The speed of sound in Wasdale

The usual problem with military planes in the mountains is not seeing them because I end up looking at the sound. Light is so fast that we assume that it arrives instantly. But sound takes about 3 seconds to travel 1km. That must have been the distance to this Hercules. We were able to find it because it travels a shorter distance during the 3 second delay than fighter jets would. Thanks to Paul for the photo!