You recently answered a question about thunderstorm frequency. What constitutes a thunderstorm occurrence?
Bill Sellers, Chicago
National Weather Service observation regulations stipulate a thunderstorm be reported when thunder is heard at the observation site. The audible confirmation is required because lightning can be visible from great distances. Most weather stations now have automated observing systems with lightning detectors. When the thunder cannot be heard because of the noise level at the station, a thunderstorm is said to have occurred if lightning is detected within five nautical miles of the station. If the lightning is detected 5-10 nautical miles from the station, the remark- thunderstorm vicinity is appended to the report. Lightning 10-30 nautical miles from the station is cataloged as distant lightning.