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Name: Kathryn
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: 8/2/2004

Several times recently, we have heard faint thunder, actually more of a grumble, when the sky has been entirely blue above us with a few cumulus clouds around the horizon. What causes this? I know you should not swim during a thunderstorm with obvious and numerous grey clouds; however, is it dangerous to be in a swimming pool when you hear faint thunder with blue skies?

Under certain atmospheric conditions thunder (and lightning -- so-called "heat lightning") can travel quite a long distance. Most supervised swimming pools or beaches will "blow the whistle" at the first sound of thunder. The reason for this is storm fronts can move very fast. Why take the chance for a few more minutes in the water.

Vince Calder


You may have already received an answer to this, but it is possible, under some atmospheric conditions, for sound to travel many miles. Once I heard the roar of a tornado when the thunderstorm cloud that it formed from could barely be seen, some 20 miles away. So, yes, you probably did hear thunder or perhaps a tornado.

It is always best to retreat to a safe place when the anvil of a thunderstorm appears overhead or a thunderstorm is within 10 miles. Lightning can come out of any part of a thunderstorm, including the anvil, which can extent many miles ahead of a storm. A few weeks ago I witnessed lightning from almost the furthest extent of an anvil, which surprised even me, although I know that it is not unusual.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory

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