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1 Highway noise changes from day to day and hour to hourânot just because of varia- tions in traffic volumes, vehicle mix, and speed, but also because of the weather. In some cases, traffic noise can increase by more than 10 dB in as little as one hour due to changes in meteorology. This research project had the following objectives: 1. Measure and document the meteorological effects on roadway noise propagation under different atmospheric conditions. a. Develop best practices and provide guidance on how to quantify meteorological effects on roadway noise propagation and explain those effects to the public. The guidance developed under this research project: 1. Identifies the most critical atmospheric parameters that affect roadway noise propagation and the distances at which their respective influences may occur; and 2. Aids in determining when meteorological effects should and should not be considered in noise analyses. The research results help practitioners understand and explain roadway noise levels under different atmospheric conditions in a roadway noise analysis. Chapter 1 provides background on how meteorology affects highway noise. It includes a brief literature review and provides a primer on meteorological effects, showing how changes in temperature and wind speed with height affect the bending of sound toward or away from the ground. It discusses different highway noise propagation models used in the United States and Europe and their approaches to incorporating meteorological effects. Chapter 2 describes the data collection and modeling conducted for this research study. It discusses the program to collect sound data along Interstate-17 (I-17) north of Phoenix, Arizona. In all, over 34,000 valid 5-minute data points were collected quantifying 230 differ- ent variables, including traffic volume, speed, vehicle mix, sound levels by 1/3 octave band, temperature, humidity, barometric pressure, wind speed, wind direction, vertical profiles of temperature, wind characteristics, and sound speed. The collected data clearly show a sound level difference between periods with similar traffic volume and vehicle mix, a marked indica- tor of the meteorological effects at work. The chapter also shows the results of sound modeling using the European Harmonoise model and the FHWA Traffic Noise Model (TNM). Chapter 3 details various approaches to conceptual models that take meteorology into account. The research team evaluates what parameters should be included in the models and provides examples of models and tools that can be used to estimate the impacts of meteo- rology. These include a tool to use the U.S. Environmental Protection Agencyâs (EPAâs) S U M M A R Y How Weather Affects the Noise You Hear from Highways