Road Weather: Pavement Forecasting

Transcription

1 Road Weather: Pavement Forecasting By Jon Tarleton Marketing Development Manager Meteorologist Surface Systems, Inc. (SSI ) QTT Quixote Transportation Technologies jdt@surface.com 2006 APWA North American Snow Conference Peoria, Illinois Monday May 1 st, 2006

2 Introduction The weather impacts us all, whether it is in our personal lives or while at work. Consciously or not, we know that every time we step outside the weather is going to affect the way we feel. Our miles of asphalt and concrete, which are our main concern, are also affected by the weather on a minute by minute basis. In the winter when we are concerned about pavement temperature the most, we spend most of time trying to guess how the weather will impact our pavement, and thus impact driving conditions. Unfortunately the pavement does not always behave like we expect. How can a bridge deck get colder than the air temperature? The temperature of a bridge or road surface is one of the most critical pieces of road information, so therefore we must learn how and why it changes. Why does frost form sometimes, but not always? Learn what to look for in the formation of frost, and how it gets on our roads. Why does the pavement temperature sometimes drop dramatically at the beginning of a snow event, and why does it drop only slightly other times? Once we know how thing change can we forecast pavement temperature? We all know how inaccurate a weather forecast can be, so why would any meteorologist try to predicting what is going to happen to an object in that weather. The Effects on Pavement As we all know anything that is exposed to the weather is somehow affected by the weather. Nothing says it better than the classic tourist trinket a small rock on a string with the instructions, If the rock is wet it is raining, if the rock is moving it s windy, and if the rock is gone it has been stolen. The human body and the bodies of animals are the only things that can attempt to regulate their own temperature when the weather changes. All other objects are incapable of this, and therefore change as the weather changes. Our roads and bridges are no different. The four major things that affect our pavement are the air, sun, moisture, and the amount of heat beneath the pavement. Air Before we had devices to measure pavement temperature we relied heavily on air temperature as our gauge as to when the pavement temperature might change. However, we have since learned through such devices as Road Weather Information Systems (RWIS) and vehicle mounted pavement sensors that trying to gauge pavement temperature using air temperature alone is not accurate. The temperature of the pavement can be dramatically different in many cases caused by our other weather parameters.

3 Our pavement surface is exposed on one side to the air, which means the air does impact our pavement temperature. To understand how the pavement is impacted we must have a very basic understanding of the balance of heat, otherwise known in science as Thermodynamics. The basic understanding is that every object is in a constant state of temperature change. For example, think of a table in a room. This table is constantly gaining and loosing heat. Now if the room is at a constant temperature, the table eventually reaches the temperature of the room. If we lower the temperature of the room dramatically the table will loose heat until it reaches the new room temperature, and because the table is exposed to the air on all four sides the temperature change happens more quickly than say a book laying flat on the floor in the room. Surface Area: The greater the exposed surface area the faster the change. A bridge is similar to a table in that it is exposed to the air on all four sides; therefore this is one reason why bridges and other elevated roadways can cool quicker than pavement with ground underneath. Another consideration for the temperature change of an object is the density and composition of that object. For example if you have two objects outside on a cold day; a metal one and a rubber one, and if you touch the metal handle with your bare hands it feels colder than the rubber object even though both are at same temperature. Why? Because metal conducts heat better than rubber, therefore the metal object draws heat out of your hand quicker than the rubber handle making it feel colder. Density/ Material: Certain objects gain and loose heat more quickly than others depending on density and composition of the object. A bridge that contains metal and concrete will gain and loose heat faster than a road surface. Is it possible for a bridge deck get colder than the air temperature? Yes. Because of the density of the bridge and the fact that it can loose heat on all sides, allows for the air to act like a vacuum and draw the heat out of the bridge. One of the biggest misconceptions with pavement temperature change is the effects wind chill has on the pavement. Wind Chill only affects the human body and has no affect on roadways, bridges, etc. Wind chill is a value developed by meteorologists to give people an idea as to how cold it feels outside. Wind does affect the pavement, but only when the air temperature is changing. As cooler or warmer air blows across the pavement it eventually changes the temperature of the pavement. If the air is getting colder because of the wind, the stronger and/or colder the wind the faster the pavement cools. Solar Radiation (Sun) Regardless of the time of year, the sun is one of the biggest controllers of pavement temperature. The sun has such a huge impact on the pavement that even on a partly cloudy day, the temperature of the pavement can fall as much as 10 degrees when the

