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A hot summer's day had finally ended, but then it ushered in a hot summer's evening. The house was too hot and stuffy to stay inside, so we sought the little relief the outdoors could offer. As we sat in the yard, chatting occasionally about the baseball exploits of the Chicago Cubs, I began to notice the northern sky glow with random flashes of light. But no thunder followed.
"Might get a thunderstorm tonight," I forecast. "Hope so," my brother replied. But although the flashing sky continued for several hours, no rain fell on us this night, no cooling relief flowed from a travelling thundershower. We only had a teasing episode of heat lightning, visible flashes of distant bolts lacking the slightest hint of thunder.
This scene played out many times in my youth, growing up in the northeastern Illinois suburb of Palatine. Today, living on Vancouver Island, thundershowers are few and far between, not seen even in the distant skies. I had not thought about such incidents for some time. But recently, I have had several questions on this weather phenomenon and decided it was time to revisit heat lightning.
Folk weather mythology suggests heat lightning is caused by hot air expanding until it sparks on sultry summer nights an incorrect hypothesis. On the other hand, you will find many credible sources telling you that heat lightning does not exist. But, this is mostly a matter of semantics.
Heat lightning, you see, is not a unique form of lightning, but normal thunderstorm lightning that flashes too far away from the observer for its thunder to be heard. It is most commonly presented as sheet lightning, which is actually a standard lightning bolt whose light is reflected off cumulonimbus cloud towers or is diffused as it passes through the atmosphere and thus loses its distinctive bolt pattern.
During sultry weather, scattered, rather short-lived thunderstorms may pop up across a region driven by the heat and humidity. Some, during their lifetimes, may travel overhead, bringing heat-relieving rain and cooling winds. Others may pass a moderate distance away but not too far. Their lightning takes on many visual forms forked, sheet, ribbon and, rumbles of thunder are heard, and perhaps a cool breeze flows from them, but no rain falls within view. Finally, there are those which pass a long distance from us whose presence is only noted by their towering cumulonimbus and flashes of yellow-tinted lightning (the blue wavelengths scattered out of the bolt's colour).
Because scattered thunderstorms do not produce a dense, sky-filling cloud deck, we are presented with a long line of sight through the mostly clear air, often extending to the horizon. Therefore, we can see the upper levels of thunderstorms even at distances below the horizon.
Within those distant thunderstorms, the lightning bolts flash, and their light can be seen as much as 160 kilometres (100 miles) from us, depending on the height of the bolt within the cloud, the clarity of the air between us and the bolt, and our elevation above the ground. Thunder, in comparison, has a much shorter range of detection -- usually less than 25 km (15 miles) in a quiet rural setting and under 8 km (5 miles) in a noisy city environment.
There are several explanations for why sound has such a limited range of detection.
Most important is the scattering and attenuation of sound by air molecules, particularly diminishing the higher pitched sound frequencies. Thus, by the time the thunder wave reaches a listener several kilometres from the lightning stroke, the predominant sound will be a low-pitched rumble. And further away, even the rumble ceases to be heard.
Vertical changes in temperature and wind speed through the lower atmosphere also affect the propagation of thunder by refracting sound waves from a straight line path toward the listener. Because the air temperature generally decreases with height and sound travels faster in warm than cold air, thunder sound waves curve upward, away from the listener. Thus, high altitude-originating thunder may bend away from the ground and never be heard. Even nearby thunder can have its sound waves bent away from a nearby observer, and in certain situations, bend back earthward some distance further away. This, in effect, creates a dead-zone for the storm's sounds.
Wind can have two different effects on thunder. First, it may alter the speed at which the sound wave moves through the air. Sound moves faster downwind that it does upwind. Second, the variation of wind with height may refract sound waves, similar to the effects of vertical temperature gradients. If wind increases with height, as is usually the case near the earth's surface, the sound wave refracts upward, bending away from the surface-based listener.
The combined effects of scattering, attenuation, refraction and, in some cases, reflection, limit the distances at which thunder may be heard by a ground-based observer. Although this distance varies with the temperature gradient, wind speed and the height of the lightning flash, thunder generally will not be heard further than 10 to 25 kilometres (6 to 15 miles) from the lightning bolt.
I'll return to folk weather wisdom to conclude this piece. An old folk saying goes:
"Yeller gal, Yeller gal, flashing through the night, Summer storms will pass you, unless the lightning's white."
And so it is with heat lighting, that yeller (yellow) gal of hot sultry nights.
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