What are the effects
on my community?
The
effects on a community depend on the distance to highways and the
frequency and duration of weather conditions. Weather patterns that change
sound levels may be more common in certain areas.
Higher sound levels will
be more common in areas where the wind t
ypically blows from a highway
toward a community (downwind) than
the opposite direction (upwind). You
may also notice that sound levels are lower during the day than at night even
though there may be
more traffic during the day. This occurs in areas where
nighttime
temperature inversions are common. Rain can also affect traffic
noise by changing the temperature profile in addition to the more obvious
increase of noise generated from vehicle tires.
What if my neighborhoo
d
has a noise barrier?
Changes in weather conditions also affect how
well a noise barrier performs.
Temperature inversions and downwind conditions can increase sound levels
in neighborhoods
protected by a noise barrier. Temperature lapses and
upwind conditions can further reduce sound levels in neighborhoods
protected by a noise barrier.
The changes in sound levels will depend on the
specific wind and temperature conditions.
CONTACT US
Benjamin R. Sperry
IDOT Noise and Air Quality Specialist
Phone: (217) 785-0202
Email: [email protected]
Web: https://idot.illinois.gov
Source: NCHRP Report 882. The National Cooperative Research Program (NCHRP) is sponsored by the individual state departments
of transportation of the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the
Feder
al
Highway Administration (FHWA). NCHRP is administered by the Transportation Research Board (TRB), part of the National Academies
of Sciences, Engineering, and Medicine.
Any opinions and conclusions expressed or implied in resulting research products are
those of
the individuals and organizations who performed the research and are not necessarily those of TRB; the National Academies of
Sciences, Engineering, and Medicine; or NCHRP sponsors.
WHY
IS IT
SO
LOUD
T
ODAY?
Understanding
how weather
affects traffic
noise levels in
your community
Have you heard?
You may have
noticed that sound levels from highways
or other sources are much louder or quieter
during particular times of the day or year.
Changes in weather conditions are often the
cause of these higher or lower sound levels.
What happens when the wind changes?
Changing wind speeds
above the ground cause sound waves to bend
toward or away from the earth
—a process called refraction. The change in
the sound level depends on the
differences in wind speeds above the
ground and the wind
direction. You might notice that sound levels are
higher when the wind is blowing from the highway toward you (downwind)
as illustrated below.
Conversely, you might notice that sound levels are
lower when the wind is blowing away from you and toward the highway
(upwind).
What happens when
the temperature changes?
The temperature of the air above the ground changes with height.
A
temperature lapse occurs
when the air above the ground is cooler than the
air near the ground
. Temperature lapses are common during the day.
Lapse
s cause sound waves to bend away from the earth and reduce sound
levels in nearby communities as illustrated below. You might notice that
sound levels are lower during the day than at night even though there may
be more traffic on the road.
Conversely, a temperature inversion occurs
when the air above the ground
is warmer than the air near the ground
. Temperature inversions are
common at night when the weather is
clear, and winds are calm. Inversions
cause sound waves to bend back toward the earth and increase sound
levels as illustrated below. You might notice that sound levels are higher at
night or in the early morning than during the day.
Have you heard?
You may have
noticed that sound levels from highways
or other sources are much louder or quieter
during particular times of the day or year.
Changes in weather conditions are often the
cause of these higher or lower sound levels.
What happens when the wind changes?
Changing wind speeds
above the ground cause sound waves to bend
toward or away from the earth
—a process called refraction. The change in
the sound level depends on the
differences in wind speeds above the
ground and the wind
direction. You might notice that sound levels are
higher when the wind is blowing from the highway toward you (downwind)
as illustrated below.
Conversely, you might notice that sound levels are
lower when the wind is blowing away from you and toward the highway
(upwind).
What happens when
the temperature changes?
The temperature of the air above the ground changes with height.
A
temperature lapse occurs
when the air above the ground is cooler than the
air near the ground
. Temperature lapses are common during the day.
Lapse
s cause sound waves to bend away from the earth and reduce sound
levels in nearby communities as illustrated below. You might notice that
sound levels are lower during the day than at night even though there may
be more traffic on the road.
Conversely, a temperature inversion occurs
when the air above the ground
is warmer than the air near the ground
. Temperature inversions are
common at night when the weather is
clear, and winds are calm. Inversions
cause sound waves to bend back toward the earth and increase sound
levels as illustrated below. You might notice that sound levels are higher at
night or in the early morning than during the day.
What are the effects
on my community?
The
effects on a community depend on the distance to highways and the
frequency and duration of weather conditions. Weather patterns that change
sound levels may be more common in certain areas.
Higher sound levels will
be more common in areas where the wind t
ypically blows from a highway
toward a community (downwind) than
the opposite direction (upwind). You
may also notice that sound levels are lower during the day than at night even
though there may be
more traffic during the day. This occurs in areas where
nighttime
temperature inversions are common. Rain can also affect traffic
noise by changing the temperature profile in addition to the more obvious
increase of noise generated from vehicle tires.
What if my neighborhoo
d
has a noise barrier?
Changes in weather conditions also affect how
well a noise barrier performs.
Temperature inversions and downwind conditions can increase sound levels
in neighborhoods
protected by a noise barrier. Temperature lapses and
upwind conditions can further reduce sound levels in neighborhoods
protected by a noise barrier.
The changes in sound levels will depend on the
specific wind and temperature conditions.
CONTACT US
Benjamin R. Sperry
IDOT Noise and Air Quality Specialist
Phone: (217) 785-0202
Email: [email protected]
Web: https://idot.illinois.gov
Source: NCHRP Report 882. The National Cooperative Research Program (NCHRP) is sponsored by the individual state departments
of transportation of the American Association of State Highway and Transportation Officials (AASHTO), in cooperation with the
Feder
al
Highway Administration (FHWA). NCHRP is administered by the Transportation Research Board (TRB), part of the National Academies
of Sciences, Engineering, and Medicine.
Any opinions and conclusions expressed or implied in resulting research products are
those of
the individuals and organizations who performed the research and are not necessarily those of TRB; the National Academies of
Sciences, Engineering, and Medicine; or NCHRP sponsors.
WHY
IS IT
SO
LOUD
T
ODAY?
Understanding
how weather
affects traffic
noise levels in
your community
