Crozet Weather Almanac: June 2017

Chart courtesy Heidi Sonen and Roscoe Shaw

Thunderstorms… How Many Seconds Do I Count?

You may have heard that you can determine how far away a lightning strike is simply by counting the seconds it takes between the flash and the sound. This is true and a fun thing to do in the summer, especially with kids. It combines observation with math and other skills. Scientifically, you are computing the difference in the speed of light and the speed of sound and then converting that into a distance.

Heidi and I starting thinking about this when we watched fireworks in Crozet Park on July 1. We were on a hilltop that is a mile and a half from the park. The sound takes a very noticeable seven seconds to travel that far. But we see the flash from the fireworks almost instantly. The speed of light is 186,000 miles per second. Therefore, the flash from lighting or fireworks can travel around the earth 52 times before the sound can even get part way across Crozet! The speed of sound is a brisk 767 mph, faster than almost any human has ever travelled, but that pales in comparison to to the speed of light.

So, when you see a flash of lightening, start counting “One Mississippi, Two Mississippi.” Or, if you are like me, you prefer “One thousand one, one thousand two” etc. If you get to five, then the strike was a mile away. Ten seconds is two miles and so on.

Thunder can sometimes be heard from as far away as 20 miles but often less. The distance depends on topography and acoustics. Counting for a storm 20 miles away would take a minute and a half, which is a lot of Mississippis. This can get confusing, too, because if there are lots of lightning strikes around, you can’t tell which thunder clap came from which lightning strike. Thunder is simply the sound generated by lightning, which is a sudden expansion of super-heated air in and along the electrical discharge path. The intensity and type of sound depends upon atmospheric conditions and distance between lightning and the listener.

If the lightning and thunder come at the same moment, you are lucky to be alive because the distance is near-zero. I was in a friend’s house watching TV once when a simultaneous flash and boom made us all jump 10 feet high. The front door blew wide open, all the power went out for blocks around and their TV never worked again.

“Heat lightning” is the same as regular lightning; it is just too far away to hear the thunder. This is common on hot summer nights. Sometimes I see constant flashing in the distant eastern sky and when I check radar, the storms are on the other side of Richmond, a full 100+ miles away.

Lightning is a dangerous killer but not nearly as bad as it used to be. Humans are indoor creatures now compared to 100 years ago when we were outside much of the day. Also, these days, most of us carry around a portable radar, also known as a phone. A good radar app can keep you safe.

June Recap

Temperatures in June were almost exactly average. We started the month cool and finished with cool, dry Canadian air. The middle of the month was hot, but not terrible. Rainfall was just 2.88,” which is below the long-term average of 4.08”. Coming on the heels of a very wet May, we hardly noticed the dryness until late month.

July is now here and it is usually the hottest month of the year. The worst part is the humidity. We like to watch the dewpoints. When the dewpoint gets above 70, the humidity is oppressive. If we are lucky in July, dry air will push the dewpoint into the 50s, like we had some of the time in June.


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