By KSN Chief Meteorologist Dave Freeman

Results of the KSN WeatherLab Severe Weather Survey strongly support using 1" diameter as the definition for "severe" hail. The National Weather Service used that criteria in an experiment this year in almost all of the KSN viewing area. The KSN WeatherLab posted a survey on www.ksn.com to gain feedback from viewers on that and other aspects of severe weather.

You may remember from previous columns that the KSN WeatherLab Team has been calling for a change to the hail warning criteria for a very long time. This is because the current hail criteria (.75", the size of a penny) does not pose a significant danger to the general public. Changing to 1" diameter hail (just a bit larger than a US quarter) will reduce the overall number of warnings issued, and so place the emphasis on storms which really do pose a danger to the public.

From March to August, the KSN WeatherLab collected more than 1300 surveys from viewers about these issues and the results show that the change to 1" hail is a solid decision. Here are some of the results of that survey. First, it is interesting to note that the majority of folks did not know what the current definition of a severe storm is.

Less than half of the people who completed the survey knew that the current definition of a "severe thunderstorm" is penny sized (.75") hail and/or wind at or above 58mph. In fact, nearly 30% of folks said that marble sized hail (.5") and 43mph wind was "severe!" This would mean a severe thunderstorm warning would have to be issued for practically every single storm from March through November!

It does seem that viewers start paying attention to weather when small hail occurs. The survey showed that more than 40% of viewers begin getting concerned when marble sized (.5") hail happens. Only about 8% of viewers said they pay no mind until the hail reaches golf ball sized (1.75"). I find that hard to believe--but that’s what they said!

We then tried to get an idea of when viewers’ own personal sense of danger kicked in. We asked what size hail prompts the viewers to go ahead and move away from windows and toward the center of the house--in other words, when do viewers feel threatened enough to seek shelter. And, we asked a straight forward question about when viewers feel a formal Severe Thunderstorm Warning should be issued. The results of these two questions are a clear mandate for the use of quarter sized hail (about 1") to define a severe thunderstorm.

Note the very strong agreement between when viewers’ own sense of personal danger kicks in and when they would like a warning issued. Both answers center right on the 1" hail criteria. This is a very "clean" answer as well because the survey questions were not weighted or prejudiced by identifying the current standard. The viewers were simply asked to answer based on their own understanding and observation.

The KSN WeatherLab Team believes that these results show that the majority of viewers would feel well-served by making the 1" hail criteria permanent. It matches their own sense of personal danger, and their desire to be warned of threatening conditions. Given that it is also consistent with scientific evidence of the size of hail necessary to cause meaningful damage (1" as well), we believe this change would be in the best interests of the public.

One final note is that these results do show that most folks become concerned when smaller sized hail falls. We also note that there are interests such as agriculture that do need documentation of smaller sized hail. But, we must point out that both of these concerns can be met--and indeed, are being met in the area where the experiment is ongoing. Storm reports of smaller sized hail are still being collected and documented, and several of the National Weather Service offices are using "Significant Weather Advisories" to give a heads-up on storms which are strong but below the severe weather threshold. We believe that this is a very sound way to meet these needs while reserving the use of a "Warning" for those storms that pose an immediate threat to life, limb and property.

The KSN WeatherLab Team urges the National Weather Service to permanently adopt the 1" hail criteria for severe thunderstorms.

Dave Freeman
KSN Chief Meteorologist
weatherlab@ksn.com