Lightning..

[ProfessorDoxus]

Here we're having a really hot dry summer, and today was a cooler overcast/cloudy day. Now, at 12:51 at night, we're having this odd thunderstorm where there are more flashes of light then noticeable thunder. Maybe I'm missing it, but I wanted to ask if this was typical.

[CaptainPanic]

I've seen that before where I live (Netherlands). If you cannot hear the thunder, this only means that the lightning is too far away to hear (and perhaps the wind direction also prevents you from hearing it). I'm guessing there also was no rain, or at least no heavy rain? Lack of rain really increases your visual range, and you can see lightning much further away. Sometimes it might be as far as 30-50 km away (note: this is a bit of a guess), and still be visible as a flash on the horizon.

[Enthalpy]

A matter of distance, that's my opinion too. Direct sight to >10km altitude goes farther than 50km: it would be 357km on a perfectly spherical Earth - a big attenuation and delay for sound.

 

Such silent thunderstorms are not uncommon in Europe.

 

I doubt wind could have an effect on it. The effect of wind on sound is intriguing, because the simple change of path length would have no noticeable effect: even 30m/s wind would make the path just 1/10th longer for sound, and let it lose 1dB only due to open-space attenuation. The best explanation I got is that near the ground, wind has a strong gradient due to viscosity; this gradient also carries sound at a speed that changes with height; upwind, the propagation speed gradient lets the wave bend upwards and vanish to us, while downwind, the gradient bends the wave downwards and guides it like fiber optics.

[EdEarl]

I have lived in tornado alley in the US where thunderstorms are frequent. Lightening without thunder is common. I do not know why. Perhaps upper atmospheric lightening does not make thunder.

[overtone]

On the prairie where I grew up we called it "heat lighting". It was common in the summer, and during drought.

 

I suspect prevaling wind, direction and strength, has something to do with the silence. Sound does not travel well upwind, and a storm moving away is more often downwind than a storm coming in (hence the absence of cooling rain or wind, "heat lightning").

 

Like this:

The best explanation I got is that near the ground, wind has a
strong gradient due to viscosity; this gradient also carries sound at a
speed that changes with height; upwind, the propagation speed gradient lets the wave bend upwards and vanish to us

Yes. And there are also air density height gradients from temperature and humidity involved in heat lightning, not wind only.

 

But there might be also something else; you can hear this from motorcycles overtaking you in a car on the freeway - from behind, sound traveling into the headwind, they have to be close to be loud; ahead, they are louder for a greater distance. The difference is much less if you are standing alongside the road - you can hear them coming a long way off, going almost as far. There is no airspeed height gradient involved in that.

.

Edited June 11, 2013 by overtone

[EdEarl]

Sound is a sequence of waves of pressure that propagates through compressible media such as air or water. (Sound can propagate through solids as well, but there are additional modes of propagation). Sound that is perceptible by humans has frequencies from about 20 Hz to 20,000 Hz. In air at standard temperature and pressure, the corresponding wavelengths of sound waves range from 17 m to 17 mm. During propagation, waves can be reflected, refracted, or attenuated by the medium.

 

From: http://en.wikipedia.org/wiki/Sound#Propagation_of_sound

 

Perhaps thunder is heard some places and is reflected, refracted and attenuated such that it is not heard in other places at similar distances from the source.

 

High altitude lightening can be seen from a greater distance than lightening within clouds and from cloud to ground; thus, it may not be heard because its thunder is too distant to be heard.

Edited June 11, 2013 by EdEarl