Single factored Environmental variables do not adequately represent climate across the diverse SoCal area. Temperature and humidity change how sound propagates. Noise effects borne by differing meteorological characteristics are not modeled correctly resulting in average noise values being incorrect up to .6dB near airports.
The Southern California Study Area is over a large and diverse geographic location and has multiple distinct climates, micro-climates and ecosystems. The FAA’s Environmental Assessment collected meteorological data from 41 years but averaged it to a single set of environmental variables- thus ignoring significant climate gradients :
|Barometric Pressure (in-Hg)||29.98|
|Relative Humidity (%)||71.5|
1.) Using Average values will not adequately represent the sound impacts. Differences in temperature and humidity effect the attenuation of sound through the air. As humidity rises, the absorption of sound energy decreases which means noise will be higher in coastal areas than the drier inland- when considering humidity. Higher temperatures, such as those encountered inland, will reduce absorption and also mean higher noise across the frequency range.2.) Temperature and humidity vary over day and night conditions, as well as during the day. In Southern California temperatures can vary over 20 degrees and Humidity 30% when comparing day to night measurements. Average Daily Values should not be used as the distribution of aircraft flights across the day is highly weighted to daylight hours. Daily temperatures include the colder nighttime hours when less aircraft fly. Humidity and temperature should be modeled with either daytime/nightime, or time-of-day weightings of aircraft flights for each airport.
The Southern California Climate:
The offshore currents help create subtropical and Mediterranean climates along the coast. Moving inland the climate becomes more continental, with humidity decreasing as areas turn more temperate, semi-arid and hot arid similar to the Sahara Desert. The temperature gradient between the cool coast and warm inland is about 7 °F in winter and 25 °F in the summer.
East LA and parts of San Gabriel Valley averages the warmest winter high temps (72 F) in all the western US while Santa Monica averages the warmest winter lows (52 F) in all the western US. During the summer the temperature gradient between the Santa Monica Airport (75° F) and Burbank Airport (95 °F) is over one degree per mile. The temperature gradient is most extreme between Santa Barbara and Death Valley, with temperatures differing by up to 35 °F in the summer– Death Valley recorded the highest temperature in the world at 134 °F. San Bernardino airport, 70 miles inland, is rated as temperate -the winters reach 40 °F and the summers 109 °F, whereas San Diego Airport is in a semi-arid warm steppe climate.
The California Energy Commission has separated the Southern California study area into 9 distinct climate zones. Yearly temperatures across zones can vary 70 degrees and humidity up to 90%.
The study’s single static environmental variables poorly model the real environmental variables of airports and communities in Southern California. Overall noise of aircraft will vary depending upon local humidity and temperature.
See additional information in comment Meteorological effects change sound propagation
3.) Operationally, an increase in temperature will decrease an aircraft’s rate of climb. This will result in a need for additional thrust or an extended period at lower altitudes- both conditions will increase noise.
4.) Below is a table of airports and the correction needed. For each Airport city I’ve included the Average Relative Humidity (%)*, Average high temp (F)*, Calculation of the Sound Attenuation (dB/m), and some of the resultant errors and correction values from using the single static environmental variables.
(*Average High Temp is used as I couldn’t readily find average time-of-day, or daytime/nighttime, temperature for all cities. Likewise the average time-of-day, or daytime/nightime, Relative Humidity was similarly not readily available. The resulting calculations will represent a conservative value and could be lower. I recommend the FAA gather this data to increase the accuracy of noise modeling.)
|Airport City||Humidity (%)||Avg high temp (F)||Sound Absorption coefficient (dB/m )||EA Error (%)||EA Error @ 45dB (dB)||EA Error @ 65dB(dB)|
The Sound Absorption coefficient can be multiplied directly on the aggregate DNL values, or the underlying data.
Using a single temperature and humidity value for all of Southern California introduces inaccuracy in the noise modeling. So Cal has a diverse climate with distinct atmospheric conditions which are not adequately being represented.
There is an error of +.6 dB @ DNL 65 dB and +.4 dB near Palm Springs. Airports in Ontario, Santa Ana, Santa Barbara and Van Nuys all have .4 dB error at DNL 65.
In the released 4(f) noise data there are (91) noise points from the 2015 proposed action that would now qualify for the DNL 45 dB criteria and (4) that would move into DNL 65 dB when adjusted for the appropriate Sound Absorption Coefficient.
The FAA needs to accurately report noise impacts by:
- Using the correct temperature and humidity, or the appropriately calculated Sound Absorption Coefficient, for each environmentally distinct area,
- Weighting Sound Absorption Coefficient to match time-of-day aircraft noise with time-of-day temperature and humidity, or at the very least a daytime/nighttime difference.
Using the the census block or grid point noise data may bring other locations in the study area into the DNL 65 dB, DNL 60 dB, or DNL 45 dB noise criteria thresholds. Further study in this area may be warranted.
 Absorption of Sound in Air versus Humidity and Temperature, Cyril Harris (1966), Journal of the Acoustical Society of America, 40, p. 148.
 Graphic from Handbook For Acoustic Ecology, Barry Truax editor (1999) http://www.sfu.ca/sonic-studio/handbook/Sound_Propagation.html
 California Building Climate Zone Map, California Energy Commission
 ISO 9613-2:1996 Attenuation of Sound During Propagation Outdoors, Part 1: General Method of Calculation.