Abstract/Summary

The urban heat island (UHI) is one of the earliest documented effects on air temperature observed in urban areas. It results from differences in the surface energy balance between urban and rural areas, caused by the interaction of urban surfaces with atmospheric processes. The focus of this guidance is the canopy layer UHI (CL-UHI). The CL-UHI is concerned with near-surface air temperatures differences (~1.5 m above ground) between urban and rural areas. With more than half of the world’s population living in cities, these temperature effects are directly experienced by many humans and can be linked to changes in human comfort, public health and human activities. Exposure to high temperatures can increase morbidity and mortality, especially during heat waves and at night, when urban CL air temperature can be elevated compared with the rural surroundings. Other types of UHIs, such as the boundary layer, surface or subsurface types, have less direct influences on human health and are therefore only briefly addressed in the present guidance. The scientific background needed to understand the processes creating and influencing the CL-UHI is presented in the present guidance, with examples of different agencies and services that need information about the CL-UHI. Critical to this is understanding the role of scale and the link to urban form. Guidance is provided on the parameters required to characterize urban areas at microscale and local scale, and the ways in which they influence the CL-UHI are discussed. The guidance reviews the “ideal” weather conditions (calm, clear days and nights) for development of large CL-UHI intensities, and the influences of meteorological factors, topography and urban features. It introduces different approaches to measure the CL-UHI using sensors placed in different locations, for example a pair of sites, traverses, meteorological networks and opportunistic sensing, with discussion of site and sensor selection criteria. Similarly, modelling approaches, including statistical, obstacle-resolving and numerical weather prediction models, are compared, along with their advantages and limitations for determining CL-UHI values. Measurements can support CL-UHI assessments and enable robust evaluations of model outputs. Models performing satisfactorily for their intended applications can be used for simulating CL-UHIs for current and future scenarios such as city development and climate change. Monitoring the CL-UHI requires both observations and modelling. Key to monitoring are long-term maintenance and documentation of changes, for example, of site and instruments (that is, metadata). Mitigation and adaptation efforts to reduce the effects of CL-UHIs are discussed, such as efforts to reduce heat stress, air pollutant concentrations and energy use. The guidance is written for WMO Members, National Meteorological and Hydrological Services (NMHSs) and their many potential partner agencies and stakeholders undertaking activities in cities that are impacted by weather and climate across a wide range of time and space scales. Information on CL-UHIs is one part of an IUS and may be part of multi-hazard early warning systems and high-resolution weather prediction systems for urban areas.