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Understanding Isokeraunic Charts.

One of the tools that scientists and insurance companies have developed to measure the potential for thunder storms is the isokeraunic (eye-so-cure-anick) map. This map (example shown above) provides a visual overview of how many thunder days occur on an annual basis.


Rather than try to count the number of lightning events (which can often be difficult), scientists collect the average number of days that have thunder storms per year. This data ranges from 1 event (Alaska) to 100 events (Central/South Florida)! Needless to say, this information is quite useful to insurance companies and electric utilities as they plan for and respond to these potentially damaging weather events.


The distribution of thunder and lightning storms is highly dependent on the location, climate and time of year. Southern states typically see the most storms from June through August. States in the Midwest tend to have higher storm activity during transitions from winter to spring and from fall to winter.


From a global perspective, thunder and lightning occur on average, 40 to 50 times per second over the entire Earth. That equates to 1.4 billion lightning flashes per year!


So, how do electric utilities protect their systems in high lightning prone areas?

They increase the number of lightning arresters on their distribution system to help mitigate the risk of lightning damage to critical equipment.

Many offer additional surge protection programs to reduce or eliminate the risk of damage from nearby thunder/lightning events

Here is a summary of the average annual number of thunder days for cities and areas around the country.

City/Area

  • Anchorage Alaska
  • Hawaii
  • California
  • New York
  • Chicago
  • Atlanta
  • Kansas City
  • Charleston, SC
  • Tampa – Orlando
  • South Florida

Thunder Days

  • 1 to 3
  • 3 to 5
  • 5 to 10
  • 20 to 30
  • 30 to 40
  • 50 to 60
  • 50 to 60
  • 60 to 70
  • 80 to 90
  • 90 to 100

City/Area

  • .
  • .
  • .
  • .
  • .
  • Yes
  • Yes
  • Yes
  • Yes
  • Yes