High flying aircraft loudly audible today
I have noticed than sometimes high flying airliners, which are normally inaudible from the ground, can be heard quite loudly as they pass overhead. An AA 777 Heathrow to NY has just passed overhead at 36,000' (according to Fight Radar), con-trail clearly visible in the cloudless sky, and with a distinct roar.
Normally such flight may be visible, more often not due cloud, but very rarely can they be heard clearly from the ground. I have noticed this before when out walking in the Peak District on cold clear days, when the noise can be quite intrusive with busy skies above evidenced by con trails..
Weather today is a NW air stream on the chart (surface wind nearer W) with a low over Norway, slight ridge in the Atlantic, low surface air temperatures due to clear skies. No visible inversion.
Anyone any idea why this should happen on rare days, whereas most of the time it doesn't?