Beautiful weather produces noisy mornings!

Yesterday morning I received an email from blog reader Emily Vilbrant wondering about an increase of train noise during the morning hours.  Distant trains sounded unusually loud and clear.  Emily asked if I might have an explanation.

And I have been noticing something similar.  While walking my dog around 6:45 AM yesterday, the sound of distant traffic was unusually loud.  Even my little dog noticed.

Interestingly, it all makes perfect sense, and believe it or not, it has to do with the clear skies and beautiful weather we have been having.   Let me explain.

The sun was about to rise at 7:20 AM this morning.  Beautiful weather led to noise start of he day.  SpaceNeedle PanoCam

The last few days we have had a ridge of high pressure over our region (see upper level map at 5 AM Wednesday morning), which resulted in mainly clear skies, sinking air aloft, and weak offshore flow at low levels.

Clear skies allowed the surface to radiant infrared radiation to space, resulting in the surface cooling rapidly.  Our nights are getting much longer now, so there is plenty of time for the earth to cool.  The atmosphere above does not radiate as well, so it stayed relatively warm.  High pressure is associated with light winds, so there was little atmospheric turbulence mixing the warm air aloft down to the surface.

The result of the intense surface cooling was the development of a strong inversion yesterday morning (and many other mornings during the past week).....and remember that an inversion is when temperatures WARMS with height. 

I can prove this to you by showing you a temperature profile with height in north Seattle from a fancy piece of equipment located on the NOAA campus near Magnuson Park:  the radar-wind profiler.  The Y-axis is height in meters and the temperature (x axis) is in Celcius (C); temperature profiles are shown from 3 AM to 9 AM yesterday. 

Wow---a HUGE inversion. About 11C warm up in 400 meters (about 1300 ft).  We are talking about 20 F increase.   So while it was in the forties near the surface, the temperature was around 68F at 1300 ft.

Pretty amazing. Can you imagine going on a short hike yesterday morning?

But what does this have to do with sound?

Well, it turns out that inversions can bend or refract sounds down toward the surface.  Sounds that would normally radiate away above your are bent down to the surface (see schematic below).  This occurs because the speed of sound depends on temperature:  sound moves faster when it warms.

As a result of the bending of sound waves, you can hear sounds from further away more clearly when a low-level inversion is around.   That is the answer to Emily's question.

Since I am a professor, let me give you an assignment.  The inversion should still be around on Saturday morning, but perhaps not as strong.  Head outside either very late at night (college students) or get up early and head outside and LISTEN.  See if you can hear the effect.

from Cliff Mass Weather and Climate Blog https://ift.tt/2J58QZj