Download Yeah, There is a Difference Measuring Road Weather and Using it!

Yeah, There is a Difference
Measuring Road Weather and Using it!
Jon Tarleton
Head of Transportation Marketing – Meteorologist
Twitter:
@jontarleton
What are we going to talk about?
 The weather of course! But when and what matters!
 The weather before, during, and after a storm.
 The weather around frost.
 Let’s start with nothing and build on it.
 When we are done you will know what information is good
information!
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There is weather…
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And then there is
road weather…
Let’s start at the beginning!
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You just got your fleet of new plows!
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But otherwise you are Anytown, USA
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Approaching winter storm
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Air temperature
Critical in telling us the type of precipitation.
How do we measure it?
 Measured from 6ft off the ground
 In a white vented enclosure
Combined with wind it has an impact on our road
surface.
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Thermodynamics 101
To understand how the air impacts our pavement we
must understand how heat transfers from objects,
and between the air and objects.
Sun
Air
Subgrade
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Wind
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Wind – An important piece of the weather
 Lows typical move from
southwest to northwest.
 System may not always
contain all of the
precipitation types.
 Best snow is usually approx.
250 miles north of center of
low.
 Greatest uncertainty with
forecast is located near
center of low.
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L
When will it begin?
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We have lots of tools
Weather Forecast
Radar
Media
Dew Point
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Moisture in the atmosphere
Meteorologists have several
parameters we use to note the
amount of moisture in the air.
 Relative humidity
 Dew point
 Wet bulb
In the winter moisture can be
hard to come by, so monitoring
it is critical.
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Dew Point
The temperature to which air must be cooled for
saturation to occur.
 It is reported in degrees F or C.
So if it is 70°F in this
room the dew point is
some number less
than or equal to 70°.
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Why is dew point easy to use?
50
40
31
28
30
39
35
29
30
36
32
20
10
0
7am
10am
Air Temp
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1pm
Dew Point
4pm
Why is dew point so important?
It gives you an easier way to see how moist is the
atmosphere.
88°F
Winter
40°F
35°F
72°F
Summer
88°F
40°F
42°F
25°F
Air /
Dew
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Air /
Dew
Air /
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Air /
Dew
In Winter it can signal the start of snow
30°F
15°F
Air /
Dew
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Radar can be misleading
Radar “sees” moisture, it
does not know if it is
reaching the ground.
Forecast models still do a
horrible job at timing
saturation.
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Watch the dew point!
Storm Overhead
35
30
25
20
15
10
5
0
28
30
30
28
27
23
18
7am
19
10am
Air Temp
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Snowing!!
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1pm
Dew Point
4pm
You don’t plow the air or 6ft off the
ground!
Using the air temperature to predict if snow will stick
is dangerous.
Thermodynamics tells us the road surface will be
different.
Chemical effectiveness is going to occur at the
surface temperature.
What can we use to detect?
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Our eyes
For most of us they have never let us down
They tell us what we need to know
Helps us make all sorts of decisions
They cannot tell differences in temperature
They cannot see how slippery is something
Our eyes are fooled by changes in light,
especially at night
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We have two choices
1. Place a thermometer in the road surface.
2. Use an infrared sensor above the surface.
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Handheld infrared gun
 Easy to use! Point and shoot!
 Cheap!
 No training necessary!
 Sensor must be acclimated to environment
 Not originally designed for outdoor use
 Typical accuracy comments:
 ± 4°F from 32°F to 55°F
 Assumes ambient operations
temperature of 73°F to 77°F
 One model does not work below 32°F
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Embedded pavement sensors
 Placed directly in the road to measure
pavement temperature.
 Detects road chemicals and freezing
point of solution
 System archives data for later
analysis.
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Mobile weather sensors
 Extremely popular because of
cost and ease of use.
 Allows user to see entire road not
just one spot.
 Basic System is air and pavement
temperature
 Advanced System adds dew point,
friction, and road condition.
 Not as accurate as fixed.
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Non-intrusive sensors
Non-Intrusive is a very popular method for all
road devices.
In science we call measuring something from a
distance remote sensing.
Benefits of sensors:
 Lower installation costs/cost to maintain
 Safer for service and install
 Accuracy similar to in-road sensors
 Provides a new value – road friction
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The Storm is here!
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Air Temperature still is important
 Near the low pressure center, or as a
front passes temperatures will vary the
most.
 Winds play a vital part in temperature
changes because “wind moves” colder
and warmer air.
