**1. Introduction**

Global climate change is one of the most pressing environmental challenges of the century. Climate change is affecting cities and their residents, especially the poor ones, and more severe impacts are expected as climate extremes. Cities already face significant climatic and environmental challenges that are independent of climate change. They are generally warmer

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than the surrounding non-urban areas, because of the higher heat absorption and the relatively limited cooling associated with vegetation and permeable surfaces. Urban areas suffer from air pollution, which is exacerbated by high temperatures. In conjunction with these existing issues, the impacts of climate change on cities will depend on the actual changes in climate, such as increased temperatures and rainfall.

the emissivity, and the fraction of vegetation cover. Thus, urban regions behave a lot different from natural ones and they cause unique physical processes, depending on many parameters like the heat retaining capacity of the construction materials, the sealing of the soil, a modified water balance and the waste heat. As a result, urban landscapes modify the original physical processes that govern any natural land surface, and also add new, unique biogeophysical and biogeochemical processes into the land surface–atmosphere, such as the storage heat flux, the

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The urban surface and morphology results in cities being relatively warmer than the rural surroundings, a phenomenon called urban heat island (UHI). The warmer city climate can have fatal consequences such as those witnessed during the summer heatwave of 2003 in Central Europe [3]. There are different kinds of UHIs, displaying different characteristics and controlled by different assemblages of energy exchange processes. These possess different scale manifestations and result from different processes. Air temperature varies with height, a phenomenon much complicated in the urban environments and the different atmospheric layers. Thermal remote sensing permits definition of an UHI named ground or surface UHI, which refers to the skin or surface temperature difference between the city and its surround-

The concept of scale is fundamental in the understanding of the surface-atmosphere interactions when it comes to urban environments. In building scale, for example, the walls and roof facets have different time-varying exposure to solar radiation, net longwave radiation exchange, and ventilation. Horizontal ground-level surfaces are a patchwork of elements, such as irrigated gardens and lawns, non-irrigated greenspace, and paved areas with contrasting radiative, thermal, aerodynamic and moisture properties, frequently including trees. These different surface elements possess diverse energy budgets that generate contrasts in surface characteristics (e.g., skin temperature), and lead to mutual interactions by radiative exchange and small-scale advection. These fundamental units may be aggregated hierarchi-

Distinctive energy balances characterize each scale that generally do not represent the areaweighted average of the budgets of individual elements. This happens because each unit interacts with adjacent ones in the same scale by advection. While spatial scale increases, the

**Figure 1.** Graphical illustration of the different scales in the city. The urban canyon scale includes building walls and elements between buildings. The city block scale includes a number of urban canyons and roofs of buildings. The neighborhood scale refers to a number of city blocks, while the land use scale refers to larger areas including many

canyon effect, and the anthropogenic heat flux [2].

ing areas.

cally, as illustrated in **Figure 1**.

similar neighborhoods.

The cities need to adapt and the climate change needs to be considered in all development plans and investments, local, national, and international. Local policy makers tend to see climate change as an environmental issue of global scale that is not of their concern. The majority of climate change specialists focus on reducing greenhouse emissions, without practically helping cities to learn how to change and adapt. Climate change science mainly deals with global and regional impacts, and it is less able to provide reliable assessments for the cities.

Datasets from Earth observation (EO) satellites are crucial for measuring key parameters relevant to the climate change. The use of satellites to observe the Earth provides the data necessary to improve our understanding of the Earth system and help predict future change. The satellite data and products may form the understanding of climate change and the quantitative estimates of its effects form the basis for policy-makers to build effective strategies for adapting to and mitigating the effects of a changing climate. Although EO data and products are mainly used for global and regional research studies, there is great potential in their use for monitoring the urban climate and thus allow cities to adapt to a changing climate.
