- NWS New Orleans and Baton Rouge, "Estimating Hail and Wind"
- NWS Weather Spotter's Field Guide (YPA-201154, June 2011)
- NWS Service Change Notice 09-52: the one-inch hail severe criterion, operational 5 January 2010
- CoCoRaHS, Measuring Hail
- NCEI State Climate Extremes Committee hail standards guide