 Due to clouds and precipitation the sun
is not really much of a factor on air
temperature.
 Changes in air temperature will have a
slow impact on road temperatures.
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Coldest
City
Warmest
Winds
Wind during a storm creates two concerns:
 Impacts air temperature changes based on direction
 (Which leads to) changes in pavement temperature,
especially in areas exposed to wind.
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Misconception of wind chill
 Wind Chill Factor was created by meteorologists to give
people an idea of how cold it feels outside.
 The wind chill value itself has no meaning to object such
as roads and bridges.
 For example -10°F wind chill will not cool the bridge to 10°F.
 However if the air is getting colder or warmer the
pavement will respond.
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Pavement temperature
 Chemicals are working or not working
based on this temperature!
 Surface is being impacted by:
 rain or snow on surface (sleet is worse)
 Plowing exposes surface to air
temperature
 Evaporation is a cooling process
 Endothermic and exothermic chemicals
are causing minor changes
 If subsurface is 40°F or above heat is
coming from below
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Measuring surface temperature
 It becomes a little trickier, infrared sensors
are now telling you the temperature of the
water or ice, which might be different.
 Bridges and elevated surfaces will tend to
match the air temperature.
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Condition of the Road
 Road condition becomes the key during an
event.
 Drivers visually see road conditions and
react.
 Vehicle performance is impacted when the
grip/friction is reduced on the surface.
Pavement
Temp.
 Condition of the road is impacted:
 Pavement temperature
 Amount of chemical present vs. amount of
ice/water
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Amount of
Chemical
Amount of
Ice/water
Measuring road condition
Road condition measuring is not easy. Why?
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What is the road condition?
Images :Courtesy of Duane Amsler
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Eyes again!
There is too much that is subjective
in a observation.
We are using our eyes again which
we know are not good at measuring
much of anything.
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Subjective
Lost Trail Pass, Idaho : Guess the driving conditions on
the following days?
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Did you get them right?
Road Temp: 30ºF
Road State: Slush
Grip: Good
Road Temp: 50ºF
Road Temp: 43ºF
Road State: Dry
Grip: Excellent
Road State: Wet
Grip: Good
Road Temp: 28ºF
Road Temp: 43ºF
Road State: Ice
Grip: Poor
Road State: wet
Grip: Good
Road Temp: 28ºF
Road State: Ice
Grip: Very poor
Road Temp: 39ºF
Road Temp: 28ºF
Road Temp: 21ºF
Road State: Dry
Grip: Excellent
Road State: Ice
Grip: Poor
Road State: Dry
Grip: Excellent
Salt Residue
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Salt Residue
Friction/Grip
 Ultimately isn’t that what we are
wanting to know? How slippery is the
street?
 Measuring friction also removes the
subjective issues with people reporting
conditions.
 Friction/grip is reported as the
coefficient of friction:
0.
0
0.
5
0.
2
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0.8
Dry
Road
Wet
Road
Icy
Road
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1.
0
The Snow has ended…
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Wind and Blowing Snow
 Northwest winds are typical
following a winter storm in most
of the U.S.
 Increased wind speeds are
usually caused by two factors
 proximity to low pressure (storm)
 diurnal (daytime) changes in wind
 Drier snowfall that has not a
chance to melt and refreeze
moves the most.
 Use weather data with wind
information to monitor and track.
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Pavement temperature
 After the precipitation has ended the
sky condition, winds and road condition
begin to cause larger changes in
temperature.
 Cooling will occur (if at night):
 skies clear
 winds weakening or blowing in cold air
 surface has been plowed
 Warming will occur (if during the day):
 skies clear
 surface has been plowed
 surface faces south
 Use pavement temperature tools to
monitor
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Dew point
 Typically in the U.S. after an event
has passed cold, dry air filters in
behind the storm.
 A good North, Northwest, or
Westerly wind keeps dew points
low or even makes them lower.
 Streets dry quicker the drier the air
(lower dew point).
 If winds weaken completely, change
to a more southerly direction, and
there was any melting of snow… it
is not all good!
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Dew Point and Frost
 At night, as the air cools, heat also leaves the road
surface. The road can cool faster than the air and thus fall
below the dew point.
33°F Air
30°F Dew
33°
F
31°
F
29°
F
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Summary
Understanding the weather is the first thing to
overcoming when dealing with weather and winter
maintenance.
Next, make sure you have the correct tools.
You can see that with each tool, or piece of information,
you can react smarter to winter.
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