Excerpt from the Illinois CoCoRaHS Newsletter March 2009...
Currently one of the criteria for a severe thunderstorm is hail three-quarters of an inch in diameter or larger. Effective April 1, National Weather Service offices in the Central Region (which includes Illinois) will be using a threshold of one inch (the size of a quarter). There has been research that shows that significant damage to property does not occur until hail reaches one inch. Additionally, raising the threshold means there will be fewer warnings issued for marginal events, and when warnings are issued with the new threshold they will be for storms that have a greater damage potential. With severe weather season already spinning up, it's a good time to download our pocket-size "Hail Measurement Guide" from the Illinois CoCoRaHS web page. It has been updated to reflect this change.
A NWS video explaining the reason for the change is available for viewing
here.
