Why were the jets so loud in SLO County in April?



PG&E meteorologist John Lindsey took this photo of an airliner flying over the Central Coast.

David Kudija of Paso Robles asked:

“Mr. Lindsey, were there different weather conditions on April 14? We are on Flight Path 4 with airliners traveling to the Orient, and although we see them often, we only hear them faintly. evening, however, they are at least five to ten times louder than normal. They almost sound like thunder outside. Are there atmospheric conditions this evening that could explain this?”

To answer this question, the first piece of information I thought of was atmospheric profiles or soundings from Vandenberg Space Force Base.

Every day, the 30th Weather Squadron from the base in western Santa Barbara County launches a weather balloon that has a tiny transmitter called a radiosonde attached to it. As the weather balloon ascends through the atmosphere, its transmitter broadcasts the temperature, dew point temperature, pressure, and GPS coordinates of the winds to the receiving station.

Atmospheric data from these radiosondes are compiled into a Skew-T chart, a vertical chart of the atmosphere from the Earth’s surface to nearly 55,000 feet above sea level. The red line is the air temperature and the green line is the dew point temperature in degrees Celsius.

You can view these graphs of Vandenberg and other locations in North America that launch daily weather balloons through the University Corporation for Atmospheric Research (UCAR) at weather.rap.ucar.edu/upper.

Atmospheric data for April 14 shows a nearly isothermal condition from the Earth’s surface down to 600 millibars, or about 15,000 feet. In other words, very little temperature change, which is unusual. After the 600 millibar mark, there was a gradual decrease in air temperatures to the stratosphere at nearly 38,000 feet that day.

Most commercial airliners fly at an altitude of between 28,000 and 41,000 feet – about 6 miles in the sky – where temperatures are freezing and winds aloft are strong.

These airliners often fly at different altitudes to take advantage of or lessen the impacts of the jet stream to optimize their ground speed on Earth.

On April 14, the airliners may have flown at a slightly lower altitude to increase groundspeed on their journey to Asia, which likely placed them in the troposphere, the lowest region of the Earth. atmosphere that extends from the Earth’s surface to a height of approximately 35,000 feet and fluctuates daily in altitude throughout the year.

One of the reasons planes fly in the stratosphere is that it’s where there is the least turbulence.

On that day, a significant amount of aircraft noise, more than normal, was probably directed towards the 600 millibar mark due to an atmospheric sound channel in the vertical plane.

You see, sound waves will always deflect from areas of higher velocity. Think of a bulldozer; if one set of its tracks rotates faster, the bulldozer will turn in the direction of the slower moving tracks.

In isothermal conditions of 600 millibars on the ground, the sound of the airliner was directed directly at David in Paso Robles.

Sound channels can also develop horizontally.

Residents of coastal communities often hear the waves crashing along our coastline. There are times when the waves are high, but the sound is barely perceptible; other times the swell is weak, but the sound along our shores is quite loud.

Let me explain.

Many years ago, I took my son to the beach, and he picked up an empty Lewis moon snail shell, put it to his ear, and said he could hear the ocean. The shell picks up ambient noise from the environment. The sound resonates inside the shell and produces a wave-like sound no matter how far from the ocean.

Just as the walls of the shell pick up and direct sound to our ears, so can the atmosphere.

Atmospheric temperature inversion layers come and go as Santa Lucia winds (northeast/offshore) and northwest winds (onshore) compete for supremacy. You can see it by the ebb and flow of low coastal clouds along the beaches.

Cool, dense air at the Earth’s surface produces a layer of temperature inversion that creates a sound channel, ranging from a few feet above the ocean surface to hundreds of feet high.

Some of the wave sounds reflect or jump from the inversion layer to the ground. In other words, the sound of crashing waves is picked up in the surface channel and propagates to our coastal communities with minor loss of intensity.

At other times, the winds blow fast enough to mix up the temperature inversion layer, allowing sound to travel in all directions with much greater loss of intensity, making it much quieter. I guess you could say the sound of the waves was lost in the wind.

Learn more about PG&E’s wildfire prevention efforts

PG&E is hosting an interactive webinar for all customers on April 20 at 5:30 p.m. to discuss our wildfire prevention efforts in our service territory. Join one of our VPs and team members to ask questions and share your feedback. Visit www.pge.com for more information.

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