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6 CHAPTER 3 SNOW AND ICE CONTROL OPERATIONAL CONSIDERATIONS The major snow and ice control operational considerations, Other important meteorological variables that have an exclusive of agency resources, are climate, weather, site con- impact on winter pavement conditions are sky, or cloud cover ditions, and traffic. Each has a profound effect on some aspect conditions (solar radiation effects); air temperature, to the of operations. extent it establishes the trend in pavement temperature; dew point temperature; condensation; pavement temperature; rel- ative humidity; wind speed and direction; and evaporation. CLIMATE CONDITIONS Condensation occurs when the pavement, or bridge deck, temperature is above 32F and below the dew point temper- Climate can be defined as the weather that occurs averaged ature. Frost, on the other hand, occurs when the pavement over a specified period of time, normally 30 years. The climate temperature is at or below 32F and below the dew point tem- is also defined by statistics about the frequency of extreme perature. It is common for bridge deck surfaces to develop events. Climate elements are the averaged meteorological frost when the adjacent highway surfaces do not. This typi- elements. cally happens in the fall and spring when these temperature Table 2 provides a listing of major climate-related issues. conditions are satisfied, the sky is free of cloud cover, and the Most of these have value in the planning phase of operations. wind speed is calm (0 to 3 mph). The early morning hours, LOS goals are to some extent climate driven in terms of what is and is not possible. Strategies and tactics that support LOS just before sunrise, are ideal times for bridge deck icing/ goals are similarly climate driven. Certain recurring site con- frosting to occur. The prediction of these icing conditions is ditions (microclimates) are climate driven and require spe- particularly difficult, especially for rural areas with elevation cific recurring operational responses. These include cold spots, changes and varied roadside vegetation coverage. Location high humidity locations, persistent windy areas, etc. of Road Weather Information Systems (RWIS) sensors in In any climate, the achievable LOS is limited by the rate of these sensitive areas has been most helpful in detecting the precipitation, cycle time capability, sustainability of the main- onset of frost conditions. tenance effort, site conditions that may cause road closure, Crosswind speeds in excess of about 15 mph may cause and materials options. Higher LOS for similar weather condi- local snow drifting and inhibit anti-icing operations. Also, tions can be provided with shorter cycle times and the use of liquid chemical sprayers should be set closer to the pavement straight ice control chemicals. during windy conditions to avoid spray loss. WEATHER CONDITIONS SITE CONDITIONS Field winter maintenance personnel in a given area are Site conditions are those local situations that affect how mostly concerned with anticipated winter weather conditions snow and ice control operations are conducted. They influ- and not with climate considerations. "Weather" usually refers ence type of equipment, materials choices, materials appli- to the measurable or identifiable meteorological events that cation rate, priority and sequence of treatment, and type of occur at a given site or in a given area at a particular point in treatment. Table 4 is a listing of important site conditions. time. Weather can be characterized by describing the meteo- rological elements associated with those events (e.g., precip- itation type and amount, visibility, wind speed and direction, temperature, and relative humidity). TRAFFIC CONDITIONS Precipitation is arguably the most important weather con- dition. Having a working knowledge of precipitation defini- The influence of traffic was discussed in the Chapter 2 sec- tions is essential when designing a snow and ice control treat- tion on Snow and Ice Control Operational Considerations ment. Those definitions appear in Table 3 and are taken from Relating to Level of Service. The traffic conditions of possible the Federal Meteorological Handbook (FMH) No. 1 (6). importance are listed in Table 5.