Urban planners and city officials interested in targeting interventions need to identify neighborhood level hotspots. However, urban microclimate measurement poses substantial challenges. For example, data taken at local airports are not representative of the conditions at the neighborhood or district level because of variation in impervious surfaces, vegetation, and waste heat from vehicles and buildings. In addition, fixed weather stations cannot be deployed quickly to capture data from a heat wave. While remote sensing can provide data on land cover and ground surface temperatures, spatial and temporal resolution remain significant limitations. In an effort to overcome some of these issues, we have designed, constructed, and validated a mobile measurement bicycle to investigate the microclimates of Cuyahoga County, Ohio.
Mobile measurement provides a simple way to gather mesoscale data along a transect that spans urban and rural land uses. Mobile surveys are a common method used in urban climate studies for assessing and quantifying canopy layer UHIs; they are also used as part of a larger observation network. Automobiles or light trucks are the most common platform for these studies; temperature sensors are typically attached in front of the engine or on the roof or to avoid thermal contamination. Advantages of mobile surveys include high temporal resolution of data, low cost compared to the installation of multiple stationary weather stations, and no requirement to cross-calibrate sensors from multiple sites.
However, while automobile-based measurement allows for analysis at the urban scale, this equipment is often too cumbersome to investigate microscale phenomena that might occur at the neighborhood- or block-level. To overcome these limitations, we borrowed insights from the thermal comfort literature to understand how researchers use mobile measurement carts to support their work. Although automobile transects and thermal comfort using carts are both well established approaches for their respective literatures, until recently there were few efforts that bridged the gap between these meso- and microscale methods.
To the best of our knowledge, this is the first time a research grade weather station has been installed on a bicycle to gather multiple types of data (e.g., ground surface temperature, solar radiation, sky view factor) for analysis. For our research, we also wanted to determine the limits to the amount of equipment that could be carried, if a bicycle was a suitable platform for this type of analysis, and what limitations non-motorized transportation imposed on rural to urban transects.
Because we needed to place meteorological equipment at least 1.25-meters above grade to avoid interference from the ground and travel up to 50 kilometers per transect, we chose a cargo bicycle as a base for the equipment. Cargo bicycles are commonly used for bicycle touring with large amounts of camping equipment. They differ from standard bicycles in that they have a heavier duty frame, spokes, and brakes, and a longer wheelbase to improve stability under load. We were especially interested in bicycles with high weight capacities (>100kg) to hold the equipment on front and rear mounted racks, multiple gears to assist pedaling from site to site, but a size small enough to be stored on a standard bicycle rack to move from site to site.
A thermocouple, hygrometer, and GPS unit was installed at the top of a 2.0-meter tall aluminum tower constructed of extruded aluminum sections. The GPS unit collected latitude, longitude, and speed, and provided a time stamp to synchronize fisheye images taken by a camera to determine Sky View Factor. A four component net radiometer and infrared radiometer were installed off the back of the bicycle at 1.25-meters above the ground to gather information about incoming and outgoing short- and longwave radiation and ground surface temperature. All of the equipment took a reading once every second; the datalogger averaged the measurements for each minute and stored it to an onboard solid state hard drive.
The bicycle performed well as a platform to gather data to analyze ground and surface temperatures. The bicycle allowed us to access locations that would be inaccessible by automobile and was less expensive than setting up multiple research grade weather stations. In addition, riding a bicycle helped us in the validation of land cover data from satellites, something that would be difficult to accomplish from the confines of an automobile.
However, bicycles do have significant limitations, such as safety, heat stress, fatigue of the rider, and difficulty scaling steep terrain. Interpretation of weather data from mobile measurement systems was more difficult than interpreting results from a static weather station because of the need to account for spatial effects like the bicycle speed, location, and autocorrelation of thermal data.
While the data we collected is helpful for understanding temperature variations and the extent of the urban heat island effect, more research is needed before the results broadly inform policy. For example, there is no document to standardize mobile measurements like the WMO guidance to obtain representative meteorological observations from urban sites, or ASHRAE Standard 55 which governs thermal comfort in buildings. We suggest that there should be more standardization among equipment, measurement heights, and protocols for data collection and analysis; this will promote the comparability of results among multiple studies.
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