For many years the National Weather Service (NWS) has considered any thunderstorm that produces a tornado, 3/4 inch (0.75") or larger hail, or damaging winds of 58 mph or higher a severe thunderstorm. For hail, common objects such as coins have been utilized to help estimate the size of hail.
In the past, we used a dime or a penny to indicate 0.75 inch hail. If you take out the calipers and take a precise measurement, you’ll find that a dime is actually just 0.69 inch across, with a penny at 0.75 inch. Thus, in the past two years, the NWS has decided to no longer consider a dime size hail as a lower limit to define severe hail.
Half inch (0.50") hail has also been associated with marbles. This is problematic as well since there are many different sizes of marbles. Thus, the term "small marble" will now be used with 1/2 inch hail to avoid confusion.
The term "Large Marbles" will not be used in storm reports. If we get a report of such, the NWS will request a measurement or a reference to another object (see table below) to better estimate the size of the hail.
When measuring hail, be sure to wear gloves so that the heat of your hand doesn’t melt the stone. Also, many hail stones have irregular shapes and are not perfectly round. Measure the stone along the longest axis (see example below).
In our preliminary storm reports, we generally round to the nearest 0.25 inch (with exception of Nickel Hail, which is 0.88 inch). So, if we get a report of dime hail, we are going to log that as 0.50 inch hail. If the hail is measured at 0.75 inch or estimated at Penny size, then it will be logged as severe.
For several years, there has been push to raise the severe hail criteria from 3/4 inch to one inch (the size of a quarter). Damage to vehicles and structures often starts once a hail stone exceeds one inch. However, aviation interests are concerned about penny size hail since flying at rapid speeds through this smaller hail can cause damage to a plane’s airfoils. Thus, it is not clear when or if the severe hail criteria will be changed in the near future.
For reference, here is a table of common objects used to estimate hail size
0.50" Small Marble
0.69" Dime (will be logged in Local Storm Report as 0.50")
1.25" Half Dollar
1.50" Ping Pong (or Walnut)
Very Large Hail
2.00" Lime (or Hen Egg)
2.50" Tennis Ball
3.00" Large Apple (or Tea Cup)
4.00" Softball (used to be 4.50")
4.50" Grapefruit (used to be 4.00")
5.00" Computer CD
Jeff Craven, Science and Operations Officer, NWS Milwaukee/Sullivan WI