Last week we sat on the slopes of Ben Nevis looking down on this cattle grid. Our seat was near the top corner of the isolated patch of woodland in the picture. We noticed that there was a big time delay between seeing the car hit the cattle grid and hearing the sound. I timed it. Now is that sad? Anyway, the values were 2.74 and 2.61 seconds respectively. If we assume the speed of sound in air to be about 335 metres per second, then that means we were sat about 90o metres away. To read about the way in which air temperature affects the speed of sound, try this Wikipedia link: http://en.wikipedia.org/wiki/Speed_of_sound