between the center of the city and surrounding fields. The difference in temperature between an inner city and its surrounding suburbs is frequently mentioned in weather reports, as in " downtown, in the suburbs". "The annual mean air temperature of a city with 1 million people or more can be warmer than its surroundings. In the evening, the difference can be as high as ." The UHI can be defined as either the air temperature difference (the canopy UHI) or the surface temperature difference (surface UHI) between the urban and the rural area. These two show slightly different diurnal and seasonal variability and have different causes. Diurnal behavior The IPCC stated that "it is well-known that compared to non-urban areas urban heat islands raise night-time temperatures more than daytime temperatures." For example, Barcelona, Spain is cooler for daily maxima and warmer for minima than a nearby rural station. A description of the very first report of the UHI by Luke Howard in the late 1810s said that the urban center of London was warmer at night than the surrounding countryside by . Though the warmer air temperature within the UHI is generally most apparent at night, urban heat islands exhibit significant and somewhat paradoxical diurnal behavior. The air temperature difference between the UHI and the surrounding environment is large at night and small during the day. The opposite is true for skin temperatures of the urban landscape within the UHI. Throughout the daytime, particularly when the skies are cloudless, urban surfaces are warmed by the absorption of solar radiation. Surfaces in the urban areas tend to warm faster than those of the surrounding rural areas. By virtue of their high heat capacities, urban surfaces act as a giant reservoir of heat energy. For example, concrete can hold roughly 2,000 times as much heat as an equivalent volume of air. As a result, the large daytime surface temperature within the UHI is easily seen via thermal remote sensing. As is often the case with daytime heating, this warming also has the effect of generating convective winds within the urban boundary layer. It is theorized that, due to the atmospheric mixing that results, the air temperature perturbation within the UHI is generally minimal or nonexistent during the day, though the surface temperatures can reach extremely high levels. At night, the situation reverses. The absence of solar heating leads to the decrease of