4 pavement is shaded by clouds. In fact, even in the middle of the winter, during the day, under heavy cloud cover, the pavement is still being impacted by the sun s radiation. Elevation is another important consideration with respect to solar radiation, because the higher the elevation the higher the solar radiation. Moisture Moisture affects the temperature of the pavement through the amount of moisture in the air, and by the precipitation falling from the sky onto the pavement. Moisture appears in the atmosphere in all three forms, as a gas in the form of water vapor, as a liquid in the form of clouds and rain, and as a solid in the form of snow and ice. The most difficult of the three to understand is the water vapor form because it is invisible to us. Water vapor is reported by weather measurements in two forms, Relative Humidity and Dew Point. Now while most people think they understand relative humidity it is really quite a confusing measurement. To better understand the amount of moisture in the air and to see its effects on the pavement, Dew Point is the preferred measurement to use. However, Dew Point is also the least understood term in weather, so we need to cover a little Dew Point 101 to aid in the understanding. Dew point is one of the most important variables to monitor before and during a winter storm and directly relates to pavement conditions. Example: Temperature 40º F (4ºC) and Dew point 30º F (-1ºC) If you cool the air temperature from 40º F (4ºC) Down to 35º F (2ºC) Down to 30º F (-1ºC) condensation occurs! When the temperature reaches the dew point it causes water molecules in the air to form into water droplets on a surface. In the atmosphere, water vapor condenses on the surfaces of pollen, dust, and pollution. Dew point can give you an indication of how moist an incoming air mass is. Example: Dry air mass Moist air mass Temp 30º F (-1ºC) Temp 30º F (-1ºC) Dew point 15º F (-9ºC) Dew point 27º F (-3ºC) Why does water sometimes condense on a soda can? Because the temperature of the soda can is below the dew point temperature. The temperature of the soda can might

5 be 35-40º F (2-4ºC) and dew point could be 50º F (10ºC). As air around the can begins to cool, the air temperature drops to 50ºF (10ºC) or below, therefore water vapor in the air now begins to condense on the can. If the surface of can was below 32 º F (0ºC), the water vapor would condense as frost instead of liquid. Keep in mind that the dew point is still 50º F (10ºC). The same thing can happen on the pavement surface. Remember, the surface temperature and dew point temperature is the key. The pavement surface can become wet or frozen depending on surface temperature. When the pavement temperature falls below the dew point it will develop condensation on the surface. If that pavement temperature is below freezing then frost will form on our surface. To form bridge frost we have some key conditions to watch for: Clear skies overnight Low level moisture from melted snow or recent rains Light winds Pavement temperature below freezing Pavement temperature less than the dew point temperature If all these things occur we may still not always have frost. Why? There are two key issues we need to watch for which may hinder our frost formation, or idea that frost could exist. The first issue is where we are measuring our pavement temperature and dew point? If we have the luxury of an RWIS system we are measuring the pavement temperature from a sensor in the bridge and the dew point from a device on the RWIS tower. But where is our RWIS tower? How close is the RWIS tower to the bridge? Remembering that an RWIS tower could be as much as 5000ft from the bridge, which is long enough to cause changes in dew point measurements. If we do not have an RWIS site the most convenient location to obtain the current dew point is from a nearby airport. As you can imagine this is OK, but many times this is miles from our bridge. Most times during frost conditions the bridge temperature only drops a degree or two below the dew point, so we have little room for error. The second issue relates to how quickly frost forms. When the bridge temperature only drops a degree or two below the dew point condensations begins to form slowly over several hours. The farther the pavement temperature drops below the dew point the quicker the frost formation. Condensation can also occur in a road surface as well, which sometimes is called road frost or a type of black ice. This typically happens when the ground underneath a road has been allowed to get very cold. Then when a sudden warm-up occurs (not caused by the sun) the road surface can not warm quickly enough, because it is sitting on a block of ice. These events typically occur late in the winter or early spring in areas of the country where cold weather has been persistent. As you can see, if we know the current dew point and pavement temperature we can determine if and when frost or black ice will form on the pavement surface. What about moisture in the form of rain or snow? How does this affect the pavement temperature? Much like when colder air moves across our pavement surface cooling the pavement, rain and snow will typically cause cooling of the pavement. The harder the rain or snow is

