October 18, 2018

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.


  1. I live in South Snohomish county about one mile in from the Sound and have noted occasionally when walking the dog outside not hearing anything at all, utter silence, when normally I hear the constant din on auto traffic on Mulkilteo Speedway, I-5 and 99 no matter what time of day. The utter silence is jarring! Must be for the opposite reason.

  2. This also has an effect on some radio frequencies.
    Tony Brocato

  3. Dear Cliff, Long ago, I did a couple of summers as an intern in atmospheric sciences at the Argonne National Labs. It was 1990 & 1991. I worked with a guy named Richard Coulter, who used low-frequency sounds to do vertical temperature profiles and wind-speed estimates. It was a big dish with a tweeter, pointed straight up and surrounded by 6-8ft stacks of hay! The effects you describe must be the same physical principles just on a bigger scale (over distances). Much like the effect of mirages, the temperature changes create a change in the index of refraction for light and sound, and the waves (acoustic or electromagnetic- light and radio frequencies) bounce off those temperature boundaries. At my home, a good 6 or 7 miles from the ferry terminal, I can hear the fog horn of the ferry at Edmonds on days like today. The sound must just be bouncing off those boundaries created by the inversion, back down to the ground. Always a pleasure to hear the fog horn. Thanks for jogging my memory! phun physics. WD

  4. Interesting that despite the inversion we seem to have clean air. I always associate inversions with the murk, clear skies with onshore flow. I’m missing something.

    Thanks again for your blog, book, and radio spots. Good popular science communication by working scientists is a national treasure.

  5. Does this mean aircraft takeoff noise will suddenly decrease when the plane climbs above the inversion altitude? Below the inversion, sound would reflect back down. Above it, sound would reflect up?

  6. In Port Townsend during this period the smell of the sea has pumped up Morgan Hill to an area that I rarely smell such a strong level of sea air. Also in other areas of town while we biked the same unusual sea smell caught us off guard.

  7. I did take a hike in the inversion, although on Tuesday rather than Wednesday. I was bracing for the worst pre-dawn when it was 32f coming up highway 2. Imagine my surprise when I got to my 4000ft trailhead and it was 54f and the sun had just barely risen. A very nice surprise indeed after hiking a similar trail in fresh snow the week before.

  8. Different experience in Port Angeles. All week I have been noticing the total absence of the usual traffic noise from US 101, a mile and a half from my house, even in the humid mornings. But the jets passing overhead to/from Alaska and Asia have been more audible.

  9. Ward: what you are referring to is Radio Acoustic Sounding System (RASS). I worked with Rich Coulter on the Atmospheric Radiation Measurement (ARM) Program. I think Rich is still working.

    Werner Ecklund, NOAA, was one of the early developers of RASS and Radar Wind Profilers. Just about any acoustic signal will work and he told stories about piping Pink Floyd through the giant speakers rather than the more mundane chirping sound you normally here. If I remember correctly, this was done in Papua New Guinea and it created quite a stir.

  10. Dear Cliff:

    I was appalled by your blog on Prop 1631. Your points on its (maybe) shortcomings are noted, but I ask you: if we vote down this climate intitiative, when ? under the Trump administration will we get something even similar; and if we come up with another initiative locally. we're already 3 years into the UN drop-dead timeline. I'd rather trust the money to left wing environmentalists than anyone on the horizon.

    Circumstances are dire. I don't need to tell you.

    Joyce Bush

  11. Joyce...sorry you are appalled. Forget Trump...we need to come up with something here. 1631 is a total disaster and undemocratic. Circumstances are NOT dire. We just need to get it right and not waste billions of dollars on doling out funds to special interest groups...as dictated in 1631..cliff

    PS: the Seattle Times has come out against it and their reason are sound


  12. Not Dire.Really?

    AMOC slowing, glaciers melting at an unexpected rate, what happens when the water cycle crashes? Does anyone know how many years before we need to deal with that crisis ? And that's not a crisis we have any control over, nor can we 'fix' it.

    Your credibility just took a nose dive, Cliff. Sorry, for both of us. I looked forward to your blogs.


  13. Planetary scientists already know how this world will end, despite all the climate change that is happening ... wet greenhouse. Also, did I just read a dig against Californians (again?). What is it with you , Cliff...? we moved from Pasadena and are very happy residents here, we are not a burden to the locals in any way, shape or form, except that we (perhaps) dress a little better. Please stop the CA bashing, it diminishes you.


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

Is Washington State REALLY In a Drought Emergency?

On Tuesday, the Washington State Department of Ecology declared a DROUGHT EMERGENCY for nearly the entire state (see map below). As I will d...