6 falling the more it increases cooling, and therefore the heat currently being held by the pavement can not keep up with the cooling effects of the rain or snow. The dew point has another important relationship with falling precipitation most people do not realize. We learned earlier that the farther apart the air temperature and dew point are the drier the air. As soon as it begins raining or snowing some of the water/ice begins to evaporate back into the atmosphere, even while it is still raining or snowing. The process of evaporation requires heat from the surrounding area, so in the case of our pavement it takes the heat from the pavement. The drier the air, the fast evaporation occurs and thus the faster the cooling process on the pavement. The best effects of this can be seen during the winter when the air is drier than it is in the summer. Heat Beneath the Pavement As we discussed earlier a road surface is affected on only one side by the atmosphere. Its other side is affected by the ground beneath. In an effort to predict the pavement temperature, meteorologists knew that to accurately predict the pavement temperature we would need to know how much heat is beneath the pavement. As RWIS were installed across the country, a temperature sensor was added beneath the pavement to measure this heat. It turned out that sub-surface temperature was one of the most important pieces of information needed to determine how the pavement would react to the weather above. When the subsurface temperature is warm, snow may still accumulate on the pavement above, but as soon as the snow stops the heat beneath the pavement, warms the surface back to above freeze allowing for melting. When the subsurface temperature is cold, rain can fall on the pavement causing the last drop in temperature needed to freeze rain on the road. Subsurface information can only come from RWIS installed by highway and street departments. Measuring Pavement Temperature Most of us have known for a while that pavement temperature is very important for us to know during the winter months, and therefore several methods have been developed to measure it. RWIS Installed Pavement Sensors Installing an actual temperature measuring device within the pavement is still the best way to measure the pavement temperature. It produces the least amount of error and the systems typically archive the information for later retrieval and analysis.

7 Vehicle Mounted Pavement Sensors By far the most popular way to measure pavement temperature due to the easy of use and inexpensive start-up cost. However this method is not without vaults. Vehicle mounted sensors use infrared technology to measure the heat of the pavement; however as with all infrared technology it can only measure the first thing it sees. If the road is wet or snow cover it is telling you the temperature of the water or snow. If the lens gets dirty it is tell you the temperature of the lens. Non-intrusive Pavement Measurements The new buzz word in the world of transportation sensors is non-intrusive or measuring a parameter from the side of the road instead of in it. Non-intrusive sensors measuring everything from traffic to weather are becoming popular because of the lure of less installation cost and easier maintenance. The proper term used by scientists is called remote sensing measuring something from afar. However no matter how good this technology will get, it will mostly likely never be more accurate than a device at the location. The questions to ask when considering this technology are: How accurate is it? At what times is it not accurate? If the device is accurate 95% of the time, but it can t measure the pavement temperature correctly during critical times what use is it? Forecasting the Pavement Change As we have learned there are many variables that go into the changing of pavement temperature, but even though these processes may seem simple enough to try predicting these changes on your own. Don t forget your own likely impression of weather forecasting, They never get it right! Forecasting the weather is challenging enough, trying to predict the air temperature at 9pm this evening can prove to be a difficult task. Now try forecasting the temperature of something out in the weather at 9pm, such as the pavement. You have to accurately predict the weather to have any chance of predicting the temperature of something out in the weather. This alone makes forecasting pavement temperature a difficult task, but certainly not impossible. Now since the pavement temperature can change so quickly over a given area because of the factors we discussed (air temperature, precipitation, clouds, thickness of the pavement, etc.) pavement forecasts are forced to be site specific; for example the I-80 at Exit 22. So all good pavement forecasts are site specific.

8 To create a pavement forecast meteorologist must first create a forecast for the atmosphere above the pavement, because as we learned the weather above the road is so important to pavement temperature change. This atmospheric forecast is then imputed into a computer program that has an understanding of all the pavement influencers we have discussed. This computer program known as a pavement model, then must know the current pavement temperature and the trend (is it getting warmer or cooler). The last value required is the current subsurface temperature, so that this model now has a complete picture of all the forces affecting the pavement, and now an accurate predication of the pavement temperature can be made. Typically these forecasts go out 24 hours into the future, but could go further out if ever required by the industry. So to get one of these pavement forecasts we need an RWIS system? NO! Of course having an RWIS system does make the forecast more accurate, but it is not necessary. A pretty good pavement forecast can be generated by using the nearest RWIS site (typically owned by a DOT, City or Airport). The meteorologist then makes an atmospheric forecast over your area of concern, and then the nearest RWIS site is used to gather the other current information, but start times of rain/snow are still for your location not the nearby RWIS site. In addition, current RWIS conditions typically only impact the first 6-8 hours of the pavement forecast, after that the atmospheric forecast has a bigger impact. Conclusion As we have learned there are a lot of forces impacting our pavement temperature and thus making our decision of what will happen difficult but not impossible! But as you can see sitting there before a storm trying to remember all the variables that affect the pavement, and measure these impacts is not worth your time. Knowing whether the pavement will freeze is one of your fundamental questions that you must answer to determine how you will deliver the best service. Simply trying to guess the answer may work sometimes, but it will make you a lot more efficient if you have a much more reliable solution. State DOTs rely heavily on these pavement forecasts to determine when to pre-treat their roads, when to schedule crews, and how much chemical will be required throughout the duration of the event. But we are a small agency with a limited budget. Isn t that more reason why you have limited room to be wrong?